EQD2 and Tumour Control in the High Dose Rate Brachytherapy of the Nasopharyngeal Carcinoma

Purpose: The aim of this study is to evaluate the decrease of EQD2 (biologically equivalent dose in 2 Gy fraction) with various overall treatment time and its correlation with local control of tumour in the treatment of nasopharyngeal carcinoma (NPC).

Materials and Methods: A retrospective study was carried out on 69 NPC (stage II, III and IV) treated with fractionated High Dose Rate (HDR)–Brachytherapy boost, following external beam radiation therapy and chemotherapy in the period from May, 2009 to December, 2015. External radiation therapy (EBRT) of all patients were given with a dose of 66 ± 4.5Gy in 2 Gy per fraction. It was followed by HDR –Intra Luminal Radiotherapy (ILRT) after having a gap. The total EQD2 prescribed was 84.8 (range:75.0 – 102.8Gy).

Results: EQD2 lost per day (in Gy) for the overall treatment time of 75, 150, 250 and 350 days are 0.04, 0.10, 0.12 and 0.13 respectively. Extra radiation dose required for maintaining the prescribed EQD2 for the overall treatment time of 75, 100, 150, 250 and 350 days are estimated as 2.7, 5.9, 12.1, 24.7 and 37.2 Gy respectively. The probabilities of disease recurrence within a median follow-up of 28 months (range 10 – 80 months) are expected as 0.06, 0.30, 0.62 and 0.94 (p=0.05) for the above overall treatment time. The relative risk of local recurrence of Stage IV NPC patient is about 2.8 times higher than stage III and stage III patients is about 2.0 times higher than stage II patients.

Conclusions: The increase of overall treatment time may be of different origin resulting in the fall of EQD2. It was observed that the recurrence of disease is more significant (p>0.50) when the overall treatment time is above 100 days where EQD2 lost becomes more than 0.07 Gy/day. Moreover, there is a significant decline of tumour control probability when overall treatment time reaches 150 days or more and the EQD2 lost = or > 0.10Gy/day. The relative risk of disease recurrence was observed highest with stage IV patients followed by stage III patients and least with stage IINPC patients.