Utility Of An Intermediate Risk Volume In Hpv Plus Oropharyngeal Cancer: A Patterns Of Failure Analysis

Recent contouring guidelines suggest the use of an intermediate risk volume and dose around the primary in head and neck cancer. Radiation oncologists often use an intermediate risk volume and dose for nodal echelons that are involved or considered at risk. However, we hypothesized that this intermediate risk volume and dose would have limited utility in patients with human papilloma virus (HPV) related oropharyngeal squamous cell carcinoma (OPSCC) where locoregional control (LRC) rates are in excess of 90% with definitive radiotherapy. After institutional review board approval was obtained, a database of HPV or p16 positive non-metastatic OPSCC patients treated with definitive RT with or without chemotherapy was evaluated. No patients were treated with intermediate risk dosing to the periphery of the primary or at-risk nodal echelons. Patients with documented locoregional recurrences (LRR) after the end of RT were identified and diagnostic imaging of recurrence (rDI) was co-registered to the patient’s original planning CT (pCT) with rigid or deformable image registration software. The recurrence gross tumor volume (rGTV) 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 line. Recurrences within the elective neck volume were defined as neck levels in the same echelon, next echelon, or beyond. Patients with marginal or out-of-field recurrences were identified and the prior 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 assigned as near-total encompassing of the rGTV by PTV-ID. A total of 388 patients were available for analysis with median follow-up of 37 months. LRC of the entire cohort was 92% at 3 years. A total of 31 patients with LRR were identified, of which 25 (81%) had available RT treatment plans and recurrence imaging. 12 patients (48%) had in-field recurrences, 4 had marginal recurrences (16%), and 9 (36%) had out-of-field recurrences. Of the 9 out-of-field recurrences, 4 were within elective neck volumes, 2 were in un-targeted neck regions, and 3 were in areas adjacent to the primary target. The combined marginal out-of-field and recurrence rate was 3.4%. In patients with marginal and out-of-field recurrences, none would have been encompassed by a PTV-ID expansion. We assessed the role of a PTV-ID in a retrospective fashion for marginal and out-of-field recurrences with available RT plans and found no potential patients whose recurrences may have been prevented. Omission of an intermediate risk volume/dose surrounding the primary or at-risk nodal echelons be reasonable to decrease treatment related toxicities without significantly compromising LRC.