ESTRO 2021 Abstract Book

S838

ESTRO 2021

receiver operating characteristic (ROC) analysis was rendered to identify the optimal cutoff of LNR. Multivariable Cox regression (MVC) analysis was employed to assess the prognostic value of categorical LNR. Impact of PORT was evaluated in respective subgroups with different LNR stratification. Results A total of 908 OCSCC patients with pT1-2N1 diseases were eligible for analysis. The 5-year overall survival (OS) and disease-specific survival (DSS) was 57.6% and 68.4% respectively. The peak hazards of overall death and disease-specific death consistently presented at 12-month after treatment. A non-linear relationship was observed between the continuous LNR and survival indices with the rough trend that higher LNR leading to increased risk of death and disease-specific death. ROC analyses for OS and DSS concordantly revealed 5% as the optimal cutoff of LNR to maximize the survival difference. In MVC analysis, significantly higher risks of death (HR = 1.577, 95% CI: 1.113 to 2.234) and disease-specific death (HR = 1.672, 95% CI: 1.055 to 2.647) were unveiled in patients with LNR > 5%. PORT was significantly associated with improved OS (5-y rate: 59.9% vs. 47.7%, p = 0.024) and DSS (5-y rate: 72.7% vs. 57.2%, p = 0.022) in LNR > 5% subgroup, whereas such trend was not found in counterpart group. Conclusion LNR is a critical determinant for prognosis stratification in pT1-2N1 OCSCC. LNR > 5% is indicative of inferior oncologic outcome, warranting the utilization of PORT. PO-1009 definitive radiotherapy for squamous cell carcinoma of the external auditory canal K. Kikuchi 1 , R. Nakamura 2 , K. Shiga 3 , T. Segawa 1 , H. Oikawa 1 , H. Ariga 1 1 Iwate medical university, Radiation oncology, Yahaba-cho, Japan; 2 Morioka red cross hospital, Radiology, Morioka, Japan; 3 Iwate medical university, Head and neck surgery, Yahaba-cho, Japan Purpose or Objective Few studies have reported the outcomes of radiotherapy for external auditory canal (EAC) cancer, which is a rare malignancy. The present study aimed to evaluate outcomes in patients with squamous cell carcinoma of the EAC treated with definitive radiotherapy. Materials and Methods The study included the retrospective analysis of 32 patients with histologically confirmed squamous cell carcinoma of the EAC who were treated with ≥60 Gy radiotherapy between January 1, 1997 to December 31, 2020 in the study institution. The median age was 68 (range, 47–87) years, there were 21 (66%) male and 11 (34%) female patients, and tumors were right- and left-sided in 15 (47%) and 17 (53%) patients, respectively. Additionally, 3, 4, 4, and 21 patients had stage I, II, III, and IV (M0) cancer, respectively, according to the 8th edition of the UICC TNM staging. Nine (28%) patients had synchronous and metachronous cancers. The median prescription dose was 70 (range, 60–74) Gy, and radiotherapy techniques were 3D-conformal and intensity- modulated radiation therapy in 28 (88%) and 4 (12%) patients, respectively. Concurrent chemoradiotherapy (docetaxel, cisplatin, and 5-FU, alone or in combination) was administered in 20 (63%) patients. The median follow-up duration was 14.8 (range, 3–113) months. Progression free survival (PFS) and overall survival (OS) rates were analyzed by the Kaplan–Meier method. Results In the entire cohort, the 1-, 3-, and 5-year PFS rates were 68.7%, 47.1%, and 47.1%, respectively. All recurrences occurred within two years after radiotherapy. During the follow-up, 8, 2, and 2 patients experienced local recurrence, cervical lymph node metastasis, and distant metastasis (lung, bone, and liver), respectively. None of the three patients with stage I cancer who received 60–66 Gy radiotherapy alone experienced recurrence. In the entire cohort, the 1-, 3-, and 5-year OS rates were 78.5%, 52.9%, and 52.9%, respectively. During the follow-up, 11 patients, including 9 and 2 patients with cancer in the EAC and other sites, respectively, died. The 3-year OS was better in patients with stage I–II cancer than in those with stage III–IV cancer (80.0% vs. 42.6%, p = 0.063) as well as in those treated with concurrent chemoradiotherapy than in those treated with radiotherapy alone (62.6% vs. 34.3%; p = 0.056). Finally, concurrent chemotherapy was associated with a significantly better 3-year OS in patients with stage IV cancer (59.3% vs. 0%; p = 0.012)

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