Entrance and exit dose regions for a Clinac-2100C.

PURPOSE: While significant advances have taken place in confirming dose in homogeneous media and accounting for changes due to distant heterogeneities, interface dosimetry is still a dose assessment problem. The entrance and exit surfaces of the patient are a prime example where dose assessment is questionable, but important, in many clinical situations. METHODS AND MATERIALS: Data were taken to examine the effect of such parameters as field size, source-to-skin distance, blocking tray, compensation materials (lead, aluminum and brass) and various patient support materials (Mylar, graphite, thermal plastic, and foam) on the surface doses from a dual energy (6 MV, 18 MV) linear accelerator. Measurements were conducted with a thin window parallel-plate chamber. RESULTS: Relative surface dose was found to have a proportional dependence on field size for both energies with surface doses ranging from 6% to 53%. A surface depth dose of 22.6% for a 15 cm field was measured for the low energy beam while a surface dose of 22.3% was observed for the 18 MV beam. The surface dose increased significantly for short source to skin distances and with the presence of a blocking tray. Compensating filter materials had little influence on surface dose. However, patient support devices such as Alpha Cradle and the graphite of the table can increase the surface dose to as much as 92%. CONCLUSION: We found there was a loss in dose at the exit surface (in comparison with percent depth dose tables, which assume infinite depth) on the order of 15% and 11% for the 6 MV and 18 MV beams, respectively. However, this loss is quickly compensated for with the introduction of most inherent backscattering media, [which in fact can increase the dose] for example, graphite in the patient support assembly tabletop.

Changes in photon dose distributions due to breast prostheses.

PURPOSE: Subcutaneous prosthetic implants had been routinely used for cosmetic augmentation and for tissue replacement following mastectomy over the last 15 years. The implants come in many forms as the gel filler material and surrounding shell material(s) vary significantly. METHODS AND MATERIALS: This study uses a thin window parallel-plate chamber and thermoluminescent dosimeters to quantify and dosimetric changes to surrounding breast tissue due to the presence of the prosthesis. A mammographic phantom was compared to four commercial prostheses, namely two silicon gel fillers within two different shells (silicon or silicon/polyurethane), a tri-glyceride within silicon and a bio-oncotic gel within silicon/polyurethane. The latter two implants were designed with a low-Z fill for diagnostic imaging benefits. RESULTS: Ion chamber results indicate no significant alteration of depth doses away from the implant with only minor canceling (parallel opposed) interface perturbations for all implants. In addition the physical changes to the irradiated prostheses were quantified by tonometry testing and qualified by color change. Each implant exhibited color change following 50 Gy, and the bio-oncotic gel became significantly less formable following irradiation, and even less formable 6 weeks postirradiation. CONCLUSION: The data indicates that prostheses do not affect the photon beam distribution, but radiation does affect the prostheses.

Surface and peripheral doses of dynamic and physical wedges.

PURPOSE: The physical and dosimetric differences between three different wedge systems on a multileaf collimator (MLC) equiped linear accelerator are discussed in this report. In particular, the in-field and peripheral surface doses from these wedge systems are measured and their clinical differences discussed. METHODS AND MATERIALS: A parallel-plate chamber was used in a solid water phantom to measure the surface doses of the wedges. Published correction factors were used to convert relative ionization to relative surface dose. Measurements were performed for 6 and 18 MV photon beams for different field sizes, source-surface distances (SSD), and distances outside the field for peripheral dose measurements. Surface-dose profiles across a field in the wedge-gradient direction were measured for the dynamic and upper wedges. Dose profiles in the nonwedge gradient direction were measured for open beam as well as the three wedges using films at depths of maximum dose (d(max)). RESULTS: At 85 cm SSD, surface doses on the central axis under a dynamic wedge or upper wedges are similar to those of an open field, while those of a lower wedged field are as much as 100% higher. Differences in surface doses due to beam energy are relatively minor compared with differences due to SSD or wedge systems. Dynamic and upper wedges produce similar peripheral doses, much lower than those produced by the lower wedges. The surface dose profile across the field under the dynamic wedge has a higher slope than that under the upper wedge, when the difference in wedge angles is compensated for by normalization to the dose profile at d(max). In the nonwedge gradient direction, the dose profiles at d(max) of both the upper and the lower wedges demonstrate a marked effect of oblique filtration of the primary beam, resulting in an off-axis ratio at 80% of the field width of 0.95, in contrast to the off-axis ratio of 1.05 in the open and the dynamic wedged fields. CONCLUSIONS: The three wedge systems produce significantly different surface and peripheral doses that should be considered in properly choosing a wedge system for clinical use. Dynamic wedge and upper wedge systems deliver surface and peripheral doses similar to those of open fields and much lower than the lower wedge system. Both physical wedge systems degrade beam profiles in the nonwedged direction.

