Room: AAPM ePoster Library
Purpose: The 50 kVp X-ray tube technique was clinically implemented by Jean Papillon, both as curative modality for early stage or as pre-operative modality for T2, and T3 rectal adenocarcinomas. Technique survived to this date using the “end-on” geometry, where the radiation is emitted through the end of a rectoscope-like applicator. Such “end-on” geometry may require applicator rotation to accommodate the target position and this process might be very uncomfortable for patients. In this work we present results of radiochromic film based dose measurements of novel “side-on” geometry based rectal applicator employing 50 kVp electronic-brachytherapy source.
Methods: For the new applicator we measured the dose distribution using radiochromic film dosimetry. Pieces of EBT3 film were irradiated in air and calibration curves were created in terms of known (from previous ion chamber measurements) air-kerma in air ((K???)???) for Xoft 50 kVp source beam quality (measured HVL = 0.4 mm Al). Subsequently, pieces of EBT3 film were placed in front of the 3 cm diameter collimator opening placed within special phantom designed to mimic clinical situations. As scatter is incorporated in the measured film signal in phantom, measured (K???)???n??? was subsequently converted into absorbed dose to water following the AAPM TG-61 dosimetry protocol.
Results: found that the most uniform dose distribution is obtained by stepping the source over 13 dwell-positions, 0.5 cm apart, with first 10 positions set at equal dwell times and the last 3 positions (towards the tip of the applicator) increased by 25%. The dose variation across the applicator opening was 5% in both longitudinal and transversal direction (with respect to source motion axis). The output with this plan was measured to be 7 Gy/min.
Conclusion: The results of the film measurements confirm that the new side-on geometry based applicator could be used clinically for rectal cancer treatments.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Xoft Inc., a subsidiary of iCAD, San Jose, CA