What is current practice for sparing a tissue strip in extremity radiation?

So far as I am aware, RTOG 0630 is the only source for a "skin strip" constraint, and as @Dr. First Last 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 stripe of normal tissue is contoured at the discretion of treating radiation oncologist.")

ARST 1431 (link, page 178) used "total skin within field (5mm thickness)", V20 < 50%, but also does not specify how...

Since @Dr. First Last 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, with post-operative radiation therapy to the entire limb circumference and had very poor outcomes related to circumferential fibrosis and distal edema. Based largely on these data, it became standar...

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 receive 20Gy. The strip of skin was to be contoured at the discretion of the treating physician. My experience has been that it's probably okay to go up to 30Gy in standard fractionation.

In this scenario...

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