Room: Exhibit Hall | Forum 6
Purpose: Proton range deviation (RD) on the surface of the clinical target volume (CTV) is correlated to dose discrepancy from the planned. Range-based registration (RR) is to minimize the RD by adjusting the patient position or beam angles. We examine the performance of RR in pelvic and abdominal regions by using daily CBCT.
Methods: 134 CBCT data sets from 8 patients with sarcoma in abdomen and pelvis treated with intensity-modulated proton therapy were investigated. CTV contours were mapped from the planning CT to the CBCT from the first treatment day after rigid registration. The location of the contours in subsequent daily CBCTs was assigned based on rigid registration between the planning CT and the daily CBCT. The water equivalent thickness along the CTV surface was calculated for each beam using the first day CBCT which was then used as the reference for the remaining treatments. The RD on the daily CBCT relative to the reference was calculated. RR was performed by searching the optimal beam angle within 5 degrees from the planned.
Results: The performance of RR depended on the target and beam angle. For lateral POP beams for prostate, RD was reduced from 3.8Â±0.6 mm (meanÂ±RMSD) to 1.3Â±0.5 mm for the 90Â° beam and from 2.8Â±0.6 mm to 1.8Â±0.8 mm for the 270Â° beam, while for a CTV on peri-rectum, RD for the 300Â° beam was decreased from 6.3Â±1.5 mm to 6.2Â±0.9 mm, but if the beam angle was 270Â°, RD was decreased from 4.6Â±0.6 mm to 2.2Â±0.4 mm.
Conclusion: RR can efficiently mitigate RD due to setup and anatomic change, especially for the RD associated with proper beam angles. We estimate that an interfractional margin of 9 mm without RR, and the margin of 4 mm with RR would be sufficient for the pelvic and abdominal volumes.