A reduction in the AAPM TG-36 reported peripheral dose distributions with tertiary multileaf collimation. American Association of Physicists in Medicine Task Group 36.

PURPOSE: The American Association of Physicists in Medicine Task Group 36 (AAPM TG-36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data apply to linear accelerators not equipped with tertiary multileaf collimators (MLCs). Peripheral dose distributions consist of internal scatter, collimation scatter, transmission through collimation, head leakage, and room scatter. Tertiary MLCs may significantly reduce the PD due to a reduction in collimation scatter, transmission through collimation, and head leakage. Measurements were performed on a multimodality linear accelerator, equipped with a tertiary MLC, to determine PD distributions as a function of energy, field size, distance from the primary radiation field edge, MLC position, and collimator orientation. METHODS AND MATERIALS: Measurements were made using an ionization chamber embedded in a 20 x 40 x 120-cm3 water-equivalent plastic phantom with the secondary collimator and MLC settings of 10 x 10, 15 x 15, 20 x 20, 25 x 25 cm2, and with the MLC fully retracted. Data were taken along the longitudinal axis of the machine for 6 and 18 MV photons. Peripheral dose distributions were evaluated with the collimator set to 180 and 90 degrees. Rotation of the collimator allowed measurements parallel and orthogonal to the direction of motion of the MLC. RESULTS: For both photon energies, peripheral doses measured on a MLC machine were lower than the TG-36 data. When the collimator is rotated by 90 degrees, placing the lower jaws and the MLC leaves along the plane of interest, PD was reduced by as much as a factor of three compared with PDs measured with the MLC fully retracted and the collimator rotated to 180 degrees. PDs measured with the MLC fully retracted and collimator rotated to 180 degrees were comparable to the TG-36 data. Measured PDs were lower when the MLC was used to shape the field than when the MLC was fully retracted. CONCLUSION: A strategic orientation of the collimator with a tertiary MLC can reduce PD distributions by more than a factor of two. This decrease significantly lessens or eliminates the need for external lead shielding to reduce the critical organ dose. This method can be used even when Lipowitz metal blocking (such as for mantle fields) is used, with the MLC leaves oriented along the longitudinal plane.

Changes in electron beam dosimetry with a new scattering foil-applicator system on a CL2100C.

PURPOSE: The optimization of clinical electron beams is a challenge to accelerator manufacturers. There are numerous variations and reports of scattering-foil and applicator configurations. The accelerator at our facility was recently updated with new foils and applicators. We conducted many dosimetric tests to critically evaluate dosimetric changes and their clinical effects. METHODS AND MATERIALS: The new dual foil systems are thicker and have shaped disks seated on the lower foils. The 12 MeV beam no longer shares a common foil used for 6 and 9 MeV. The applicators now have denser collimating plates, and Fiberglas no longer connects the plates. The new applicator set includes a rectangular 10 x 6 cm applicator that uses one photon jaw setting for all energies. After the electron beam energies were tuned to previous specifications (energy according to ionization depths, symmetry to +/- 2%, and flatness to +/- 6%), recommissioning took place. Electron beam output checks at various source-to-skin distances (SSD) were conducted for all energies and applicators. Computer-driven water scanning provided percent depth dose, profile, isodose, and Bremsstrahlung data. Surface doses, in-air electron dispersion, effective SSDs, and leakage were also measured. All results compared the previous and updated systems. RESULTS: We found little change in relative percent depth doses for 100 cm SSD between the two systems. The differences in PDD due to increasing SSD, however, decreased with the updated system. Surface doses decreased in most cases, while Bremsstrahlung increased in all cases (typically by a factor of two). Beam uniformity indices increased significantly, while penumbra widths decreased. Diagonal profiles are now quite flat for large fields. For a 20 MeV beam, the 90% width along the diagonal axis for a 25 x 25 applicator at dmax depth has increased from 25 to 32 cm. There was little or no change in 'effective SSD' or in-air dispersion. Leakage outside the applicators was reduced by a factor of two to three. The flatness characteristics of the 10 x 6 cm applicator were poor in comparison to the improved flatness of the new square applicators. CONCLUSIONS: The updated scattering foil-applicator electron beam system has yielded many dosimetric changes. Major improvements have been made in beam flatness and leakage. These positive changes have not been accompanied by any clinically significant dosimetric deficiencies.

1995 survey of physics teaching efforts in radiation oncology residency programs.

