Accuracy of methionine-PET in predicting the efficacy of heavy-particle therapy on primary adenoid cystic carcinomas of the head and neck

BACKGROUND We evaluated whether or not PET or PET/CT using L-methyl-[11C]-methionine (MET) can allow for the early prediction of local recurrence and metastasis, as well as the prognosis (disease-specific survival), in patients with adenoid cystic carcinoma of the head and neck treated by carbon ion beam radiotherapy. METHODS This was a retrospective cohort study of sixty-seven patients who underwent a MET-PET or PET/CT study prior to and one month after the completion of carbon ion radiotherapy (CIRT). The minimum follow-up period for survivors was 12 months. The MET accumulation of the tumor was evaluated using the semiquantitative tumor to normal tissue ratio (TNR). A univariate analysis was conducted using the log-rank method, and the Cox model was used in a multivariate survival regression analysis. RESULTS The average TNR prior to and following treatment was 4.8 (±1.5) and 3.0 (±1.3), respectively, showing a significant decrease following treatment. In the univariate analysis, a high TNR prior to treatment (TNRpre) was a significant factor for predicting the occurrence of metastasis and the disease-specific survival. A high TNR following treatment (TNRpost) was a significant factor for predicting the development of local recurrence. The residual ratio of TNR changes (TNRratio) seemed to be less useful than the TNRpre. In the multivariate analysis, the TNRpost and tumor size were the factors found to significantly influence the risk of local recurrence. The TNRpre, TNRratio and tumor size were all significant factors influencing the occurrence of metastasis. Regarding the disease-specific survival, the TNRpre and age were the only factors with a significant influence on the outcome. CONCLUSIONS The TNRpre was a factor that was significantly related to the occurrence of metastasis and the disease-specific survival after CIRT for adenoid cystic carcinoma of the head and neck. The TNRpost was a factor that was significantly related to the development of local recurrence. Thus, MET-PET or PET/CT can be useful for predicting or determining the therapeutic efficacy of CIRT.