No abstract

Electron radiotherapy fields are commonly used to treat superficial cancers. Field shaping can be achieved by placing lead on the patient surface to minimise the dose to surrounding areas. However, significant dosimetry changes under high density material edges for electron fields have been reported in the literature. This project evaluated the dosimetry of small dimension electron fields shaped with lead placed on the surface. Comparisons were made between circular lead cutouts placed on the skin and low melting point alloy cutouts placed in an applicator. Depth doses, profiles and output factors were measured using a diode detector in a water phantom. Film was also used to determine surface dose delivered when the lead cutouts were placed on the surface. Minimal differences were observed between the different setups for the depth dose curves, although significant differences were seen in the penumbra and the surface doses. The penumbra is smaller for the lead cutouts placed on the surface, however, significant dose increases at the edge of the field were observed for larger fields and energies; this may result in undesirable clinical effects.

Purpose: Best fitting Tomotherapy treatment planning parameters for nine different lesion sites. Method and Materials: Tomotherapy treatment planning and delivery depends on parameters that are not necessarily familiar to a radiotherapy physicist. It is important for planners to familiarize themselves with these parameters and their impact on the time required for delivery: 51 Prostate plans, 268 lungs, 197 Brain, 21Liver, 38 Head & Neck, 46 Breast, 51Pelvis and 59 Pancreas plans for parameters like Pitch, Gantry period, treatment time, delivery modulation, total dose, calculated treatment length and slice width were analyzed for the best fit. Results: For Prostate average modulation delivery factor of 1.8 with average delivered dose of 51.53Gy and average treatment time of 342.4 seconds was used. Average numbers of fractions were 27. For Lung average dose of 50.70Gy with average modulation of 1.726. Average treatment time was 338.6 seconds and average numbers of fractions was 24. For Brain lesions average dose of 28.42Gy with average modulation of 1.71 was used. Average treatment time was 401.45 seconds. Average numbers of fractions was 11. For liver lesions average dose of 28.42Gy was used. Average modulation factor of 1.71 and average treatment time of 401.45 seconds. Average numbers of fractions was 11. For Head & Neck average dose of 36.08Gy with average modulation factor of 1.81 was used. Average treatment time was 420.64 seconds. Average numbers of fractions was 20. For Breast average dose of 41.44Gy with average modulation of 1.97. Average treatment time of 429.27 seconds with average numbers of fractions of 24. For Pelvis average dose of 38.8Gy with average modulation of 1.95. Average treatment time was 344.91 seconds. Average numbers of fractions was 19. For Pancreas average dose of 46.88Gy with average modulation of 1.78. Average treatment time was 266.64 seconds. Average numbers of fractions of 24.

Purpose: The purpose of this study is to determine dosimetric properties of Play‐Doh as bolus in clinical electron beam radiotherapy. Methods: Electron beam energies of 6, 9 and 12 MeV were used. Play‐Doh with a thickness of 0.5 and 1cm were placed on top of a plastic water phantom. During all PDD curve measurements the SSD was maintained at 100cm, while utilizing a PTW Markus Model N23343 parallel plate chamber for the measurements. Shifts and absolute dose ratio at Dmax between the Play‐Doh PDD curve and plastic water PDD curve were calculated using an unconstrained nonlinear optimization method. The same procedure was repeated using the Superflab bolus material for comparison. Results: The comparison of depth dose curves normalized to Dmax of the plastic water for each electron energy and bolus material show that the Play‐Doh and Superflab cause the PDD curve to shift upstream. For a bolus of 0.5 cm of Play‐Doh or Superflab, the shifts were approximately 0.1cm upstream relative to plastic water. However, for 1cm of Play‐Doh the shift was approximately 0.5cm upstream for all energies while Superflab still had a shift of about 0.1cm. The absolute dose at Dmax for each set of measurements using the different bolus materials were all within ±1% of plastic water. Conclusion: The PDD curve shifts for 0.5cm of the bolus material were within 0.1cm for all configurations. For 1cm of Play‐Doh the shift was about 0.5cm upstream; while shifts for Superflab were within 0.1cm. The absolute dose with different bolus material was within 1% of plastic water. Based upon this investigation, it is important that clinicians understand the effect of bolus material on the PDD for electrons.

Purpose: To investigate the adequacy of an existing 6 MV vault for shielding and to verify that head leakage is the primary source of radiation emanating from a helical tomotherapy unit. Methods & Materials: Before installation of a helical tomotherapy unit in an existing 6 MV accelerator vault, the vendor provided isodose plot was analyzed. Estimated exposure was calculated by scaling the exposure on the plot, correcting for inverse square, and using a 6 MV TVL for 2.35 g/cm3 concrete from NCRP 49. Sinograms were generated for a rotational (20 second period) and fixed gantry (0°, 90°, 180°, 270°) to deliver radiation with all leaves closed and opened for a 5×40 cm2 field size and fixed couch position. At 26 locations the exposure rate was measured at height of isocenter with an ionization chamber. Results: The maximum exposure occurred when the accelerator was nearest to the measurement point. A maximum exposure rate of 1.75 mR/hr was measured in the accelerator control area behind 30″ concrete at a 45° angle relative to the axis of the accelerator and a distance of 5 meters. The lowest exposure rate at the same position occurred when the accelerator was at the farthest distance. Assuming 30 minutes of irradiation time per hour, the maximum exposure would be less than 1750 mR/yr. No significant differences (∼10–15%) between field settings were observed. The calculated values are higher than the measured and loosely agree with the highest measured values at each position. Conclusions: The existing vault is adequately shielded for the tomotherapy unit. Data indicate that leakage is the primary radiation source. Modulation does not affect the leakage significantly. The calculated and measured values disagree and discrepancy may be attributed to the use of a 6 MV TVL.