3724 - Safe Deviation from the "5+5" Expansion Rule for the Delineation of Primary Tumor Clinical Target Volume (CTV-P) in Oropharyngeal Cancer: A Prospective Cohort Study

V. Salvestrini¹, C. Becherini¹, M. Zani², I. Desideri³, L. P. Ciccone⁴, B. Bettazzi¹, D. Cela¹, S. Pallotta⁵, G. Simontacchi¹, M. Loi¹, D. Greto¹, G. Francolini¹, L. Livi³, and P. Bonomo¹; ¹Radiation Oncology, Careggi University Hospital, University of Florence, Florence, Italy, ²Medical Physics, Careggi University Hospital, University of Florence, Florence, Italy, ³Department of Experimental Clinical and Biomedical Sciences “Mario Serio”, University of Florence, Florence, Italy, ⁴Centro Nazionale di Adroterapia Oncologica, Fondazione CNAO, Pavia, Italy, ⁵University of Florence, Department of Biomedical, Experimental and Clinical Sciences “Mario Serio", Florence, Italy

Purpose/Objective(s): Over the past decades, the optimal target volume for patients with head and neck squamous cell carcinoma has been widely investigated. The “5+5” expansion rule for CTV-P was postulated by the latest international consensus guidelines [1]. As per our local standard of practice (SoP), since Jan 2015 the 10 mm margin expansion from gross tumor volume (GTV) of the primary tumor (CTV-P2) was not part of target delineation for oropharyngeal cancer (OPC). Thus, in the context of a controlled, prospective, single-center cohort of patients (pts) we aimed to assess the pattern of failure after the omission of CTV-P2 while exploring its impact on toxicity and potential dosimetric implications Materials/

Methods: Target definition for pts affected by OPC has been performed in accordance with our SoP. The CTV-P1 consisted of primary GTV with an isotropic expansion of 5 mm margin, edited for air cavities and anatomical barriers. A 3-mm margin was added to CTV-P1 (PTV-P1). An IMRT-SIB 3-D CRT and IMRT system technique was adopted. A 3-dose level prescription was applied: high, intermediate and low risk PTVs received 69.9, 59.4 and 52.8 Gy in 33 fractions, respectively. Concurrent cisplatin-based chemotherapy (cht) was administered as per SoP. According to international guidelines [1], a CTV-P2 was retrospectively delineated by 3 radiation oncologists, blinded to the clinical outcome data, and then expanded by 3 mm (PTV-P2). Median local control (LC), progression-free survival (PFS) and overall survival (OS) were assessed by the Kaplan-Meier method. To describe whether the incidence of >G2 oral mucositis was correlated with PTV-P2 coverage, a Mann Whitney test was used.
Results: From Jan 2015 to Sep 2020, 100 pts were included in our cohort. The median age was 63 years. The vast majority (69%) had HPV-related OPC and received concurrent cht (91%). According to TNM 7^th edition, 80% of pts had stage III/IV disease. At a median follow-up of 55 months (range: 1-109), 77 patients were alive and 57 had no evidence of disease. Median LC and PFS were not reached while 3-year LC and PFS rates of 85 % and 68 %were found. The median OS was 97 months (95%CI 81.2-96.6). The main pattern of failure was distant (23%). Overall, a low crude rate of local failure was reported (15%). Out of 15 local events (6 HPV + and 9 HPV - cases), 12 in-field and 3 marginal relapses, referring to the high-risk volume (PTV-P1), were collected. No correlation was found when comparing the incidence of >G2 oral mucositis in respect to PTV-P2 coverage above or below 95%, corresponding to 56.4 Gy (p=0.68).
Conclusion: In a cohort of OPC pts treated uniformly with a 3-dose level prescription, our SoP based on the omission of 10 mm expansion for CTV-P2 did not result in a detrimental effect on LC. Longer follow-up is needed to ascertain its impact on oncologic outcome and late toxicity. Reference

1. Grégoire V et al., Radiother Oncol. 2018