Recommendations for clinical electron beam dosimetry: supplement to the recommendations of Task Group 25.

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

Fractionated stereotactic radiotherapy for pituitary adenomas following microsurgical resection: safety and efficacy.

The treatment of pituitary adenomas following medical management has historically involved surgical excision or stereotactic radiosurgery, with the two modalities often utilized collectively. However, there have been only a limited number of reports on the use of fractionated stereotactic radiotherapy (FSRT) for the treatment of pituitary adenomas. To enhance the existing knowledge regarding the safety and efficacy of this treatment modality, we describe our initial experience with FSRT for residual pituitary adenomas following microsurgical resection. From 1999 to 2005, 14 patients (7F, 7M) with residual pituitary adenomas (7 nonsecretory, 2 growth hormone secreting, 2 prolactin secreting, 2 thyrotropin secreting, 1 adrenocorticotropic hormone secreting) underwent FSRT. All patients were planned using the Radionics X-Knife 3D planning system, and received a median dose of 50.4 Gy in daily 1.8 Gy fractions administered to the 90% prescription isodose line. Treatments were delivered stereotactically using a dedicated Varian 6/100 linear accelerator, with immobilization achieved with the Gill-Thomas-Cosman relocatable head frame. Mean tumor size was 3.6 cm (median, 3.2 cm), and mean patient age was 44.6 years (median, 47 years). The mean dosages to the optic chiasm and brainstem were 0.159 and 0.040 Gy (median, 0.163 and 0.031 Gy) per fraction. All patients were evaluated with visual field testing and pre- and postgadolinium-enhanced magnetic resonance imaging at a minimum of one year follow-up (median, 22.5 months; mean, 27.8 months). Following FSRT, local control (defined as absence of tumor progression) was achieved in all fourteen patients. Three patients developed hypopituitarism (average, 30 months after treatment), with no patient experiencing visual changes or acute complications following FSRT. These results demonstrate the efficacy and safety of FSRT for achieving long-term local tumor control for pituitary adenomas, further validating this technique as an appropriate treatment modality for residual adenomas following microsurgery.

Treatment of a facial nerve neuroma with fractionated stereotactic radiotherapy.

BACKGROUND: Facial nerve neuromas are extremely rare and are often mistaken for acoustic neuromas when located near the vestibular nerve. Usually presenting with facial weakness and hearing loss, facial nerve neuromas of the cerebellopontine angle have commonly been managed by surgery. We present the first reported case of a facial nerve neuroma treated with fractionated stereotactic radiotherapy (FSRT). METHODS: The patient was a 40-year-old woman who presented with tinnitus, dizziness and decreased hearing that was associated with a right intracanalicular mass on magnetic resonance imaging (MRI). She underwent a middle fossa craniotomy only to reveal a facial nerve tumor rather than an acoustic neuroma that was not resected due to the high risk of facial paralysis. Following surgery, her facial function worsened and was associated with tumor enlargement on MRI. She was referred for FSRT and received 54 Gy in daily 1.8-Gy fractions with a prescription isodose line of 90%. RESULTS: Three months after treatment she had no worsening of her pretreatment symptoms, and at the 1-year follow-up, she experienced facial weakness improvement accompanied by an absence of tumor growth on MRI. These clinical and imaging findings persisted at 48 months of follow-up. CONCLUSION: In the first report of a facial nerve neuroma treated with FSRT, this treatment resulted in excellent long-term (4-year) tumor control with improvement of pretreatment symptomatology and absence of morbidity. This report demonstrates the potential for using FSRT to treat facial nerve neuromas of the cerebellopontine angle that could otherwise be associated with significant operative morbidity.

The use of a commercial QA device for daily output check of a helical tomotherapy unit.

Helical tomotherapy radiation therapy units, due to their particular design and differences from a traditional linear accelerator, require different procedures by which to perform routine quality assurance (QA). One of the principal QA tasks that should be performed daily on any radiation therapy equipment is the output constancy check. The daily output check on a Hi-Art TomoTherapy unit is commonly performed utilizing ionization chambers placed inside a solid water phantom. This provides a good check of output at one point, but does not give any information on either energy or symmetry of the beam, unless more than one point is measured. This also has the added disadvantage that it has to be done by the physics staff. To address these issues, and to simplify the process, such that it can be performed by radiation therapists, we investigated the use of a commercially available daily QA device to perform this task. The use of this device simplifies the task of daily output constancy checks and eliminates the need for continued physics involvement. This device can also be used to monitor the constancy of beam energy and cone profile and can potentially be used to detect gross errors in the couch movement or laser alignment.

