FACTORS DETERMINING OUTCOME IN PATIENTS TREATED WITH INTERSTITIAL IMPLANTATION AS A RADIATION BOOST FOR BREAST-CONSERVATION THERAPY

Purpose: To evaluate the relative utility of interstitial implant as a technique for tumor bed dose escalation and assess technical factors related to outcome. Methods and Materials: From 1982-1994, a prospecti vely applied institutional policy of margin-directed boost dose escala tion to the tumor bed was followed whereby interstitial implantation w as commonly employed for a final margin status (FMS) less than or equa l to 2 mm. There were 509 treated breasts, of which 127 received an im plant boost. For purposes of comparison, cases were broadly classed as ''implant'' (all FMS less than or equal to 2 mm) and ''nonimplant'' ( FMS less than or equal to 2 mm or FMS > 2 mm). The implant target volu me was determined at completion of whole breast irradiation by clinica l assessment. All implants were constructed in accordance with a prepl anning algorithm designed to maximize dose homogeneity within a prescr iption isodose goal of 0.50 Gy/h for 40 h. Local control and cosmetic outcome were evaluated with respect to extent of tumor, histopathology , FMS, extent of surgery, and systemic adjuvant therapy. Implant quali ty was assessed using four calculated parameters: strand separation qu otient (SSQ), planar separation quotient (PSQ), global separation quot ient (GSQ), and dose homogeneity index (DHI). The mean implant volume was 48.3 +/- 20 cc, the mean prescribed dose rate was 0.46 +/- 0.08 Gy /h, and the mean total implant dose was 19.94 +/- 1.52 Gy. Results: Co smetic outcome was good/excellent in 90% of implant and 83% of all non implant cases, which was not statistically different. Cosmesis was sig nificantly superior with implant when compared to nonimplant cases rec eiving an external boost of 20 Gy. Logistic regression analyses of imp lant cases revealed that reexcision volume and decreased DHI were asso ciated with adverse cosmesis. There were 10 local failures in the impl anted patients (4 within the prescribed isodose volume, 5 at the perip hery, and 1 elsewhere in the breast). The local failure rate at 5 and 7 years in the implanted group was 3.9 and 9.0%, respectively, compare d to nonimplant cases with a margin less than or equal to 2 mm of 3.2 and 3.2%, respectively. These differences were not significant. The cr ude local failure rate in patients with an associated DCIS component w as 12% a compared to 3% in patients with pure invasive histology (p = 0.06). A proportional hazards survival model revealed a significant as sociation of local failure with the performance of a reexcision and yo ung age. Conclusion: We conclude that interstitial implant boost for b reast conserving irradiation is associated with cosmesis that is super ior than the same nominal dose of external beam boost, although this i s highly dependent upon the technical quality of the source position a nd the relative uniformity of dose deposition. Breast implantation res ults in a rate of local control no better than dose-matched external b eam boost in patients with a final margin less than or equal to 2 mm. Local control with implantation might be further enhanced by increasin g implant volume and/or improved target localization. (C) 1997 Elsevie r Science Inc.