TH-D-303A-05: Role of Image Guided Patient Repositioning and Online Planning in Localized Prostate Cancer IMRT

Purpose: Compare image‐guidedIMRT for localized prostate cancer involving on‐line patient repositioning and online planning (replanning). Method and Materials: Ten early‐stage prostate cancer patients receive approximately 10 CT scans each, totaling 108 studysets. Each CT is segmented manually to identify the prostate, bladder and rectum. Using a Philips Medical, Pinnacle 8.1x RTPS,image‐guided repositioning starts with an IMRT plan to irradiate a PTV resulting from a 3‐mm margin around the prostate, on the first CT scan of each patient, which is then registered on the 2‐N, serial scans. For replanning, an IMRT plan is made on each of the serial (2‐N) CT scans using 0 and 3 mm margins. The dose distributions from scans 2‐N are then deformed to the initial CT using a mesh‐based B‐Spline deformation method, for each method. The deformed doses are added on the initial CT scan of each patient for DVH and isodose analyses. Results: Fractional volumes of rectum receiving 90 and 95% of the prescription dose (V90 and V95) range from 2–3% for 3 mm margins with repositioning and replanning, and 1–1.5% for 0 mm margins replanning. The difference in doses to rectum and bladder in repositioning and replanning with 3 mm margins are statistically insignificant. The V95 to prostate is 96.0, 97.4 and 93.8 % for repositioning with 3 mm margins, replanning with 3 mm margins and replanning with 0 mm margins, respectively. Conclusions:Image guided IMRT using 3 mm PTV margins with patient repositioning and replanning are largely comparable in target coverage and critical organ sparing, while replanning with 0 mm margins shows a statistically significant but small reduction in the doses to rectum and bladder and target coverage. Thus, a limited need exists to replan localized prostate IMRT, under image guidance.