Performance evaluation and quality assurance of Varian enhanced dynamic wedges.

Dynamic wedges have been used in clinical practice for many years. Obvious superiority of dynamic over physical wedges is accompanied by the increased overhead involved in verifying the accuracy and reliability of their use. Contrary to very limited QA required to ensure proper functioning of the physical wedges, dynamic wedges, like any other dynamic treatment, require a robust QA program. This work expands upon previous suggestions and describes a comprehensive QA program for Varian enhanced dynamic wedges (EDWs) and presents the results of an 18-month evaluation of these wedges. The QA program includes daily, monthly, and yearly tests and individual treatment QA at the onset of use of the EDWs. The results of the 18-month evaluation show reproducibility in the wedge factors of better than 1% and in dose profiles of better than 2% on a monthly basis. Daily output measurements are generally within 2% of expected values.

Fractionated stereotactic radiotherapy for pediatric patients with retinoblastoma.

In this report we discuss the application of a modified Gill-Thomas-Cosman (GTC) relocatable head frame to enable fractionated stereotactic radiotherapy (SRT) of infants under anesthesia. This system has been used to treat two infants, ages 12 and 18 months for bilateral retinoblastoma on a Varian 6/100 linear accelerator. The GTC head frame was used to reproduceably position and treat the orbits of these children to between 2520 and 3960 cGy in 180 cGy fractions. A standard head and neck tray, with accompanying thermoplastic mask, was adapted to mount to the head frame to enable these treatments. We found the maximum average deviation in the repeat fixations, as compared with the initial fitting data, to be +/- 2 mm. The overall average difference and standard deviation in measurement was 0.47 +/- 0.63 mm for the first case, and 0.19 +/- 0.94 mm for the second case with a combined average of 0.35 +/- 0.79 mm overall from a total of 381 point measurements. The stereotactic treatment plan (Radionics) incorporated a single isocenter for each orbit and 3-4 arcs per isocenter. Inter-comparisons have been made between this technique and a standard lateral field technique, designed using the SRS planning system. Dose-volume histograms and corresponding normal tissue complication probabilities (NTCP) based on pediatric bone growth inhibition have been calculated for each method for the orbital bone areas. We have found that the NTCP is reduced from 95-100% in the standard treatment method to 16% or less with SRT. Use of the modified head frame provides excellent setup reproducibility, facilitates access to patients for anesthesia and reduces the chances of a poor cosmetic result in these growing children.

Implementation of enhanced dynamic wedges in Pinnacle treatment planning system.

Enhanced dynamic wedges (EDW) provide many advantages over traditional hard wedges for linear accelerator treatments. Along with these advantages comes the responsibility of ensuring that this complex technology delivers the correct dose to patients. This involves determining the enhanced dynamic wedge factors for various field sizes and depths for use in the hand calculation of monitor units (MUs). The accurate representation of dynamic wedges in the treatment planning computer must also be ensured. This is required so that the final isodose distributions are correct and the MUs calculated by the treatment planning computer match those determined by hand calculation. We have commissioned and implemented the use of EDW in the Pinnacle radiation therapy planning system. The modeled dose profiles agree with the measured ones with a maximum difference of 2%. The MUs generated by Pinnacle are also within 2% of those calculated independently. The process of data collection and verification, beam modeling, and a discussion of a potential pitfall encountered in this process are presented in this paper.

Effect of lung and target density on small-field dose coverage and PTV definition.

