Room: Exhibit Hall | Forum 5
Purpose: We investigate the influence of interfractional motion on target coverage and rectum tissue sparing. It answers the questions, "Is CTV and PTV coverage preserved despite interfractional motion? Are we sparing rectal-tissue more than planned?"
Methods: 1306 bony-matching and prostate-matching table coordinates of photon radiotherapy prostate cancer patients were collected. The difference between these coordinates (interfractional motion) were fit to normal distributions in each DICOM axis. We assume rigid interfractional motion around the prostate and neglect minor changes in tissue stopping power. These models were randomly sampled 16 times per patient to simulate daily interfractional motion for 40 carbon ion radiotherapy (CIRT) prostate cancer patients. Shifted beams were summed to obtain `blurred' total dose. Planned and blurred DVH were obtained from planned contours in MIM Maestro 6.7 and analyzed in MATLAB 2017b.
Results: Relative to mean planned DVH, mean blurred rectum DVH was 18% lower at 20Gy(RBE), 27% lower at 30Gy(RBE), and 54% lower at 45Gy(RBE). CTV was 0.8% lower at 50Gy(RBE), 5% lower at 55Gy(RBE), and 38% lower at 58Gy(RBE). Prostate was 0.1% lower at 50Gy(RBE) and 29% lower at 58Gy(RBE). PTV2 blurred DVH lies between Prostate and CTV blurred DVH. PTV1 was 2% lower at 40Gy(RBE), 7% lower at 50Gy(RBE), and 39% lower at 58Gy(RBE). Differences between planned and blurred DVH other than bladder were statistically significant (Wilcoxon rank sum test, p<=0.023). Differences between blurred DVH given GI Toxicity grades were statistically significant except for the bladder (Wilcoxon rank sum test, p<=0.0307). Differences between planned DVH given GI Toxicity grades were less statistically significant (p>=0.0575).
Conclusion: Interfractional motion resulted in tissue sparing. Smaller margins are possible. In-room CBCT should be used to verify coverage and lower entrance dose to healthy tissue. Blurred DVH reveal greater statistical differences between toxicity grades. More accurate toxicity prediction from bony-alignment is possible.
Funding Support, Disclosures, and Conflict of Interest: Daniel Bridges received a scholarship from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) for his PhD program at Gunma University.