Selection of techniques for orthovoltage radiation therapy.

Selection of appropriate orthovoltage radiation techniques (peak kilovoltage plus total filtration) may be based on the following factors: (a) central axis percent depth dose, (b) exposure rate, (c) sensitivity of backscatter factor to size of a blocked field, (d) differential bone energy absorption, (e) penumbra characteristics, and (f) field uniformity. An analysis of these factors as a function of beam quality (i.e. half-value layer) has been performed and a recommendation of a minimal number of techniques (consistent with widely available techniques) is presented.

Decreasing the dosimetric effects of misalignment when using a mono-isocentric technique for irradiation of head and neck cancer.

PURPOSE: The purpose of this study was to quantify and develop methods to decrease inhomogeneities created with field edge mismatch when using a mono-isocentric beam-split technique. METHODS AND MATERIALS: We validated techniques to determine dose across a half-blocked field edge and quantified potential sources of systematic matchline error. Then, two methods were used to evaluate matchline doses. The first used film dosimetry data from a half-beam field and a spreadsheet. Duplication and reversal provided two columns, each representing a beam-split field edge. Summation simulated perfect abutment and shifting created various gaps and overlaps. The second method involved obtaining dose profiles at midfield along the ray perpendicular to abutted, overlapped, and gapped beam-split fields on six linear accelerators. To enlarge the penumbra, we designed several field edge modifiers, then re-evaluated matchline doses. The field edge modifiers applicability to a 3-field head and neck treatment technique was also examined. RESULTS: Film-determined dose profiles provide similar information across a beam-split field edge as an ionization chamber. With the mono-isocentric beam-split technique, a 4-mm overlap or gap produces inhomogeneities nearly 60% above or below the intended dose. A 2-mm overlap or gap produces inhomogeneities nearly 30% above or below the intended dose. A customized penumbra generator decreased the magnitude of these inhomogeneities to 20% and 10%, respectively. CONCLUSION: The two methods of evaluating matchline dose described above gave similar results. When using the mono-isocentric half-field technique, small misalignments produce worrisome regions of inhomogeneity. Our penumbra generator substantially decreases the magnitude of the dose inhomogeneities, although the volume receiving an inhomogeneous dose increases.

Doses to radiation sensitive organs and structures located outside the radiotherapeutic target volume for four treatment situations.

This study documents dosage to radiation sensitive organs/structures located outside the radiotherapeutic target volume for four treatment situations: (a) head and neck, (b) brain (pituitary and temporal lobe), (c) breast and (d) pelvis. Clinically relevant treatment fields were simulated on a tissue-equivalent anthropomorphic phantom and subsequently irradiated with Cobalt-60 gamma rays, 6- and 18-MV x-ray beams. Thermoluminescent dosimeters and diodes were used to measure absorbed dose. The head and neck treatment resulted in significant doses of radiation to the lens and thyroid gland. The total treatment lens dose (300-400 cGy) could be cataractogenic while measured thyroid doses (1000-8000 cGy) have the potential of causing chemical hypothyroidism, thyroid neoplasms, Graves' disease and hyperparathyroidism. Total treatment retinal (400-700cGy) and pituitary (460-1000 cGy) doses are below that considered capable of producing chronic disease. The pituitary treatment studied consisted of various size parallel opposed lateral and vertex fields (4 x 4 through 8 x 8 cm). The lens dose (40-200 cGy) with all field sizes is below those of clinical concern. Parotid doses (130-1200 cGy) and thyroid doses (350-600 cGy) are in a range where temporary xerostomia (parotid) and thyroid neoplasia development are a reasonable possibility. The retinal dose (4000 cGy) from the largest field size (8 x 8 cm2) is in the range where retinopathy has been reported. The left temporal lobe treatment also used parallel opposed lateral and vertex fields (7 x 7 and 10 x 10 cm). Doses to the pituitary gland (5200-6200 cGy), both parotids (200-6900 cGy), left lens (200-300 cGy) and left retina (1700-4500 cGy) are capable of causing significant future clinical problems. Right-sided structures received insignificant doses. Secondary malignancies could result from measured total treatment thyroid doses (670-980 cGy). Analysis of three breast/chest wall and regional nodal irradiation techniques demonstrated a 25-50% decrease in secondary lung dose with use of independent collimation compared to use of custom alloy blocking material. However, it is unlikely that a reduction in secondary dose of this magnitude would reduce the risk of treatment sequellae. In four-field "box" pelvic irradiation, secondary testes dose may result in temporary (clamshell shield) or permanent azoospermia, but is unlikely to impair androgen production.

