A Case-Study Investigation of the Feasibility of PTV Expansion Reduction for Dose Escalation for Anal Cancer
A Yorke*, V Williams, E Kim, S Geneser, University of Washington Medical Center, Seattle, WA
Presentations
PO-GePV-M-106 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: Several trials have investigated the impact of dose escalation on anal carcinoma and demonstrated increased colostomy-free survival. However, the high incident of treatment cessation due to acute toxicity may limit disease-free survival gains. ACCORD 03 and RTOG 0529 recommend PTV expansions of 3.0 and 3.5 cm, respectively. This study aims to assess the impact and feasibility of reducing these expansions.
Methods: We assessed various PTV expansions (3.5, 3.0, 2.5, 2.0, and 1.5 cm) for a T3N1 locally advanced anal cancer patient treated with 54 Gy in 30 fractions. Daily CBCTs were registered to the planning CT using the treatment shift, and GTVs were contoured by a physician on each CBCT. GTV volumes ranging outside the PTVs were assessed and analyzed.
Results: Daily GTVs remained well-within the 3.5 to 2.0 cm PTV expansions. GTVs were outside the 1.5 cm expansion PTV for 7 treatment days, with a maximum non-overlap volume of 1.1 cc and < 0.1 cc volume outside on two days. The patient’s GTV volume on CBCT decreased markedly over the course of treatment, from 134.6 to 55.0 cc -- a reduction of 40.9%.
Conclusion: In this case study, the daily GTVs remain well within the 2.0 cm expansion PTV with minimal volumes outside the 1.5 cm PTV expansion, indicating the feasibility to improve OAR sparing while retaining coverage. Given the 3.5 and 3.0 cm PTV volumes are 1166.8 and 930.1 cc, respectively, compared with the initial planning GTV volume of 109.534.6 cc, the 2.0 cm PTV expansion resulting in 548.1 cc is reasonable. The GTV exhibited a substantial volume decrease, indicating it may be possible to monitor and safely further reduce the PTV expansion. This study demonstrates the feasibility of reducing PTV expansions for the purpose of anal cancer dose escalation.
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