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Evaluation of Superficial Dosimetry Between Treatment Planning System and Measurement Among Several Breast Cancer Treatment Techniques

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Y Akino

Y Akino1,2*, I Das1, E Thompson1, Y Peng1, H Zhang1, G Bartlett1, J Zook1, T Teshima2, (1) Indiana University School of Medicine, Indianapolis, IN, (2) Osaka University Graduate School of Medicine, Suita, Osaka, Japan

TH-F-211-5 Thursday 2:00:00 PM - 2:50:00 PM Room: 211

Purpose: Accurate superficial dosimetry in breast cancer treatment is critical for the evaluation of the cosmetic and recurrence free survival. It is often believed that treatment planning systems (TPS) may not be able to provide accurate dosimetry in buildup region as it is not properly modeled in most TPS. Breast dosimetry in various treatment techniques; tangential, field-in field, electronic compensator and IMRT are studied using EBT2 film and compared with TPS.

Methods: A humanoid unsliced phantom was chosen for this study. Multiple layers of superflab bolus were placed on the breast for EBT2 film dosimetry. Treatment plans were generated using 4 techniques with 2 different grid sizes (1x1 and 2.5x2.5 mm²) to provide optimum dose distribution using AAA algorithm. Films were exposed with selected techniques and analyzed after 24 hrs. A calibration curve for dose versus pixel values was also generated at the same day as of the phantom measurement. The calculated dose and image data are imported to in-house software developed using Visual C++, and the dose profiles on the film positions were collected. The calculated profiles were stretched to match their resolutions to that of film dosimetry.

Results: At 6 mm and 11 mm depths, film dosimetry showed good agreement with the TPS calculation. On the other hand, the measured dose at 3 mm depth was higher than calculation by 15-30% for all techniques. For tangential and IMRT techniques, the calculation with 1x1 mm² grid size showed smaller difference than 2.5x2.5 mm² grid size.

Conclusions: It is concluded that TPS do not provide accurate dosimetry in the buildup region as verified by EBT2 film for all treatment techniques. For all cases TPS and measured dose are in agreement from 6 mm in depths but differ at shallower depths. Grid size does play role in dose calculation.

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