Short-term variation in labeling index as a predictor of radiotherapy response in human oral cavity carcinoma

In vitro determination of [3H]thymidine labeling index (LI) was carried out on squamous cell carcinomas of the oral cavity from 52 patients before and during radiotherapy. Pretreatment LI values ranged from 0.01% to 50%. After administration of the first 10 Gy in five consecutive daily fractions, a decrease in LI was observed in 39 cases and an enhancement in 13 cases, with an overall median 70% decrease in the initial value. The type of variation induced by radiotherapy was not related to pretreatment LI except for tumors with a very low proliferative activity (LI less than or equal to 1.9%), which all showed a marked increase in LI. Pretreatment LI was not indicative of short- or long-term response to radiotherapy, whereas the variation induced on LI after 10 Gy was related to the clinical outcome. A variation in LI of more or less than 70% was not significantly associated (p = 0.077) with clinical objective response (respectively, 85 and 53%). However, all 8 patients who reached a complete regression, notwithstanding an enhancement or a slight decrease in LI, had a local recurrence within 19 months. Conversely, the probability of disease-free survival was 82% for the 11 patients whose tumors had a significant decrease in LI (greater than or equal to 70%) after the first 10 Gy.