Matching intraoperative electron-beam fields: dosimetric and clinical considerations.

In the setting of a large or irregularly shaped tumor, adjacent or intentionally overlapped intraoperative electron fields may be required to give adequate coverage of the intraoperative target volume. The matching of such intraoperative electron fields present special dosimetric problems because of the divergence of electron isodose curves with depth. In the intraoperative setting, where large, single-fraction doses are delivered, the low- and high dose areas which result from gaps or overlaps between the diverging isodose curves of electron fields matched at depth or the surface may translate into decreased local tumor control or excessive normal tissue toxicity. This study examines the dosimetry of gapped, adjacent, and overlapped 8 X 9 cm2 rectangular intraoperative fields, for 9 to 18 MeV electrons, using film densitometry. "Ideal" methods of matching rectangular intraoperative electron fields are presented, and include: 1) a 2-mm gap plus surface bolus for adjacent fields, and 2) placing a tenth-value layer shaped lead cutout in the overlap region for intentionally overlapped fields.

Intraoperative electron beam radiation therapy: technique, dosimetry, and dose specification: report of task force 48 of the Radiation Therapy Committee, American Association of Physicists in Medicine.

Intraoperative radiation therapy (IORT) is a treatment modality whereby a large single dose of radiation is delivered to a surgically open, exposed cancer site. Typically, a beam of megavoltage electrons is directed at an exposed tumor or tumor bed through a specially designed applicator system. In the last few years, IORT facilities have proliferated around the world. The IORT technique and the applicator systems used at these facilities vary greatly in sophistication and design philosophy. The IORT beam characteristics vary for different designs of applicator systems. It is necessary to document the existing techniques of IORT, to detail the dosimetry data required for accurate delivery of the prescribed dose, and to have a uniform method of dose specification for cooperative clinical trials. The specific charge to the task group includes the following: (a) identify the multidisciplinary IORT team, (b) outline special considerations that must be addressed by an IORT program, (c) review currently available IORT techniques, (d) describe dosimetric measurements necessary for accurate delivery of prescribed dose, (e) describe dosimetric measurements necessary in documenting doses to the surrounding normal tissues, (f) recommend quality assurance procedures for IORT, (g) review methods of treatment documentation and verification, and (h) recommend methods of dose specification and recording for cooperative clinical trials.

Specifying and evaluating the performance of computed tomography (CT) scanners.

Specifying and evaluating the performance of a computed tomographic (CT) scanner is necessary in order to ensure that a "typical" level of performance is achieved as well as providing baseline values for a program of quality assurance. Performance to be specified and tested include parameters related to: (a) slice geometry, (b) patient dosage, (c) artifactual behavior and (d) contrast--detail performance. Measures of performance are defined and specified. Tests which provide for performance evaluation in these four areas are detailed and discussed.

Computerized therapy registry: the Mayo Clinic experience.

Establishing a computerized tumor registry involves careful consideration of a number of factors. Included in this consideration are the cost of both software and hardware, the type and selection of data needed, and the ongoing cost of the program. A representative listing of the data to be collected for the radiotherapy tumor registry is included.

Photon attenuation in computed tomography.

The analysis and understanding of results of computed tomography (CT) require an understanding of photon attenuation in matter. The high sensitivity and resolution of these devices coupled with the use of a polychromatic photon source require a level and breadth of understanding about photon attenuation not usually required in any particular subspecialty of radiological physics. With this goal in mind, a discussion of narrow-beam photon attenuation in matter is given and related to those problems currently underway in the field of computed tomography. Measurements and calculations of tissue properties are presented. Calculations of descriptive quantities relevant to polychromatic source attenuation and CT scanning are described and presented.

Improving agreement between radiation-delineated field edges on simulation and portal films: the edge tolerance test tool.

The anatomy at the edge of a treatment machine portal film may differ from that shown by the delineator lines on a simulator film by up to 10 mm, even if both the simulator and the treatment machine meet accepted criteria for mechanical tolerances. To assure that this possibility is minimized requires some form of overall alignment check between the simulator and the treatment machine. A new test device, the edge tolerance test tool (ET3), has been designed to permit a quick and accurate check on whether portal film disagreement with simulator films is due to an accumulation of tolerances. Its use should eliminate one source of this common problem.

A chamber and electrometer calibration factor as determined by each of the five AAPM accredited dosimetry calibration laboratories.

