Implementing Plan of the Day for Cervical Cancer: A Comparison of Target Volume Generation Methods

Abstract

Background and purposeDue to substantial interfraction motion in cervix cancer, Plan-of-the-Day (PotD) adaptive radiotherapy may be of benefit to patients. Implementation is limited by uncertainty over how to generate the planning target volumes (PTVs). We compared published methods on our own patients. Materials and methodsForty patients each had three planning scans with variable bladder filling and daily cone beam CTs (CBCTs) during radiotherapy; 5-11 CBCTs were selected to represent interfraction motion. Clinical target volumes (CTVs) and organs at risk (OARs) were contoured following EMBRACE-II guidelines. A literature search identified 30 adaptive and non-adaptive solutions to PTV generation, which we applied to our patients. PTV sizes and mean coverage of the daily CTV were determined. For 11 patients, the clinically-implemented subjectively-edited plan library was also investigated. ResultsEleven studies assessed 15 PotD strategies against non-adaptive comparators on median 14 patients (range 9-23). Some PotD approaches applied margin recipes to the CTV on each planning scan, some modelled the CTV against bladder volume, and others applied incremental isotropic margins to the CTV with a single planning scan. Generally, coverage improved as PTV size increased. The fixed isotropic margin required to provide 100% coverage of all patients was 44mm, with mean PTV size 3316cc. The PotD strategy with the best coverage was a two-plan library formed by modelling the CTV against bladder volume with extrapolation; it provided 98% mean coverage with 1419cc mean PTV size. A three-plan library consisting of the CTV on each planning scan with 10mm margin provided 96% mean coverage with 1346cc mean PTV size. The clinically-implemented solution which employed subjective extrapolation, had mean 100% coverage and 1282cc PTV size on the 11-patient subset. Coverage provided by the best non-adaptive strategies was not statistically superior to the best PotD strategy (p = 0.13) but PTVs were larger (p = 0.02). ConclusionWe identified a modelled two-plan method, and a simple three-plan method, which both provided excellent coverage with small PTVs compared to non-adaptive strategies.