We examined the interfractional variations of clinical target volumes (CTVs), planning target volumes (PTVs), and organs-at-risk (OARs) in patients receiving MRI-guided stereotactic body radiotherapy (SBRT) to the prostate bed and evaluated the potential role of adaptive planning.
31 patients received 30-34 Gy in five fractions to the prostate bed on a phase II clinical trial. OARs, CTVs, and PTVs were retrospectively contoured on daily pretreatment MRIs (n=155). Geometric comparisons were made between initial planning contours and daily pretreatment contours. Predicted treatment plans for each fraction were evaluated using the following constraints: CTV V95%>93%, PTV V95%>90%, bladder Dmax<36.7Gy, bladder V32.5Gy<35%, rectum Dmax<36.7Gy, rectum V27.5Gy<45%, rectum 32.5Gy<30%, and rectal wall V24Gy<50%. Adaptive planning was simulated for all fractions that failed to meet these criteria. Plans were then re-evaluated.
Median change in volume was 0.48% for CTV, -24.5% for bladder, and 6.95% for rectum. Median DSC was 0.89 for CTV, 0.79 for bladder, and 0.76 for rectum. 145/155 fractions (93.5%) met CTV V95%>93%. 75/155 fractions (48.4%) failed at least one OAR dose constraint. Overall, 83/155 fractions (53.5%) met criteria for adapting planning. This affected 24/31 patients (77.4%). Following adaptive planning, all fractions met CTV V95%>93% and PTV V95%>90% and 120/155 fractions (77.4%) met all OAR constraints.
Due to significant interfractional variations in anatomy, a majority of fractions failed to meet both target volume and OAR constraints. However, adaptive planning was effective in overcoming these anatomic changes. Adaptive planning should be routinely considered in prostate bed SBRT. This was a post-hoc analysis of the following registered clinical trial: NCT03541850.
Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology. 2023 Mar 17 [Epub ahead of print]
John Nikitas, Lauren M Smith, Yu Gao, Ting Martin Ma, Ankush Sachdeva, Stephanie M Yoon, Tommy Jiang, Daniel A Low, Leslie K Ballas, Michael L Steinberg, Minsong Cao, Amar U Kishan
Department of Radiation Oncology, University of California, Los Angeles, 200 Medical Plaza, Ste B265, Los Angeles, CA, 90025, USA., Department of Radiation Oncology, University of Southern California, 1441 Eastlake Avenue, Los Angeles, CA, 90033, USA., Department of Radiation Oncology, University of California, Los Angeles, 200 Medical Plaza, Ste B265, Los Angeles, CA, 90025, USA. Electronic address: .