So far as I am aware, RTOG 0630 is the only source for a "skin strip" constraint, and as @Krishnan Patel mentions, the protocol does not fully specify how this was defined. ("No more than 50% of a longitudinal stripe of skin and subcutaneous tissue of an extremity should receive 2000 cGy. This strip...

Since @Gene Kopelson mentioned me in his post, I thought I would provide a bit of additional information. As he mentioned, Herman Suit, when he was at MD Anderson and pioneering much of the practice of radiation therapy in soft tissue sarcoma, treated a series of patients to high doses, often 70 Gy,...

I don't know of any solid data which establishes a dose or volume relationship for toxicity related to circumferential radiation, but in RTOG 0630 [protocol link], which is a pre-op radiation for sarcoma protocol, the specified dose constraint for sparing a strip of skin was no more than 50% to rece...

The use of skin/subcut strip sparing was pioneered by Dr. Herman Suit, initially at MDAH and then at MGH. In the 80s, Dr. @Joel E. Tepper joined Dr. Suit in a comprehensive Red Journal article on this and other techniques for sarcoma xrt.