Repair of Potentially Lethal Damage Following Irradiation with X Rays or Cyclotron Neutrons: Response of the EMT-6/UW Tumor System Treated under Various Growth Conditions in Vitro and in Vivo

Postirradiation potentially lethal damage (PLD) repair was examined in the EMT-6/UW tumor system under a variety of in vitro and in vivo growth conditions. Following X irradiation, surviving fraction increased (i.e., PLD was repaired) in fed and unfed plateau cultures if subculture and plating were delayed; in exponentially growing cultures if they were covered with depleted medium for the first 6 hr postirradiation; and in tumors in vivo if excision for preparation of a cell suspension was delayed. Following irradiation with 21.5 MeV ($d^{+}\rightarrow {\rm Be}$) neutrons, PLD repair was measurable only in unfed plateau cultures when subculture was delayed and in exponentially growing cells exposed to depleted culture medium immediately after irradiation. In X-irradiated EMT-6/UW cells, the greatest repair capacity and the highest surviving fraction ratios were measured in unfed plateau cultures; the least repair was observed in exponentially growing cells exposed to depleted medium. Thus post-neutron repair was not limited to situations where the amount of repair of photon PLD is large. The demonstration of PLD repair in tumors irradiated in vivo with X rays and the absence of such repair after neutrons could have important implications in radiotherapy if this is a general phenomenon. The repair mechanisms and their modifiers in various growth states must be identified before the therapeutic relevance can be determined.