We have studied the effect of target and lung density on block margin for small stereotactic body radiotherapy (SBRT) targets. A phantom (50 × 50 × 50cm(3)) was created in the Pinnacle (V9.2) planning system with a 23-cm diameter lung region of interest insert. Diameter targets of 1.6, 2.0, 3.0, and 4.0cm were placed in the lung region of interest and centered at a physical depth of 15cm. Target densities evaluated were 0.1 to 1.0g/cm(3), whereas the surrounding lung density was varied between 0.05 and 0.6g/cm(3). A dose of 100cGy was delivered to the isocenter via a single 6-MV field, and the ratio of the average dose to points defining the lateral edges of the target to the isocenter dose was recorded for each combination. Field margins were varied from none to 1.5cm in 0.25-cm steps. Data obtained in the phantom study were used to predict planning treatment volume (PTV) margins that would match the clinical PTV and isodose prescription for a clinical set of 39 SBRT cases. The average internal target volume (ITV) density was 0.73 ± 0.17, average local lung density was 0.33 ± 0.16, and average ITV diameter was 2.16 ± 0.8cm. The phantom results initially underpredicted PTV margins by 0.35cm. With this offset included in the model, the ratio of predicted-to-clinical PTVs was 1.05 ± 0.32. For a given target and lung density, it was found that treatment margin was insensitive to target diameter, except for the smallest (1.6-cm diameter) target, for which the treatment margin was more sensitive to density changes than the larger targets. We have developed a graphical relationship for block margin as a function of target and lung density, which should save time in the planning phase by shortening the design of PTV margins that can satisfy Radiation Therapy Oncology Group mandated treatment volume ratios.

Application of measured pencil beam parameters for electron beam model evaluation.

Most current electron beam models, as are used in commercial treatment planning systems, combine measured broad beam central axis depth dose data with measured or modeled functions to approximate radial scatter and heterogeneity effects. In this paper, we extend a recently developed pencil beam model to calculate doses outside the field edge and doses in heterogeneous media. We have also explored use of this model as a tool for evaluating commercial electron planning programs. The algorithm we have developed, based on the concept of the lateral buildup ratio (LBR), enables calculation of dose at any point in an irregular electron field, and is capable of generating both on- and off-axis depth dose curves and isodose profiles. This model includes the effects of density and mass-angular scattering power in measured broad beam central axis depth dose data, which when combined with small field reference data, can be used to generate LBR ratios. From these ratios one can infer the depth dependent, effective pencil beam radial spread parameter a in water or other materials, which can be used to model any arbitrary field. We have used this approach to calculate fractional depth doses for small fields incident on aluminum and cork, which we have then compared against measurements and the calculations of several commercial planning systems.

Single dose versus fractionated stereotactic radiotherapy for meningiomas.

OBJECTIVE: To evaluate the safety and efficacy of stereotactic radiosurgery (SRS) compared to fractionated stereotactic radiation therapy (FSRT) for meningiomas treated over a seven year period. METHODS AND MATERIALS: Of the 53 patients (15 male and 38 female) with 63 meningiomas, 35 were treated with SRS and the 18 patients with tumors adjacent to critical structures or with large tumors were treated with FSRT. The median doses for the SRS and the FSRT groups were 1400 cGy (500-4500 cGy) and 5400 cGy (4000-6000 cGy) respectively. Median target volumes for SRS and FSRT were 6.8 ml and 8.8 ml respectively. The median follow-up for the SRS and FSRT groups were 38 months (4.1-97 months) and 30.5 months (6.0-63 months) respectively. RESULTS: The five-year tumor control probability (TC) for benign versus atypical meningiomas were 92.7% vs. 31% (P = .006). The three-year TC were 92.7% vs. 93.3% for SRS vs. FSRT groups respectively (P = .62). For benign meningiomas, the three-year TC were 92.9% vs. 92.3% for the SRS group (29 patients) vs. FSRT group (14 patients) respectively (P = .77). Two patients in the SRS group and one in the FSRT group developed late complications. CONCLUSION: Preliminary data suggest that SRS is a safe and effective treatment for patients with benign meningiomas. Fractionated stereotactic radiation therapy with conventional fractionation appeared to be an effective and safe treatment alternative for patients not appropriate for SRS. A longer follow-up is required to determine the long-term efficacy and the toxicity of these treatment modalities.

Linac-based stereotactic radiosurgery for treatment of trigeminal neuralgia.

Trigeminal neuralgia (TN) is a disabling pain condition that has classically been treated using either surgical or medical techniques. Several researchers have shown that stereotactically delivered radiation can be an effective tool in the amelioration of this condition. For these studies, the Gamma Knife was used to deliver the radiation treatment. The target location was designated as the proximal nerve at the root entry zone, and doses greater than 70 Gy to the maximum point in a single fraction were found to be effective in controlling pain in 80% of the patients treated. LINAC-based stereotactic radiosurgery has been notably absent from the treatment of TN, even though it has many similarities to Gamma Knife-based stereotactic radiosurgery. The aim of this paper is to describe our LINAC-based stereotactic technique for treatment of TN. We also compare treatment of TN using our technique to that using the Gamma Knife. We found that a LINAC-based treatment of TN can be accomplished with accuracy comparable to treatments delivered using the Gamma Knife. The dose distributions are essentially equivalent for the two treatment approaches. The LINAC-based system is easy to plan and offers the ability to reduce the involvement of sensitive structures from the treatment fields as well as the Gamma Knife system does. A disadvantage of the LINAC-based system is the time involved for treatment.