A physics teaching survey was constructed and sent to the 83 radiation oncologist training programs. The survey requested program information regarding size, staffing, curriculum, lab/rotation programs, organization, requirements, instructor makeup, teaching materials, and board certification examination results. The surveys were sent to the physicist responsible for the physics program. Forty-nine (59%) institutions returned completed surveys, of which 43 (88%) were university-associated programs, and 27 (55%) were 4-year programs. On average, there were two residents/year. Most programs (39) taught physics exclusively during the first year (PG2). Some programs taught different subjects (or levels) to different year residents. Radiation dosimetry, treatment planning, and brachytherapy constituted nearly half of the teaching hours. On average the total classroom time expended by physicists was 61.4 h/year with a range of 24-118 h. The mean for laboratory/demonstration time was 27 h/year with 18 programs providing none. Physics orientation/rotations ranged from 1 to 480 h with a mean of 170 h for a physics rotation taking place in year 2 (PG3). Mandatory attendance was 80% for first-year residents and decreased in later years. Homework was assigned in 76% of the programs, and 65% of the programs were graded. The primary instructors averaged 18.2 years of experience, and the majority were ABR/ABMP certified. Khan's textbook was the most prevalent resource for most subjects. No correlation could be made between teaching hours and ABR physics percentile scoring. The survey results reveal enormous differences in national teaching efforts.

The influence of air cavities on interface doses for photon beams.

PURPOSE: As the quantification of dose in homogeneous media is now better understood, it is necessary to further quantify effects from heterogeneous media. The most extreme case is related to air cavities. Although dose corrections at large distances beyond a cavity are accountable by attenuation differences, perturbations at air-tissue interfaces are complex to measure or calculate. These measurements helps understand the physical processes that govern these perturbations. METHODS AND MATERIALS: A thin window parallel-plate chamber and a special diode were used for measurements with various air cavity geometries (layer, channel, cubic cavity, triangle) in x-ray beams of 4 and 15 MV. RESULTS: Underdosing effects occur at both the distal and proximal air cavity interfaces. The magnitude depends on geometry, energy, and field sizes. As the cavity thickness increases, the central axis dose at the distal interface decreases. Increasing field size remedied the underdosing, as did the introduction of lateral walls. Following a 2.0 cm wide air channel for a 4 MV, 4 x 4 cm2 field there was an 11% underdose at the distal interface, while a 2.0 cm cubic cavity yielded only a 3% loss. Measurements at the proximal interface showed losses of 5% to 8%. For a 4 MV parallel opposed beam irradiation the losses at the interfaces were 10% for a channel cavity (in comparison with the homogeneous case) and 1% for a cube. The losses were slightly larger for the 15 MV beam. Underdosage at the lateral interface was 4% and 8% for the 4 MV and 15 MV beams, respectively. CONCLUSION: Although reports suggest better clinical results using lower photon energies with the presence of air cavities, there is no reliable dose calculation algorithm to predict interface doses accurately. The measurements reported here can be used to guide the development of new calculation models under nonequilibrium conditions. This situation is of clinical concern when lesions such as larynx carcinoma beyond air cavities are irradiated.

A mono isocentric technique for breast and regional nodal therapy using dual asymmetric jaws.

PURPOSE: Definitive radiation therapy for breast cancer with regional nodal involvement often introduces treatment of adjacent abutted regions. Many methods describe techniques to achieve an effective transverse plane match. Our facility recently adopted a matching technique using asymmetric jaws to beam-split all portals along the central axis plane. Our technique uses one isocenter to treat the opposed tangential breast fields, the supraclavicular port and the posterior axillary field. METHODS AND MATERIALS: Our linear accelerator has four collimator jaws capable of being set independently. The longitudinal (Y) jaws beam-split all the portals at the match plane, namely the upper border of the tangential beams and the bottom border of the nodal fields. The transverse (X) jaws define the lateral borders of the nodal fields, and in a near beam-split fashion in conjunction with customized Cerrobend, block the lung for the tangential beams. The unique isocenter is chosen along the mid-bridge through the tangential match plane. Dosimetric qualities and calculational techniques of the asymmetric beams were analyzed with ionimetric water scans, ion chamber studies, and film. The match-line is clinically confirmed with composite port films. RESULTS: Our dosimetric studies show asymmetric jaws provide nearly equivalent field edge definition and superior absorption in comparison with Cerrobend blocks. The use of one isocenter results in a reduction of in-room treatment time by a factor of two. The burden of lifting heavy Cerrobend blocks has been removed. A composite port film, which includes the medial tangential and supraclavicular ports, shows a perfect match-line in all cases. Similar composite port films taken with our previous technique of geometric matching with collimator and table angulation exhibit slight overlap or underdose regions in many cases. CONCLUSION: Our treatment technique takes full advantage of dual asymmetric jaws to achieve a perfect match-line, necessitates only one isocenter and set-up point, and supplies more absorption in reference to lung and contralateral breast dose. The pure match-line is accompanied by the fact that the patient does not have to move in any direction.