The same calibration grade ionization chamber and electrometer were sequentially sent to all five currently AAPM Accredited Dosimetry Calibration Laboratories (ADCL). A Cesium-137 based check device was utilized to ensure chamber and electrometer factor constancy and showed a maximum deviation of 0.32% (typically less than 0.1%) over the 227 days needed to complete the intercomparison. The chamber and electrometer calibration factor provided by each of the five ADCLs were analyzed for consistency. The maximum percent difference in reported chamber factor between all five ADCLs was 1.40%. The reported electrometer factor had a maximum discrepancy of only 0.50%. System (chamber plus electrometer) factors as provided by three of the five ADCLs had a maximum discrepancy of 1.51%.

Acceptance testing and quality assurance of automated scanning film densitometers used in the dosimetry of electron and photon therapy beams.

Scanning film densitometry routinely used for obtaining dosimetric information about therapy treatment beams is subject to several sources of inaccuracies. The most significant of these are described and appropriate methods of testing are presented. This establishes the need for adequate acceptance testing followed by a quality assurance program if these devices are to be used to provide accurate relative dose information over extended periods of time.

Exposure values around an x-ray scanning transaxial tomograph (EMI scanner).

Measurements of exposure accumulated in a one-month period in and around a scanning x-ray transaxial tomograph are reported. For the unit studied (the EMI neurological scanner) values measured indicate that the shielding required is "minimal."

The dosimetric properties of an applicator system for intraoperative electron-beam therapy utilizing a Clinac-18 accelerator.

A prototype electron applicator system providing circular and rectangular fields for use in intraoperative electron beam therapy with a Varian Clinac 18 linear accelerator has been fabricated. The dosimetric properties of this system for a variety of electron-beam energies, applicator sizes, and x-ray collimator settings was documented. Significant findings include: (a) surface dose values are in excess of 90% for electron energies of 12 MeV and above; (b) for the 18-MeV beam, the deepest depth where the central axis dose in 90% of its maximum value is in excess of 50 mm for circular applicators whose diameters are in excess of 5 cm; and (c) the treatment time to deliver 1000 rads "given dose" (at given dose rate of 300 MU/min) is on the order of 3-4 min. Cross-field behavior is acceptable for the intended application and x-ray contamination is less than 4% for any applicator/electron energy combination. A system for irregular field blocking and TLD verification dosimetry has been developed.

Airborne concentrations of toxic metals resulting from the use of low melting point lead alloys to construct radiotherapy shielding.

Determinations of airborne concentrations of lead, cadmium, bismuth, and tin were made above vessels containing a "fusible" lead alloy (158 degrees F melting point) commonly used for construction of radiotherapy blocks. Fume concentrations were determined by collection on a membrane filter and analysis by atomic absorption spectrophotometry. Samples were obtained for alloy temperatures of 200 degrees, 400 degrees, and 600 degrees F. In all instances, concentrations were much lower than the applicable occupational limits for continuous exposure. The results of this study indicate that the use of a vented hood as a means of reducing air concentrations of toxic metals above and near vessels containing low temperature melting point lead allows commonly used in construction of radiotherapy shields appears unjustifiable. However, proper handling procedures should be observed to avoid entry into the body via alternate pathways (e.g., ingestion or skin absorption). Transmission data of a non-cadmium containing lead alloy with a melting point of 203 degrees F was ascertained and is reported on.

Absolute fluence measurement for a prototype neutron radiotherapy source.

An inexpensive recoil-proton counter telescope has been designed for absolute fluence measurements of a gas-target neutron source for radiation therapy. The detector has an absolute efficiency of 1.1 times 10-minus 9 at 20 cm from an isotropic source and is useful for production rates of 10-9 minus 10-13 neutrons per second. The telescope consists of a thin hydrogenous irradiator foil and a surface-barrier detector to count recoil protons within a defined solid angle. The telescope provides n-gamma discrimination as well as discrimination against scattered neutrons. Initial tests of the counter telescope were performed using the DD reaction employed in development stages of the gas target. A clear separation of full-energy recoil protons from background and scattered neutron events was evident in the pulse-height spectra without the use of coincidence gating techniques.

X-ray-transmission computed tomography.

The immediate goal of clinically based x-ray-transmission computed tomography (CT) is to provide a measurement of the x-ray linear attenuation coefficient in cross section with the ultimate goal of impacting on patient management and care. To do this with the accuracy needed for clinical goals requires the careful integration of x-ray physics, detector technology, and mathematical reconstruction theory. Performance evaluation and quality assurance are necessary adjuncts to a CT scanning program. A number of investigative studies are underway.