Comparison of dynamic and step-and-shoot intensity-modulated radiation therapy planning and delivery.

Intensity-modulated radiation therapy (IMRT) is commonly delivered using the dynamic or segmental mode of multileaf collimators (DMLC or SMLC). Both methods are designed to deliver intensity-modulated beams as determined by inverse planning software. In this study, we have used the Helios IMRT planning system to generate ideal treatment plans for 10 cases of 2 common treatment sites (prostate and head and neck) and have investigated the actual treatment fluence distributions generated for each of the MLC leaf motion choices. The 2 dose delivery techniques were dosimetrically compared to each other and to the treatment plans. For each technique, point doses were measured in a water phantom using ionization chambers. Also for each technique, 2-dimensional dose distributions at a selected depth in a plastic phantom were obtained, using extended range film. The total delivery time and the number of monitor units (MU) delivered by each method were also compared. Our results indicate that the 2 delivery methods produce comparable results dosimetrically. For the cases reviewed, the delivery time was an average of 15% longer for SMLC deliveries, while the number of MUs (beam-on time) required by SMLC was an average of 15% fewer, than that for the DMLC. In the interest of simplicity, lower beam-on time, and potentially fewer mechanically-related problems, we think that the SMLC delivery technique may be the better choice when Helios is used for planning and Varian linear accelerators are used for delivery.

Patient specific 3D printed phantom for IMRT quality assurance.

The purpose of this study was to test the feasibility of a patient specific phantom for patient specific dosimetric verification.Using the head and neck region of an anthropomorphic phantom as a substitute for an actual patient, a soft-tissue equivalent model was constructed with the use of a 3D printer. Calculated and measured dose in the anthropomorphic phantom and the 3D printed phantom was compared for a parallel-opposed head and neck field geometry to establish tissue equivalence. A nine-field IMRT plan was constructed and dose verification measurements were performed for the 3D printed phantom as well as traditional standard phantoms.The maximum difference in calculated dose was 1.8% for the parallel-opposed configuration. Passing rates of various dosimetric parameters were compared for the IMRT plan measurements; the 3D printed phantom results showed greater disagreement at superficial depths than other methods.A custom phantom was created using a 3D printer. It was determined that the use of patient specific phantoms to perform dosimetric verification and estimate the dose in the patient is feasible. In addition, end-to-end testing on a per-patient basis was possible with the 3D printed phantom. Further refinement of the phantom construction process is needed for routine use.

The accuracy of inhomogeneity corrections in intensity modulated radiation therapy planning in Philips Pinnacle system.

The degree of accuracy of inhomogeneity corrections in a treatment planning system is dependent on the algorithm used by the system. The choice of field size, however, could have an effect on the calculation accuracy as well. There have been several evaluation studies on the accuracy of inhomogeneity corrections used by different algorithms. Most of these studies, however, focus on evaluating the dose in phantom using simplified geometry and open/static fields. This work focuses on evaluating the degree of dose accuracy in calculations involving intensity-modulated radiation therapy (IMRT) fields incident on a phantom containing both lung- and bone-equivalent heterogeneities using 6 and 10 MV beams. IMRT treatment plans were generated using the Philips Pinnacle treatment planning system and delivered to a phantom containing 55 thermoluminescent dosimeter (TLD) locations within the lung and bone and near the lung and bone interfaces with solid water. The TLD readings were compared with the dose predicted by the planning system. We find satisfactory agreement between planned and delivered doses, with an overall absolute average difference between measurement and calculation of 1.2% for the 6 MV and 3.1% for the 10 MV beam with larger variations observed near the interfaces and in areas of high-dose gradient. The results presented here demonstrate that the convolution algorithm used in the Pinnacle treatment planning system produces accurate results in IMRT plans calculated and delivered to inhomogeneous media, even in regions that potentially lack electronic equilibrium.

Monitor unit checking in heterogeneous stereotactic body radiotherapy treatment planning.

