MR CHARACTERIZATION OF POSTIRRADIATION SOFT-TISSUE EDEMA

Objective. Radiation therapy is often used to treat bone und soft tiss ue neoplasms, and commonly results in soft tissue edema in the radiati on field, However, the time course, distribution and degree of this ed ema have not been well characterized. Our study was carried out to bet ter define these features of the edema seen following neutron and phot on radiation therapy. Design and patients. Two hundred and twenty-six patients underwent radiation therapy as part of combined modality mana gement for musculoskeletal sarcomas between 1985 and 1993. Of these, 1 5 had surgical resection of their neoplasm, had no clinical evidence o f recurrent disease, and had adequate MR followup that allowed sequent ial assessment of soft tissue following irradiation. Ten patients rece ived photons with an average dose of 52.8 Gy, Five patients received n eutrons with an average dose of 17.3 nGy. Sequential MR follow-up was available in these patients for an average of 22.8 months following ra diation therapy. On each of the serial MR imaging studies, subcutaneou s fat, muscle, and the intramuscular septa/fascial planes were graded subjectively as to size and signal intensity. Results. In general, sof t tissue signal intensity in the radiation field initially increased o ver time, peaking at about 6 months for neutron-treated patients and a t about 12-18 months for photon-treated patients. Signal intensity the n decreased slowly over time. However, at the end of the follow-up per iod, signal intensity remained elevated for most patients in both grou ps. Signal intensity in a particular tissue was greater and tended to persist longer on STIR sequences than on T2-weighted sequences. Surviv al analysis of signal intensity demonstrated much longer edema surviva l times for neutron-treated patients than for photon-treated patients. Signal intensity increase in the intramuscular septa persisted for mu ch longer than for fat or muscle. A mild increase in size was noted in the subcutaneous fat and intramuscular septa, Muscle, on the other ha nd, showed a decrease in size following treatment. This was mild for t he photon-treated group and more marked for the neutron-treated group. Conclusions. There is a relatively wide variation in the duration and degree of post-irradiation edema in soft tissues. This edema seems to persist longer in the intramuscular septa than in fat or muscle. Alth ough the duration of follow-up was limited, our study suggests that th is edema resolves in roughly half the photon-treated patients within 2 -3 years post-treatment and in less than 20% of neutron-treated patien ts by 3-4 years post-treatment. Muscle atrophy was seen in both photon - and neutron-treated patients, but was more severe in the neutron-tre ated group.