Determination of accurate dosimetric parameters for beveled intraoperative electron beam applicators.

Intraoperative electron beam therapy requires accurate dose maximum/monitor unit (Dmax/MU) values for both flat and beveled ended applicators (cones). Measurement of Dmax/MU values with either solid or nontilting scanning water systems may give rise to inaccuracies due to the difficulty in locating an accurate position for dmax, since the ionization chamber usually cannot be scanned along the central axis of the beveled intraoperative applicator. A linear one-dimensional scanner (which permits ionization measurements to be made as a function of water depth) has been modified to provide scanning along a line up to 30 degrees from the perpendicular to the phantom surface. This modification has proven helpful in improving the accuracy of certain dosimetric parameters (e.g., Dmax/MU) of beveled applicators. For example, we found inaccuracies which arose when we measured Dmax/MU of beveled intraoperative radiation therapy cones in either solid or other scanning water systems were greatly reduced, especially for the lower electron beam energies (e.g., 6 and 9 MeV).

Acceptance testing computerized radiation therapy treatment planning systems: direct utilization of CT scan data.

The availability of computerized radiation therapy treatment planning systems that utilize computed tomography (CT) scan data requires testing additional to that routinely needed for non-CT systems. These additional items include dimensioning verification, establishing CT number-to-tissue property conversions, verifying the accuracy of heterogeneity corrected dose predictions and autocontouring. One testing protocol is presented and sample results from an Atomic Energy of Canada Theraplan L system are presented.

A depth dependence determination of the wedge transmission factor for 4-10 MV photon beams.

The depth dependence (up to 25 cm) of the in-phantom wedge transmission factor (WTF) has been determined for three medical linear accelerator x-ray beams with energies of 4, 6, and 10 MV containing 15 degrees-60 degrees (nominal) brass wedges. All measurements were made with a cylindrical ionization chamber in water, for a field size of 10 X 10 cm2 with a source-skin distance of 80 or 100 cm. We conclude that, for the accelerators studied, the WTF factor at depth is less than 2% different from that determined at dmax (for the nominal wedge angles and photon energies studied) unless the depth of interest is greater than 10 cm. Up to the maximum depth studied (25 cm) the relative wedge factor--that is, wedge factor at depth compared to that determined at dmax--was about equal to or less than 1.02 for the 15 degrees and 30 degrees wedges and any of the photon beam energies studied. For the seldom utilized combination of a nominal wedge angle in excess of 45 degrees with a depth greater than 10 cm, the WTF at depth can differ from the WTF determined at dmax, by up to 5%. Since the wedge transmission factor is reflective of relative percent dose data, our results also indicate that it is in error to use open field percent depth doses for certain combinations of wedge angle, photon energy, and depth.

Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique.

Dose per monitor unit in photon fields generated by clinical linear accelerators can be affected by the backscattered radiation into the monitor chamber from collimator jaws. Thus, it is necessary to account for the backscattered radiation in computing monitor unit setting for a treatment field. In this work, we investigated effects of the backscatter from collimator jaws based on Monte Carlo simulations of a clinical linear accelerator. The backscattered radiation scored within the monitor chamber was identified as originating either from the upper jaws (Y jaws), or from the lower jaws (X jaws). From the results of Monte Carlo simulations, ratios of the monitor-chamber-scored dose caused by the backscatter to the dose caused by the forward radiation, R(x,y), were modeled as functions of the individual X and Y jaw positions. The amount of the backscattered radiation for any field setting was then computed as a compound contribution from both the X and Y jaws. The dose ratios of R(x,y) were then used to calculate the change in photon output caused by the backscatter, Scb(x,y). Results of these calculations were compared with available measured data based on counting the electron pulses or charge from the electron target of an accelerator. Data from this study showed that the backscattered radiation contributes approximately 3% to the monitor-chamber-scored dose. A majority of the backscattered radiation comes from the upper jaws, which are located closer to the monitor chamber. The amount of the backscatter decreases approximately in a linear fashion with the jaw opening. This results in about a 2% increase of photon output from a 10 cm x 10 cm field to a 40 cm x 40 cm field. The off-axis location of the jaw opening does not have a significant effect on the magnitude of the backscatter. The backscatter effect is significant for monitor chambers using kapton windows, particularly for treatment fields using moving jaws. Applying the backscatter correction improves the accuracy of monitor-unit calculation using a model-based dose calculation algorithm such as the convolution method.