Estimation of the α/β ratio for metastatic melanomas in a patient based on actual responses to radiation therapy delivered with various fractionation schedules.

BACKGROUND

Radiation response of 41 metastatic melanomas treated with various fractionation schedules in a single patient was analyzed to identify an α/β ratio that fits the clinical data.

MATERIALS AND

METHODS

Of the 41 tumors, 37 were located within 5 cm from the body surface, while 4 were deep-seated. The patient received radiation doses between 32 and 52.5 Gy in 4-18 fractions over 7 series of radiotherapy during 5 months. Since no tumors developed recurrence between 4 and 7 months after radiation, local status at 4 months was used as an endpoint. Local control was analyzed in relation to gross tumor volume (GTV) and biological effective dose (BED)2 Gy, 6 Gy, 10 Gy, 15 Gy, 20 Gy, and 25 Gy to identify a boundary threshold to classify controlled from uncontrolled tumors.

RESULTS

All tumors exhibited partial or complete responses; however, 11 developed local recurrence. In figures plotting GTV vs. BED for all 41 tumors, upward-sloping lines to discriminate controlled from uncontrolled tumors could be drawn for all BEDs between 2 and 25 Gy, but 34-68% of the tumors failed to meet the threshold. When non-deep-seated and deep-seated tumors were analyzed separately, upward-sloping lines could be drawn using α/β ratios of 10 and 15 Gy; no tumors recurred above the lines whereas some of the tumors recurred below the lines. In contrast, it was impossible to draw such lines using lower or higher α/β ratios.

CONCLUSIONS

An α/β ratio of 10-15 Gy appeared to more accurately reflect the actual radiation response of metastatic melanomas in this patient.