RCR6: Accumulated dose to the rectum, calculated from auto-contoured image guidance CT scans, is lower than planned dose in the majority of patients treated radiotherapy for prostate cancer
1Cancer Research UK VoxTox Research Group, University of Cambridge, Cambridge, UK,2University of Cambridge Department of Oncology, Cambridge Biomedical Campus, Addenbrooke’s Hospital, Cambridge, UK,3Medical Physics Department, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK,4University of Cambridge Department of Physics, Cavendish Laboratory, Cambridge, UK,5University of Cambridge Department of Engineering, Cambridge, UK,6Cambridge Clinical Trials Unit, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK
The radiotherapy (RT) dose plan considers a single point in time, and cannot allow for variation in target or organ position during a course of treatment. In prostate RT, the ability to calculate the accumulated dose (DA) actually delivered to the rectum, which is dose limiting, has the potential to reduce toxicity or permit dose escalation.
We devised an automated method for contouring the rectum on daily mega-voltage (MV) image guidance CT scans, with median conformity index 0.64 (IQR 0.53-0.71) compared with expert manual contours. We also devised a method to recalculate and summate dose on the daily CTs. The system was automated using expertise developed at CERN. Data for 109 prostate cancer patients (4033 scans) were processed. The volume of rectum that received 20, 30, 40, 50, 60, 65, 70 and 75 Gy, and the accumulated generalized equivalent uniform dose (gEUD), were compared with the treatment plan.
Using our system, results for the 109 patients were generated in less than 4 days. In 33/109 cases (30%), we found accumulated volume was ? planned volume, for all of the dose levels assessed. In 106/109 cases (97%), accumulated volume was ? planned volume for at least one dose level. Accumulated gEUD was < planned gEUD in 97/109 participants (89%); median difference in gEUD was -1.2 Gy (IQR -0.5 to -2.3 Gy).
This automated system gives the ability to track dose-volumes in real time during a course of prostate RT. DA to the rectum was found to be less than that planned in the majority of patients; this could offer the potential to escalate dose to the prostate on an individual adaptive basis. We have assessed toxicity at least 2 years (median 4 years) after RT in these patients; future work will correlate this with the dose data.