A volumetric study of measurements and calculations of lung density corrections for 6 and 18 MV photons.

PURPOSE: For treatment of lung cancer, dose heterogeneity corrections and subsequent prescription alteration remain controversial. Previous dosimetry studies based on slab geometry with a single beam geometry do not represent the clinical situation. A circumscribed tumor within lung poses a more complex problem. Energy choice also remains controversial. METHODS AND MATERIALS: An anthropomorphic phantom was modified by replacing lung cylinders (2.5 and 5.0 cm diameters by 5.0 cm length) with muscle-equivalent cylinders. The phantom was scanned on a CT simulator. Gross, clinical, and planning target volumes (GTV, CTV, PTV1 including tumor and regional nodes, PTV2 including tumor only) were designated slice-by-slice. Three-dimensional planning was performed with large fields (AP/PA/RPO) covering PTV1 and boost fields optimized for each PTV2, for 6 and 18 MV photons. Homogeneous, Ratio-Tissue-Air-Ratio (RTAR), and convolution-adapted RTAR (CARTAR) calculation algorithms were tested. Film was placed between phantom slices at the "tumor" levels. The phantom was irradiated with monitor units corresponding to homogeneous calculations, based on a homogeneous prescription. Measured and calculated doses were compared by isodoses and dose volume histograms. Ionization chambers and TLDs were also used for some test cases. RESULTS: The measured minimum dose covering PTV2 was within 5% of the homogeneous prescription dose of 70 Gy for 6 MV photons, while a lower dose (89% of prescription dose) was measured for 18 MV. The algorithms overpredicted the minimum dose to PTV2 by 6-18%. If the monitor units had been reduced according to simplistic heterogeneous calculations, the small PTV2 would have only been covered by 58 Gy for 18 MV irradiation. Based on this, a clinician may opt to actually increase the prescribed dose, thereby offsetting decreased monitor units. None of the algorithms predicted the diffuse penumbra associated with 18 MV photons in lung. CONCLUSION: Before adjusting dose prescriptions based on heterogeneity corrections, realistic phantom studies must be performed. The accuracy and effect of the corrections must then be assessed. The deficient coverage of PTV2 by the 18 MV beam compares unfavorably with the slight increase (5%) in hot spots associated with 6 MV. Our studies support strong caution before reducing dose prescriptions based on simple algorithms.

A change in treatment process with a modern record and verify system.

PURPOSE: With the introduction of new treatment devices, such as a multileaf collimator (MLC) and dynamic wedge (DW), therapists have an increased responsibility to ensure correct treatment. Simultaneously, three-dimensional treatment planning (3DTP) has led to an increased number of portals and table movements. To counteract this challenge and maintain efficiency, a comprehensive record and verify (R&V) system is mandatory. We evaluated a commercial system (Varis) for reliability, ease of use, efficiency, and integration with our planning systems. METHODS AND MATERIALS: Some key elements of the Varis system are: integration of MLC and DW; auto setup for MLC, jaw, collimator, gantry, and limited table parameters; direct download of simulation beam data; and a regimented field scheduling system that prescribes all beam data for particular fractions. Evaluation of the system was driven by treatment time analysis, error rates, and an increased workload. These issues were governed by how we disseminated duties and how the system accommodated or changed our processes. RESULTS: Most data entry is performed by our dosimetry staff. Data can be downloaded from the simulator, but more patients now move from CT simulation and/or 3DTP to the treatment machine. Varis does not link to these systems. The physics staff confirms all entries to correct data entry errors. The workload for dosimetrists increased by an average of 8 minutes/patient entry; physics time increased by 7 minutes/patient entry; the weekly electronic chart check takes approximately 3 minutes/patient. Therapists who used Varis efficiently showed a slight decrease in treatment times, attributed to MLC integration and auto-setup. Some therapists experienced a decrease in efficiency, because of unfamiliarity and excess intervention. On a positive note, notable events have decreased by a factor of 10 since full initiation. Unfortunately, the remaining errors are often the result of a therapist relying on incorrect electronic information. CONCLUSION: The Varis R&V system has had an impact on our clinic's process and efficiency. Checking of all beam data and related field scheduling have helped reduce errors and misconceptions. We feel a dual-energy machine can be operated with two experienced therapists and an up-to-date R&V system more accurately and efficiently than with three therapists working without an integrated R&V. We anticipate future Varis releases will further promote efficiency and accuracy.

Dosimetry and clinical implementation of dynamic wedge.

