Accumulating daily‐varied dose distributions of prostate radiation therapy with soft‐tissue–based kV CT guidance

Even with daily image guidance based on soft tissue registration, deviations of fractional doses can be quite large due to changes in patient anatomy. It is of interest to ascertain the cumulative effect of these deviations on the total delivered dose. Daily kV CT data acquired using an in-room CT for five prostate cancer patients were analyzed. Each daily CT was deformably registered to the planning CT using an in-house tool. The resulting deformation field was used to map the delivered daily dose onto the planning CT, then summed to obtain the cumulative (total delivered) dose to the patient. The delivered cumulative values of prostate D100 on average were only 2.9% less than their planned values, while the PTV D95 were 3.6% less. The delivered rectum and bladder V70s can be twice what was planned. The less than 3% difference between delivered and planned prostate coverage indicates that the PTV margin of 5 mm was sufficient with the soft-tissue-based kV CT guidance for the cases studied.