Tumor basal area and metastatic death after proton beam irradiation for choroidal melanoma

Abstract

Background: Tumor dimension is an established prognostic factor for metastasis-related death after radiotherapy for uveal melanoma. Objective: To compare various methods of modeling the relationship between tumor dimension and metastatic death. Patients and Methods: The analyses were based on a consecutive series of 1204 patients with primary choroidal melanoma treated with proton beam irradiation (70 cobalt-gray equivalent in 5 fractions) at the Harvard Cyclotron Laboratory, Boston, Mass, between January 1985 and December 1998. Largest basal diameter and largest perpendicular basal diameter were recorded at the time of surgical placement of tantalum rings used for tumor localization during proton treatment. The height of the tumor and the axial diameter of the eye were measured by ultrasonography prior to treatment. Using proportional hazards regression, we compared the prognostic influence of different indices of tumor size with estimated risk ratios and death rates according to tumor basal area and largest basal diameter. All estimates were adjusted for other established prognostic factors. Results: Patients were followed up annually through June 30, 2000. Of the 1204 patients analyzed, 193 died of melanoma metastasis. The median follow-up among survivors was 7.9 years. The 5- and 10-year metastatic death rates were 12.8% and 20.7%, respectively. Among various approaches for modeling tumor dimension, the logarithm of tumor basal area had the highest log-likelihood and performed better than other approaches in 85% of the simulations. Based on this model, the covariate-adjusted rate ratio for any doubling in tumor basal area was 1.92 (95% confidence interval, 1.62-2.28). Conclusion: Tumor basal area is a better prognostic indicator than largest tumor diameter and tumor volume in the prediction of metastatic death after proton beam irradiation for uveal melanoma.