PURPOSE: Wedge-shaped isodoses are desired in a number of clinical situations. Physical wedge filters have provided nominal angled isodoses with dosimetric consequences of beam hardening, increased peripheral dosing, nonidealized gradients at deep depths, along with the practical consequences of filter handling and placement problems. Dynamic wedging uses a combination of a moving jaw and changing dose rate to achieve angled isodoses. The clinical implementation of dynamic wedge and an accompanying quality assurance program are discussed in detail. METHODS AND MATERIALS: The accelerator at our facility has two photon energies (6 MV and 18 MV), currently with dynamic wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees. The segmented treatment tables (STT) that drive the jaw in concert with a changing dose rate are unique for field sizes ranging from 4.0 cm to 20.0 cm in 0.5 cm steps, resulting in 256 STTs. Transmission wedge factors were measured for each STT with an ion chamber. Isodose profiles were accumulated with film after dose conversion. For treatment-planning purposes, dmax orthogonal dose profiles were measured for open and dynamic fields. Physical filters were assigned empirically via the ratio of open and wedge profiles. RESULTS: A nonlinear relationship with wedge factor and field size was found. The factors were found to be independent of the stationary field setting or second order blocking. Dynamic wedging provided more consistent gradients across the field compared with physical filters. Percent depth doses were found to be closer to open field. The created physical filters provided planned isodoses that closely resembled measured isodoses. Comparative isodose plans show improvement with dynamic wedging. CONCLUSIONS: Dynamic wedging has practical and dosimetric advantages over physical filters. Table collisions with physical filters are alleviated. Treatment planning has been solved with an empirical solution. Dynamic wedge is a positive replacement for physical filters, and a first step for commercial introduction of dynamic conformal therapy.

Simultaneous delivery of electron beam therapy and ultrasound hyperthermia using scanning reflectors: a feasibility study.

PURPOSE: The feasibility of simultaneously delivering external electron beam radiation and superficial hyperthermia using a scanning ultrasound reflector-array system (SURAS) was experimentally investigated and demonstrated. METHODS AND MATERIALS: A new system uses a scanning reflector to distribute the acoustic energy from a planar ultrasound array over the surface of the target volume. External photon/electron beams can be concurrently delivered with hyperthermia by irradiating through the scanning reflectors. That is, this system enables the acoustic waves and the radiation beams to enter the target volume from the same direction. Reflectors were constructed of air-equivalent materials for maximum acoustic reflection and minimum radiation attenuation. Acoustically, the air reflectors were compared to brass reflectors (assumed ideal) for reflectivity and specular quality using several single transducers ranging in frequency from 0.68 to 4.8 MHz. The relative reflectivity was determined from acoustic power measurements using a force-balance technique. The specular quality was assessed by comparing the acoustic pressure fields reflected by air reflectors with those reflected by brass reflectors. Also, acoustic pressure fields generated by a SURAS prototype for two different arrays (2.24 and 4.5 MHz) were measured to investigate field distribution variations as a function of the distance separating the array and the scanning reflector. All pressure fields were measured with a hydrophone in a degassed water tank. Finally, to determine the effect of the air reflectors on electron dose distributions, these were measured using film in a water-equivalent solid phantom after passage of a 20 MeV electron beam through the SURAS. These measurements were performed with the reflector scanning continuously across the electron beam and at rest within the electron beam. RESULTS: The measurements performed using single ultrasound transducers showed that the air reflectors had power reflectivities of 87-96% that of brass, and that for smooth surfaces the reflections from air reflectors were as specular as those from brass reflectors. Acoustic pressure fields measurements of the SURAS for two different arrays showed that the 50% pressure amplitude contours were well-distributed across the projected surface area of the array for different distances separating the array and the reflector. Finally, film dosimetry showed that the electron dose distribution was not affected by the air reflector of the SURAS either for the scanning case or the stationary case. This indicates that the reflectors as made are basically water-equivalent in terms of high energy ionizing radiation. The measured isodoses also indicate that the constructed SURAS prototype would allow the delivery of adequate radiation (90% isodose) to a depth of 2.0 cm. CONCLUSIONS: The results presented show that the SURAS design has the potential to deliver hyperthermia to large superficial tumors, while allowing simultaneous irradiation with 20 MeV electron beams without adverse effects on the radiation dose delivery.

Clinical implementation of a commercial multileaf collimator: dosimetry, networking, simulation, and quality assurance.

