2747

Abstract

Offline image guided adaptive treatment technique with single treatment plan modification has been designed to manage patient specific anatomical motion that has a stationary feature during the process of external beam radiation delivery. However, this feature cannot be always guaranteed, and therefore it is important to evaluate the potential site-specific treatment quality prior to clinical implementation. Offline adaptive radiation treatment of prostate cancer with single inverse planning modification was mimicked using multiple daily CT images (15 ∼ 18 per patient) from 21 prostate cancer patients. A pretreatment planning CT image and four extra CTs obtained during the first week treatment were used for adaptive inverse planning. The remaining CT images, acquired twice per week during the entire treatment course, were used to evaluate this adaptive treatment plan. Daily subvolume displacements of prostate (target), rectal wall and bladder wall were calculated by model based deformable organ registration. The subvolume displacements of the first five days were used to approximate a probability density function and included in the objectives of the adaptive inverse planning. Optimal intensity of five co-planar beams was then iteratively searched maximizing the target EUD while minimizing the EUD of rectal wall and bladder wall. The differences between the estimated (planned) EUD and the ‘treatment EUD’ for each organ of interest were used to evaluate this image guided treatment technique. In addition, the evaluation was also performed as a time-sequence (weekly) of the treatment delivery to determine the potential time trend effect of the anatomical variation, as well as the confidence of image guided treatment evaluation. The EUD differences between the treatment and estimation are -0.07±0.45 Gy, -0.76±2.47 Gy and -0.31±3.65 Gy for prostate, bladder wall and rectal wall respectively. The adaptive plan demonstrates a high confidence on target dose estimation, meanwhile the bladder and rectal wall EUD calculated from the adaptive inverse planning have relatively weaker representation. However, their estimation power can be improved using weekly image-guided treatment evaluation (Figure). Target dose in offline image guided radiotherapy with single adaptive inverse planning modification can be guaranteed with very high confidence. Comparably, the bladder and rectal wall doses estimated from the adaptive planning could have relatively large biases. However, these biases can be minimized during the treatment using weekly image feedback.