Utility of Intermediate Risk Volumes in Human Papilloma Associated Oropharyngeal Cancer: A Pattern of Failure Analysis

Abstract

<h3>Purpose/Objective(s)</h3> Head/neck radiotherapy contouring guidelines recommend the use of an intermediate risk volume and dose around the primary target in head and neck radiotherapy. Patients with human papilloma virus (HPV) related oropharyngeal squamous cell carcinoma (OPSCC) have locoregional control (LRC) rates that are in excess of 90% with definitive radiotherapy. We hypothesized that intermediate risk volume and doses would have limited utility in these patients. <h3>Materials/Methods</h3> Following institutional review board approval, a database of p16+ and/or HPV OPSCC AJCC Stage I-III patients treated with definitive radiotherapy with or without chemotherapy was queried. Patient with documented locoregional recurrence (LRR) after the end of RT were identified. Diagnostic imaging at recurrence (rDI) was co-registered to the patient's simulation CT (sCT). The recurrence gross tumor volume (rGTV) on the rDI was contoured. In-field failure was defined as ≥95% of the recurrence volume, marginal as 20–95%, and out-of-field as ≤20% lying within the 95% high-dose prescription isodose lines. Patients with marginal or out-of-field recurrences were identified, and the initial treatment GTV was expanded by 1.3 cm to create a PTV intermediate dose (PTV-ID). The coverage of the rGTV by the PTV-ID was assessed, with potential benefit defined as near-total encompassing of the rGTV by the PTV-ID. <h3>Results</h3> A total of 534 patients with a median follow-up of 30 months were available for analysis. Actuarial LRC at 3 years was 91.2%. A total of 57 patients with LRR were identified, of which 41 had available RT treatment plans and recurrence imaging. 25 patients (61%) had in-field recurrences, 6 (14.6%) had marginal recurrences, and 10 (24.3%) had out-of-field recurrences. Of the 10 out-of-field recurrences, 4 were within elective neck volumes, 3 were in un-targeted neck regions, and 3 in areas adjacent to the primary. The combined crude marginal and out of field recurrence rate was 3%. In patients with marginal and out-of-field recurrences, none would have been ≥95% encompassed by the PTV-ID expansion. <h3>Conclusion</h3> In this large retrospective evaluation of an intermediate dose volume for patients with HPV-related OPSCC, we found no potential patients whose recurrences may have been prevented with a PTV-ID. The omission of an intermediate risk volume/dose around the primary or at-risk nodal echelons may be reasonable.