Purpose: To investigate the effects of split-dose irradiation on primary and metastatic uveal melanoma cell lines, with a clonogenic survival assay.
Methods: Appropriate cell concentrations of four primary and four metastatic human uveal melanoma cell lines were cultured for irradiation with single doses and with two equal fractions separated by 5 hours. After irradiation, colony formation was allowed for 7 to 21 days. Two cutaneous melanomas were also tested for comparison. All survival curves were analyzed using the linear quadratic (LQ) model. Specific parameters for the intrinsic radiosensitivity (alpha-component, SF2), for the capacity of repair of DNA damage (beta-component), as well as the alpha/beta ratio were calculated.
Results: After single-dose irradiation a wide range in the values of the alpha- and beta-component was obtained for both primary and metastatic uveal melanomas, which resulted in a wide range of alpha/beta ratios. In contrast, calculations based on split-dose data, with which the beta-component could be estimated independent of the alpha-component, indicated that estimates for the capacity of sublethal DNA damage repair was very similar in all cell lines. This indicated that intrinsic factors dominated the radiosensitivity of these cell lines. Split-dose irradiation had little influence on the intrinsic radiosensitivity (alpha-component), but cell survival increased for all cell lines. For the two cutaneous melanomas comparable split-dose results were obtained.
Conclusions: For both primary and metastatic uveal melanoma cell lines, data from single and fractionated doses indicate large variations in radiosensitivity, which are mainly dominated by the intrinsic radiosensitivities. Doses of approximately 8 Gy in five fractions would be sufficient to eradicate 10(9) cells (approximately 1 cm3) of the most radioresistant tumor cell lines, but this schedule is an overkill for the radiosensitive tumor cell lines. Based on specific morphologic and histologic tumor markers, more individualized dose fractionation schedules could improve the therapeutic ratio for uveal melanomas.