Dosimetric Outcomes of Definitive Radiation Therapy to Anal Squamous Cell Carcinoma in a Single Institution Using Volumetric Modulated Arc Therapy Evaluated Against Australian and International Treatment Planning Guidelines

INTRODUCTION: Volumetric Modulated Arc Therapy (VMAT) is the technique of choice for anal Squamous Cell Carcinoma (SCC), offering highly conformal dosimetry while improving dose sparing to Organs at Risk (OARs). Achieving the dose constraints listed in national and international guidelines can be challenging. This report validates the achievability of these constraints and brings a clinical context to these guidelines. METHODS: A retrospective analysis of 43 clinician approved plans was conducted. These plans were evaluated against the Radiation Therapy Oncology Workgroup (RTOG) 0529, eviQ, the Assessment of New Radiation Oncology Treatments and Technologies (ANROTAT) and departmental protocol. A constraint was considered 'pass' if it met an ideal objective or minor violation. A constraint was considered 'failed' if a major violation occurred. RESULTS: RTOG 0529 Planning Target Volumes (PTV) constraints D98 ≥ 80% and D95 ≥ 90% of the Total Dose (TD) were achieved in 100% of our patient cohort, with a median of 93.8% and 96% TD for the high-risk PTV (54Gy) and 92.6% and 95.2% TD for the low-risk PTV (45Gy). The stricter ANROTAT D98 ≥ 90% TD was achieved in 93% of our patient cohort for the high-risk PTV (54Gy) and 77% for the low-risk PTV (45Gy). Major violations of the gastrointestinal (GI) OAR constraints were observed in up to 40% of the plans. While there were no strict major violation specifications for skeletal and genitourinary (GU) OARs, dose constraint non-compliance occurred in up to 19% and 56% of the plans respectively. CONCLUSIONS: Although VMAT is the gold standard for anal SCC, achieving the recommended dose constraints was challenging. Further refinement of dose constraints is essential to ensure the clinical achievability of planning objectives.