Treatment of lung cancer using very-high-dose fractionation in small fields requires well-tested dose modeling, a method for density-averaging compound targets constructed from different parts of the breathing cycle, and monitor unit verification of the heterogeneity-corrected treatment plans. The quality and safety of each procedure are dependent on these factors. We have evaluated the dosimetry of our first 26 stereotactic body radiotherapy (SBRT) patients, including 260 treatment fields, planned with the Pinnacle treatment planning system. All targets were combined from full expiration and inspiration computed tomography scans and planned on the normal respiration scan with 6-MV photons. Combined GTVs (cGTVs) have been density-averaged in different ways for comparison of the effect on total monitor units. In addition, we have compared planned monitor units against hand calculations using 2 classic 1D correction methods: (1) effective attenuation and (2) ratio of Tissue-Maximum Ratios (TMRs) to determine the range of efficacy of simple verification methods over difficult-to-perform measurements. Different methods of density averaging for combined targets have been found to have minimal impact on total dose as evidenced by the range of total monitor units generated for each method. Nondensity-corrected treatment plans for the same fields were found to require about 8% more monitor units on average. Hand calculations, using the effective attenuation method were found to agree with Pinnacle calculations for nonproblematic fields to within ±10% for >95% of the fields tested. The ratio of TMRs method was found to be unacceptable. Reasonable choices for density-averaging of cGTVs using full inspiration/expiration scans should not strongly affect the planning dose. Verification of planned monitor units, as a check for problematic fields, can be done for 6-MV fields with simple 1D effective attenuation-corrected hand calculations.

In vivo diode dosimetry for IMRT treatments generated by Pinnacle treatment planning system.

Dose verification using diodes has been proposed and used for intensity modulated radiation therapy (IMRT) treatments. We have previously evaluated diode response for IMRT deliveries planned with the Eclipse/Helios treatment planning system. The Pinnacle treatment planning system generates plans that are delivered in a different fashion than Eclipse. Whereas the Eclipse-generated segments are delivered in organized progression from one side of each field to the other, Pinnacle-generated segments are delivered in a much more randomized fashion to different areas within the field. This makes diode measurements at a point more challenging because the diode may be exposed fully or partially to multiple small segments during one single field's treatment as opposed to being exposed to very few segments scanning across the diode during an Eclipse-generated delivery. We have evaluated in vivo dosimetry for Pinnacle-generated IMRT plans and characterized the response of the diode to various size segments on phantom. We present results of patient measurements on approximately 300 fields, which show that 76% of measurements agree to within 10% of the treatment-plan generated calculated doses. Of the other 24%, about 11% are within 15% of the calculated dose. Comparison of these with phantom measurements indicates that many of the discrepancies are due to diode positioning on patients and increased diode response at short source-to-surface distances (SSDs), with the remainder attributable to other factors such as segment size and partial irradiation of the diode.

Safety and efficacy of fractionated stereotactic radiotherapy for acoustic neuromas.

BACKGROUND: The treatment of acoustic neuromas (AN) has historically involved surgical excision or stereotactic radiosurgery, with a relatively limited number of reports available describing the use of fractionated stereotactic radiotherapy (FSRT). To enhance the existing knowledge regarding the safety and efficacy of this treatment modality, we describe our initial experience with FSRT for AN. METHODS: From 1999-2005, 20 patients (12F, 8M) with AN underwent FSRT. All patients were treated using the Radionics X-Knife 4.0 3D planning system, receiving 54 Gy in 1.8 Gy daily fractions with a prescription isodose line of 90%. Treatments were delivered stereotactically using a dedicated Varian 6/100 linear accelerator, with immobilization achieved via the Gill-Thomas-Cosman relocatable frame. Median tumor size (maximum diameter) was 2.1 cm (range, 1.1-3.4 cm). Median patient age was 49.5 years, with median follow-up of 22 months (range, 1-66 months). All patients were evaluated with pre- and post-gadolinium-enhanced magnetic resonance imaging. RESULTS: Following FSRT, local tumor control was achieved in every patient, with the treatment well-tolerated by all patients. No patient experienced acute complications or facial nerve weakness. Two patients experienced permanent trigeminal nerve morbidity manifesting as facial numbness. All nine patients with preserved hearing before treatment had hearing preservation at last follow-up, although four of these patients experienced hearing decline following FSRT. CONCLUSION: In our series of 20 patients with AN, all had local tumor control following FSRT, with minimal morbidity. These results support the growing body of literature demonstrating the safety and efficacy of FSRT in achieving local control for AN, further validating the viability of FSRT as a treatment modality for this patient population.