ESTRO 2024 - Abstract Book

S4104

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

Figure 2: Elective pelvic lymph node target volume coverages ( V 95 %). Each box summarises the fraction coverages for one patient, and the patients are ordered by increasing PTVn_E coverage.

PTVn_E coverage loss is found to be predominantly due to prostate shifts and patient rotation causing a misalignment of the treated volume and the planned target volume. As such, loss of PTV coverage is in line with the intention of the PTV and reflect the dose conformity. The high CTVn_E coverages show that the plans are generally robust to the interfraction setup variation observed in this study. It should be noted that a volume larger than the CTVn_E needs to be covered to ensure true target coverage as delineation uncertainty and machine uncertainties are not taken into account in these calculations.

Conclusion:

For the 22 patients in this study, the fraction average dose to the CTVp is with only few exceptions calculated to be within 2% of the planned dose. The observed deviations from the planned dose is a result of abdominal girth variation as well as interfractional prostate motion. For the elective lymph node targets, the median fraction coverage ( V 95% ) of the PTVn_E falls below falls below 95% for 5 of the 22 patients. This is a direct result of the seed based match strategy used in our clinic to target the prostate even when prostate motion is observed. The CTVn_E V 95% remains close to the target of 100%, suggesting that the PTV margins and plan robustness effectively ensure that the dose to the lymph node target is as intended.

Keywords: Dose calculation, prostate cancer