PURPOSE: Clinical implementation of multileaf collimation (MLC) includes commissioning (including leaf calibration), dosimetric measurements (penumbra, transmission, calculation parameters), shaping methods, networking for file transfer, verification simulation, and development of a quality assurance (QA) program. Differences of MLC and alloy shaping in terms of penumbra and stair-step effects must be analyzed. METHODS AND MATERIALS: Leaf positions are calibrated to light field. The resultant decrement line, penumbras, leaf transmission data, and isodoses in various planes were measured with film. Penumbra was measured for straight edges and corners, in various media. Ion chambers were used to measure effects of MLC on output, scatter, and depth dose. We maintain midleaf intersection criteria. MLC fields are set 7 mm beyond planning target volumes. After shaping by vendor software or by our three-dimensional planning system, files are transferred to the MLC workstation by means of sharing software, interface cards, and cabling. A MLC emulator was constructed for simulation. Our QA program includes file checks, monthly checks (leaf position accuracy and interlock tests), and annual review. RESULTS: We found the MLC leaf position (light field) corresponds to decrement lines ranging from 50 to 59%. Transmission through MLC (1.5-2.5%) is less than alloy (3.5%). Multileaf penumbra is slightly wider than for alloy. Relative penumbra did not increase in the lung, and composite field dosimetry exhibited negligible differences compared with alloy. Verification simulations provide diagnostic image quality hard copies of the MLC fields. Monitor unit parameters used for alloy held for MLC. DISCUSSION: Clinical implementation for MLC as a block replacement was conducted on a site-by-site basis. Time studies indicate significant (25%) in-room time reductions. Through imaging and dosimetric analysis, the accuracy of field delivery has increased with MLC. The most significant impact of MLC is the ability to increase the number of daily treatment fields, thereby reducing normal tissue dosing, which is vital for dose escalation.

Room shielding for intensity-modulated radiation therapy treatment facilities.

PURPOSE: The traditional assumptions used in room-shielding calculations are reassessed for intensity-modulated radiation therapy (IMRT). IMRT makes relatively inefficient use of monitor units (MUs) when compared to conventional radiation therapy, affecting the assumptions used in room-shielding calculations. For the same single-fraction tumor dose delivered, the total number of MUs for IMRT is much greater than for a conventional treatment. Therefore, the exposure contribution from the linear accelerator head leakage will be significantly greater than with conventional treatments. METHODS AND MATERIALS: We propose a shielding calculation model that decouples the concepts of workload, MUs, and target dose when determining primary and secondary barrier thicknesses. The workload for primary barrier calculations for conventional multileaf collimator (MLC) IMRT treatments is determined according to patient tumor doses. The same calculation for accelerator-based serial tomotherapy IMRT requires scaling by the average number of treatment slices. However, rotational therapy yields a small use factor that compensates for this increase. We further define a series of efficiency factors to account for the small field sizes employed in IMRT. For secondary barrier calculations, the patient-scattered radiation is assumed to be the same for all IMRT modalities as for conventional therapy. The accelerator head leakage contribution is proportional to the number of MUs. Knowledge of the average number of MUs per patient is required to estimate the head leakage contribution. We used a 6-MV linear accelerator photon beam to guide the development of this technique and to evaluate the adequacy of conventional barriers for IMRT. Average weekly IMRT workload estimates were made based on our experience with 180 serial tomotherapy patients and published data for both "step and shoot" and dynamic MLC delivered treatments. RESULTS: We found that conventional primary barriers are adequate for both dynamic MLC and serial tomotherapy IMRT. However, the excessive head leakage produced by these modalities requires an increase in secondary barrier shielding. CONCLUSION: When designing shielding for an IMRT facility, increases in accelerator head leakage must be taken into account for secondary shielding. Adequacy of secondary shielding will depend on the IMRT patient load. For conventional facilities that are being assessed for IMRT therapy, existing primary barriers will typically prove adequate.

Multiple machine implementation of enhanced dynamic wedge.

PURPOSE: After acquiring 4 years of experience with Dynamic Wedge, a software-driven one-dimensional (1D) compensation system, we implemented a new software version called Enhanced Dynamic Wedge (EDW). The EDW allows larger (30 cm) and asymmetric field sizes and additional angles for wedged fields. We implemented this software on four similar dual-energy accelerators that also possess upper and lower physical wedge sets. Our goal was to implement EDW with one common wedge factor (WF) table and one set of treatment-planning files. METHODS AND MATERIALS: We measured WFs with an ionization chamber and isodose profiles with both film and a diode array. We used a calculation scheme that requires only entry of the wedge angle and fixed jaw value. Filters for computerized treatment planning were configured for each wedge angle. We also examined to what degree the multileaf collimation (MLC) orientation, which is orthogonal to the EDW direction, was compromised for specific treatment sites. As a comparative test, we examined the dosimetric consistency for the 8 sets of physical wedges on the four machines. Finally, we updated our DW quality assurance program for EDW. RESULTS: The measured EDW WF was common for all four machines to within +/- 1.5% and the calculation scheme held to within 1.5%. The EDW isodoses were consistent among the machines as measured by film and diode array. The treatment-planning filters provided computed isodose profiles that were nearly identical to measured profiles. Regarding MLC orientation, we found that the collimator angle needed for EDW did not compromise isodose distributions, as apparent in measured isodoses and calculated dose-volume histograms. The consistency of the physical wedges did not fare as well. Two of the lower wedge sets had Wfs and profiles different (> 3%) from the other wedge sets. CONCLUSIONS: We have successfully implemented EDW on four machines using only one WF table and one set of treatment-planning filters. The EDW provides for improved treatment techniques for particular sites due to the large field sizes and additional angles available. Daily treatment efficiency has increased because of the remote capability provided by EDW.

