P-0150 - Intensity Modulated Versus Conformal Radiotherapy for Anal Cancer: Comparison of Plan Quality and Acute Toxicity Using a Novel Plan Comparator

Abstract

Background: Chemoradiation is standard of care for anal cancers with organ preservation rates of approximately 70%. Toxicity due to dose to organs at risk (OAR) has a significant impact on quality of life and can lead to inferior outcomes through treatment delays. Intensity-modulated radiotherapy (IMRT) can reduce doses to OAR in comparison with conformal radiotherapy (CRT), reducing toxicity and potentially allowing for dose escalation. We compare planning target volume (PTV) dose homogeneity, plan quality and OAR dose between IMRT and CRT plans along with acute toxicities. Methods: Nine randomly selected patients received chemoradiation for anal cancer. 5 received CRT and 4 IMRT. A second plan was created retrospectively for each patient, such that each had a CRT and an IMRT plan (Eclipse version 10.0). Dose volume histogram (DVH) data was exported to a novel plan comparison matrix to calculate and compare plan quality - specifically plan quality index (PQI), homogeneity index (HI), conformality index (CI), merit (M) and penalty (P). Doses to OAR were obtained from the DVH. Acute toxicity information was obtained from case notes. Results: 6/9 patients were stage IIIB, receiving full prescription dose to a nodal volume in addition to the primary tumour. All patients had a maximum prescribed dose of 50.4Gy with dose delivered to 1% volume ranging from 52.2Gy to 54.9Gy. All patients received concurrent chemotherapy (mitomycin C 6-12mg/m2 D1 and capecitabine 600-625mg/m2 D1-14 and D22-36). Dose homogeneity (HI) was superior with CRT for all high dose PTV volumes. Doses to all OAR (small bowel, bladder, genitalia, iliac crests, and femoral heads) were lower with IMRT. All plans had M of >0.99. PQI and CI were superior for all IMRT plans and P was better for the IMRT plan for 8/9 patients, suggesting a significant improvement in the therapeutic ratio for treating anal cancers using IMRT. The only patient who experienced G3 skin toxicity was receiving CRT. No other G3 toxicity was encountered and no treatment delays occurred. Conclusion: OAR dose was uniformly lower with IMRT, most notably to external genitalia, and correspondingly significant skin toxicity was not seen in IMRT patients. The relative sparing of critical structures implies potential for dose escalation. In spite of inferior HI with IMRT, no local recurrences have occurred. Longer term follow up data is needed to evaluate local control and survival with IMRT versus CRT. Quantitative plan quality comparisons were possible through the use of a novel, evidence-based matrix, which could be used to compare plan quality when evaluating newer techniques such as volumetric modulated arc therapy in the treatment of anal cancers.