PLD-repair in human melanoma xenografts following single dose and fractionated irradiation.

PLD-repair following single dose and fractionated irradiation was studied in vivo using five human melanoma xenograft lines. Tumours given single graded radiation doses were excised immediately after or 24 h after the radiation exposure for assay of cell survival in vitro. All melanoma lines showed PLD-repair after single dose irradiation: the PLD-repair factors, i.e. the ratio of the Do values for tumours excised 24 h after and immediately after irradiation, ranged from 1.2 +/- 0.1 to 1.4 +/- 0.1. PLD-repair following fractionated irradiation was studied by giving tumours seven fractions of 2.0 Gy over 7 days and then, after an interval of 24 h, single graded radiation doses in the range 6-21 Gy. Cell survival was assayed in vitro immediately after or 24 h after the last radiation exposure. The Do values as well as the surviving fractions were approximately equal after immediate and delayed cell seeding, i.e. none of the melanoma lines showed significant PLD-repair after fractionated irradiation. The lack of PLD-repair after fractionated irradiation was possibly a consequence of radiation-induced recruitment of quiescent tumour cells into the cell cycle. Consequently, PLD-repair is probably not a major cause of failure in the radiation therapy of malignant melanoma when treated with 2.0 Gy fractions.