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Dosimetric Impact of Interfractional Anatomy Changes On Breast Radiotherapy Based On Accumulative Dose: IMRT Versus 3DCRT

N Morrow*, E Ahunbay, A Currey, X Li, Medical College of Wisconsin, Milwaukee, WI

SU-E-J-129 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Interfractional anatomy changes (e.g., changes in breast shape and volume) in breast radiotherapy (RT) can be significant and cannot be fully accounted for by the current IGRT repositioning practice. This work aims to quantify dosimetric impact of these changes based on accumulative dose for IMRT and 3DCRT.

Methods:Daily CT data acquired using a CT-on-Rails (CTVision, Siemens) during IGRT for 5 representative breast cancer patients treated in prone position were analyzed. Each daily CT was registered with corresponding planning CT by aligning the lumpectomy cavity, mimicking the IGRT repositioning. Contours of breast, lumpectomy PTV and critical structures on daily CTs were generated using an auto-segmentation tool (ABAS, Elekta) based on deformable image registration. For each patient, plans for IMRT and 3DCRT with wedge were developed, and were applied to each registered daily CT set to reconstruct the daily doses with the deformation fields from ABAS which were used to compute the accumulative dose.

Results:Daily variations in D95 (dose received by 95% of treated breast volume) and D50 were not significantly different between 3DCRT and IMRT plans (within 0.5-2%). Variation of daily dose maximum for IMRT was statistically significantly higher than that for the 3DCRT plans, with some fractions increased by up to 15-20%. However, the variations of the accumulated doses from the original plans for both 3DCRT and IMRT plans are comparable.

Conclusion: Although daily doses from IMRT are more sensitive to the interfractional anatomy changes than those from 3DCRT, the variations of the accumulated doses from the original plans are comparable, as the higher variation in the hot spots for IMRT were "washed out" due to the random interfractional changes.

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