Commissioning and periodic quality assurance of a clinical electronic portal imaging device.

PURPOSE: An electronic portal imaging device (EPID) was recently installed on our dual-energy linear accelerator. Commissioning and quality assurance techniques were developed for the EPID. METHODS AND MATERIALS: A commissioning procedure was developed consisting of five parts: (a) physical operation and safety; (b) image acquisition, resolution, and sensitivity calibration; (c) image storage, analysis, and handling; (d) reference image acquisition; and (e) clinical operations. RESULTS: The physical operation and safety tests relate to the motions of the unit, stability of the unit supports, safety interlocks, and interlock overrides. Imager contrast and spatial resolutions are monitored by imaging a contrast-detail phantom. The imager calibration procedure consists of a no-radiation image to compensate for signal offsets, as well as a "flat-field image." The flat-field image is taken with 5.0 cm of homogeneous phantom material placed at isocenter to provide some photon scatter and to approximate the presence of a patient. Daily quality assurance procedures consists of safety tests and the acquisition and inspection of images of the contrast-detail phantom. After 1 year, the frequency of the daily procedure was reduced to weekly. Quarterly QA procedures are conducted by the physicist and consist of the same procedures conducted in the weekly test. The annual QA procedure consists of a duplication of the commissioning procedure. CONCLUSION: The procedures discussed in this article were applied to an ionization-chamber device. They have been useful in identifying difficulties with the EPID operation, including the need for recalibrating and monitoring the accelerator output stability.

Technology assessment of multileaf collimation: a North American users survey.

PURPOSE: The American Association of Physicists in Medicine (AAPM) initiated an Assessment of Technology Subcommittee (ATS) to help the radiotherapy community evaluate emerging technologies. The ATS decided to first address multileaf collimation (MLC) by means of a North American users survey. The survey attempted to address issues such as MLC utility, efficacy, cost-effectiveness, and customer satisfaction. METHODS AND MATERIALS: The survey was designed with 38 questions, with cross-tabulation set up to decipher a particular clinic's perception of MLC. The surveys were coded according to MLC types, which were narrowed to four: Elekta, Siemens, Varian 52-leaf, and Varian 80-leaf. A 40% return rate was desired. RESULTS: A 44% (108 of 250) return was achieved. On an MLC machine, 76.5% of photon patients are being treated with MLC. The main reasons for not using MLC were stair stepping, field size limitation, and physician objection. The most common sites in which MLC is being used are lung, pelvis, and prostate. The least used sites are head & neck and mantle fields. Of the facilities, 31% claimed an increase in number of patients being treated since MLC was installed, and 44% claimed an increase in the number of fields. Though the staffing for block cutting has decreased, therapist staffing has not. However, 91% of the facilities claimed a decreased workload for the therapists, despite the increase in daily treated patients and fields. Of the facilities that justified MLC purchase for more daily patients, 63% are actually treating more patients. Only 26% of the facilities that justified an MLC purchase for intensity-modulated radiotherapy (IMRT) are currently using it for that purpose. The satisfaction rating (1 = low to 5 = high) for department groups averaged 4.0. Therapists ranked MLC as 4.6. CONCLUSIONS: Our survey shows that most users have successfully introduced MLC into the clinic as a block replacement. Most have found MLC to be cost-effective and efficient. The use of MLC for IMRT has progressed slower, but users anticipate escalated use.

Independent collimator dosimetry for a dual photon energy linear accelerator.

