Biological tumor volume (BTV) as a predictor for local control in nasopharyngeal carcinoma (NPC)

Abstract

Background: Treatment of NPC is mainly guided by TNM stage. This study aims to evaluate if the active tumor volume delineated as BTV on PET scan is a predictor of local disease control. Methods: This is a retrospective review of all patients who had baseline PET scan performed before treatment in our center from Aug., 2010 to Mar., 2013. Patients with distant metastases at diagnosis were excluded. Total 77 patients were included. The BTV were automatically delineated by a signal to background ratio method we reported before. All patients completed radiotherapy (RT) with intensity modulated radiotherapy to at least 70Gy to primary tumor. 7 patients had RT alone and 70 patients had chemotherapy (cisplatin or carboplatin) concurrent with RT with either additional induction or adjuvant chemotherapy. Local control was assessed by endoscopy and biopsy at 10 weeks after completion of RT. Local failure include patients with persistent disease in NP after RT and those who had local relapse after initial disease remission. Results: T stage distributions of the group were 20 T1, 7 T2, 35 T3 and 15 T4. The BTV ranged from 2.68 to 147.5cc with a median of 14.4cc. BTV in general increase with T stage with mean BTV of 12.9, 13.6, 26.3 and 40.7cc respectively for T1, 2, 3 and 4 respectively. Median follow up of the group were 32.8 months. There were 2 patients with persistent loco-regional disease after RT and another 2 patients with NP relapse. Mean BTV for patients with local control were 22.4cc compared with 61.5cc among patients with local failure (t-test, p: 0.002). 4 years local control rates (LCR) according to T stage were 100% for T1 and T2, 91.3% for T3 and 50% for T4. Patients with BTV < 15cc had 4 year local control rate (LCR) of 100% vs 73.6% among those with BTV of 15cc or more (log rank test: p = 0.33). Multivariate analysis with Cox regression including T stage, BTV, SUV max, SUV mean and TLG in primary showed only BTV is predictive of LCR (p: 0.015). T stage and other factors were not significant for LCR. Conclusions: BTV is a good predictor for local control. Patients with BTV > 15cc are at risk of local failure and should be considered for radiation dose escalation.