ESTRO 2021 Abstract Book

S829

ESTRO 2021

PO-0997 Lack of CTV_P2 is not associated with a detrimental outcome in oropharyngeal cancer: a cohort study L.P. Ciccone 1 , M. Zani 2 , E. Scoccimarro 1 , M. Aquilano 1 , M. Banini 1 , L. Caprara 1 , M. Mariotti 1 , V. Salvestrini 1 , C. Becherini 1 , I. Desideri 1 , L. Marrazzo 2 , S. Pallotta 3 , L. Livi 4 , P. Bonomo 5 1 University of Florence, Department of Experimental and Clinical Biomedical Sciences "M. Serio", Florence, Italy; 2 Azienda Ospedaliero Universitaria Careggi, Department of Medical Physics, Florence, Italy; 3 University of Florence, Department of Experimental and Clinical Biomedical Sciences "M. Serio", Florence, Italy. Azienda Ospedaliero Universitaria Careggi, Department of Medical Physics, Florence, Italy; 4 University of Florence, Department of Experimental and Clinical Biomedical Sciences "M. Serio", Florence, Italy. Azienda Ospedaliero Universitaria Careggi, Radiation Oncology Unit - Oncology Department, Florence, Italy; 5 Azienda Ospedaliero Universitaria Careggi , Radiation Oncology Unit - Oncology Department, Florence, Italy Purpose or Objective The “5+5” expansion rule for the delineation of primary tumor clinical target volume (CTV) was defined by consensus in the latest international guidelines (Gregoire V, Radiother Oncol 2018). Since before their publication, the delineation of CTV_P2 was not part of our clinical standard of practice (SoP) for oropharyngeal cancer (OPC). The aim of this work was to assess the pattern of failure of a prospective cohort where no CTV_P2 was defined, and to explore its potential dosimetric implications. Materials and Methods In our center, primary target definition for patients affected by OPC has been performed in accordance with the following SoP: CTV_P1 consisted of primary GTV plus an isotropic 5 mm margin, manually edited for air cavities and anatomical barriers, without any additional expansion for microscopic tumor burden. A 3-mm PTV margin was then added to CTV_P1. An IMRT-SIB technique with daily image guidance was employed with Tomotherapy®, as follows: high, intermediate and low risk CTVs received 69.9, 59.4 and 52.8 Gy in 33 fractions, respectively. Concurrent cisplatin was administered as per standard practice. Progression-free survival (PFS) was defined as the time from the last treatment day to disease progression or death from any cause. Local control (LC) was defined as the time from the last treatment day to local failure. Following Gregoire’s guidelines, a CTV_P2 was retrospectively delineated by consensus by 3 physicians, blinded to the clinical outcome data, and then expanded by 3 mm to PTV_P2 (intermediate risk). To assess whether the incidence of >G2 oral mucositis was correlated with PTV_P2 coverage, a Mann Whitney test was used. Descriptive statistics were used to report dosimetric parameters. Results From January 2015 to September 2020, a total of 100 patients were included in our study, with a median age of 63 years. The vast majority (69%) had HPV positive OPC and received concurrent chemotherapy (91%). According to TNM 8th edition, 49% had stage III/IV disease. At a median follow-up of 28 months (range: 3-86), 85 patients were alive. The median PFS and LC were of 26 (range: 2-80) and 25 months (range: 2-80), respectively: the main pattern of failure was distant (Figure 1). Overall, a very low crude rate of local failure was reported (7%), with 5/7 recurrences occurring in HPV negative patients: by analysing the dose distribution, all were in-field in respect to the high risk volume (PTV_P1). In addition, 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). Figure 2 summarizes the dosimetric results.