PURPOSE: The independent collimator feature in medical linear accelerators can define radiation fields that are asymmetric with respect to the flattening filter and oblique to the incident surface. Prior to clinical implementation, it is necessary to evaluate the dosimetry of this non-standard treatment delivery technique. An investigation of the independent collimator dosimetry for 6 MV and 18 MV x-ray beams has been undertaken. METHODS AND MATERIALS: Dose to tissue in free space, percent depth dose and dose distribution were measured and compared to that for symmetric field collimation. RESULTS: The dosimetry results were consistent for both photon modes. Dose in free space with asymmetric collimation can be calculated from the corresponding symmetric field dose in free space to within 1.2 +/- 0.7% by applying an appropriate off-axis factor. Asymmetric field percent depth dose differs from symmetric field percent depth dose on average by 1.1 +/- 0.7% for 6 MV and by 0.7 +/- 0.5% for 18 MV for field sizes ranging from 5 x 5 to 20 x 20, centered 3 cm and 10 cm off-axis. The measured isodose curves demonstrate divergence effects and reduced doses (less than 3%) adjacent to the field edge closest to the flattening filter center. This dose asymmetry result is identical to that from secondary collimation. CONCLUSION: The methodology for clinical implementation of the independent collimator feature is straightforward. However, accurate representation of the isodose distributions by commercial radiotherapy treatment planning systems requires special dose calculation algorithms.

Pre-installation empirical testing of room shielding for high dose rate remote afterloaders.

PURPOSE: Many facilities are acquiring high dose rate remote afterloading units. It is economical that these units be placed in existing shielded teletherapy rooms. Scatter-radiation barriers marginally protect uncontrolled areas from a high dose rate source especially in a room that houses a non-dynamic Cobalt-60 unit. In addition the exact thickness and material composition of the barriers are unknown and therefore, a calculation technique may give misleading results. Also, it would be impossible to evaluate an entire wall barrier by taking isolated core samples in order to assist in the calculations. A quick and inexpensive measurement of dose equivalent using a rented high activity 192Ir source evaluates the barriers and locates shielding deficiencies. METHODS AND MATERIALS: We performed transmission calculations for primary and scattered radiation based on National Council on Radiation Protection and Measurements Reports 49 and 51, respectively. We then rented a high activity 21.7 Ci (8.03 x 10(11) Bq) Ir-192 source to assess our existing teletherapy room shielding for adequacy and voids. This source was placed at the proposed location for clinical high dose rate treatment and measurements were performed. RESULTS: No deficiencies were found in controlled areas surrounding the room, but large differences were found between the calculated and measured values. Our survey located a region in the uncontrolled area above the room requiring augmented shielding which was not predicted by the calculations. A canopy shield was designed to potentially augment the shielding in the ceiling direction. CONCLUSION: Pre-installation testing by measurement is an invaluable method for locating shielding deficiencies and avoiding unnecessary enhancement of shielding particularly when there is lack of information of the inherent shielding.

Differential dosing of prostate and seminal vesicles using dynamic multileaf collimation.

PURPOSE: We have investigated the potential of applying different doses to the prostate (PTV2) and prostate/seminal vesicles (PTV1) using multileaf collimation (MLC) for intensity modulated radiation therapy (IMRT). Current dose-escalation studies call for treatment of the PTV1 to 54 Gy in 27 fractions followed by 20 Gy minimum to the PTV2. A daily minimum PTV dose of 2 Gy using a 7-field technique (4 obliques, opposed laterals, and an ant-post field) is delivered. This requires monitor unit calculations, paper and electronic chart entry, and quality assurance for a total of 14 fields. The goal of MLC IMRT is to improve efficiency and deliver superior dose distributions. Acceptance testing and commissioning of the dynamic MLC (DMLC) option on a dual-energy accelerator was accomplished. Most of the testing was performed using segmental MLC (SMLC) IMRT with stop-and-shoot sequences built within the dynamic mode of the DMLC. METHODS AND MATERIALS: The MLC IMRT fields were forward planned using a three-dimensional treatment planning system. The 14 fields were condensed to 7 SMLC IMRT fields with two segments each. In this process, steps were created by moving the leaves to the reduced field positions. No dose (<0.01%) was delivered during this motion. The monitor units were proportioned according to the planned treatment weights. Film and ionization chamber dosimetry were used to analyze leaf positional accuracy and speed, output, and depth-dose characteristics. A geometric phantom was used for absolute and relative measurements. We obtained a volumetric computerized tomography (CT) scan of the phantom, performed 3D planning, and then delivered a single treatment fraction. RESULTS: The acceptance testing and commissioning demonstrated that the leaves move to programmed positions accurately and in a timely manner. We did find an approximately 1 mm offset of the set leaf position and radiation edge (50%) due to the curved-end nature and calibration limitations. The 7-field SMLC IMRT treatment duplicated the 14-field static plan dose distribution with variations no greater than 1.5%. CONCLUSIONS: The MLC IMRT approach will improve efficiency because the number of electronic and chart entries has decreased by a factor of 2. Portal images are able to capture the initial and final MLC segments. The question of differential daily dose to the prostate and seminal vesicles remains.