EQD2 and Overall Treatment Time in the High Dose Rate Brachytherapy of the Advanced Nasopharyngeal Carcinoma

Abstract

The aim of this study is to evaluate the decrease of EQD2 (biologically equivalent dose in 2 Gy fraction) and its correlation with local control of tumour in the treatment of the advanced nasopharyngeal carcinoma (NPC) when the overall treatment time is prolonged. A retrospective study was carried out on 43 advanced NPC (stage Ⅲ and Ⅳ) treated with fractionated High Dose Rate (HDR)–Brachytherapy boost, following external beam radiation therapy and chemotherapy in the period from 2009 to 2015. All patients were irradiated using a cobalt unit and a technique that employed two laterals, opposed fields with a dose of (66 ± 4) Gy. It was followed by HDR –Intra Luminal Radiotherapy after having a gap. The total EQD2 prescribed was (84.75 ± 7) Gy (range, 75–100.5 Gy). The probabilities of disease recurrence within a median follow-up of 24 months (range, 10–78 months) are expected as 0.12, 0.48 and 0.97 (P=0.05) for the overall treatment time of 75, 150 and 250 days respectively. The relative risk of local recurrence of Stage IV NPC patient is about 2.30 times higher than stage Ⅲ patients. The increase of total treatment time may be of different origin resulting in the fall of EQD2. It was observed that the recurrence of disease is significant (P < 0.05) when the overall treatment time is above 100 days where EQD2 lost becomes more than 0.10 Gy/day. The relative risk of disease recurrence of Stage Ⅳ is observed higher than stage Ⅲ nasopharyngeal patients.