3656 - Clinical Outcomes after Dose De-Escalation for Prophylactic Nodal Irradiation in Nasopharyngeal Carcinoma: A Long-Term, Single-Center Experience

Q. Guo¹, X. ChenXinlan², X. Yang³, Z. Yan³, W. Huang⁴, and S. Lin¹; ¹Department of Radiation Oncology, Fujian Medical University Cancer Hospital, Fujian Cancer Hospital, Fuzhou, China, ²Department of Radiation Oncology, Clinical Oncology School of Fujian Medical University, Fujian Cancer Hospital, Fuzhou, Fujian, China, Fuzhou, Fujian, China, ??, China, ³Department of Radiation Oncology, Clinical Oncology School of Fujian Medical University, Fujian Cancer Hospital, Fuzhou, Fujian, China., Fuzhou, China, ⁴Fujian Medical University Cancer Hospital, fuzhou, fujian, China

Purpose/Objective(s): Although the International Guideline for the delineation of clinical target volume(CTV) in nasopharyngeal carcinoma (NPC) has provided recommendation for the dosage of prophylactic nodal irradiation, namely 50-60Gy, there are marked variations in practice among different institutions, in addition, some even advocated to further reduce the elective node irradiation(ENI) dose to be less than 50Gy, as recommended by the 2020 Danish Head and Neck Cancer Group (DAHANCA) guideline. The purpose of this study was to evaluate the long-term outcomes of NPC patients using a even lower prophylactic irradiation dose of about 45Gy and provide evidence for further de-escalation of radiotherapy. Materials/

Methods: Patients with histologically-proven NPC who received curative IMRT with or without chemotherapy at our attending group between November 2014 and March 2018 were candidates for this study. Other eligible criteria were listed as following: (1) has no history of previous treatment or prior malignancy; (2) has avaliable imaging data for re-staging according to the TNM-8. Of note, the nodal CTVs were subdivided into CTVn1 (70Gy equivalent, GTVn without any margin) and CTVn2[45Gy equivalent, nodal gross target volume (GTVn)+3-5mm+ elective irradiation area) according to our attendig group’s principle. Elective irradiation area in N0 and N1 disease based on purely retropharyngeal lynph nodes(RPN) would include bilateral levels RPN, II, III and Va. For those with positive cervical LNs, ipsilateral levels IVa and Vb would be included as well. Survival outcomes focus on regional relapse free survival (RRFS) were analyzed.
Results: A total of 501 patients were finally included. With a median following up time of 67.3 months, 5-year RRFS, local relapse free survival, distant metastasis free survival and overall survival were 97.7%, 97.2%, 88.5% and 89.9%, respectively. At time of censorship, 13 cases developed regional recurrence, the sites of regional failure in relation to the target volume were exclusively inside GTVn/CTVn2 in 5(1.0%) patients, exclusively outside GTVn/CTVn2 in 5(1.0%) patients, synchronously inside and outside GTVn/CTVn2 in 3 cases(0.6%). For the 8 cases who developed inside GTVn/CTVn2 relapse, only 3 patients occurred inside CTVn2(outside GTVn), the remaing 5 reccurrence all occurred within GTVn in situ. Eight patients had outside GTVn/CTVn2 failure, there were 5, 1 and 2 located at levels VIII, level Ib and IV/Vc, respectively.
Conclusion: The excellent clinical outcomes of the current study supported the feasibility of our policy of dosage es-escalation for prophylactic nodal irradiation, an elective nodal irradiation dose of 45Gy equivalent maybe sufficient for NPC patients to control undetected nodal metastasis during IMRT. Further well-designed multi-center prospective trials should be conducted to confirmed our results.