IS MACHINE ENERGY (4-8 MV) ASSOCIATED WITH OUTCOME FOR STAGE I-II BREAST-CANCER PATIENTS

Purpose: To assess the relationship between machine energy (4-8 MV) an d treatment outcome in patients treated with conservative surgery and radiation therapy. Methods and Materials: Between 1968 and 1985, 1624 patients were treated for clinical Stage I or II invasive breast cance r. The study population was limited to 1380 patients who underwent com plete gross excision and received greater than or equal to 60 Gy to th e tumor bed. Of these, 1125 were treated on a 4 MV, 153 on a 6 MV, and 102 on an 8 MV linear accelerator. Patients were selected for treatme nt on the 8 MV machine based on chest wall separations greater than 24 cm. Of patients treated on the 8 MV, netting was used for 42% and bol us was used for 26%. The median dose with bolus was 14 Gy in seven fra ctions (range: 2-34.2 Gy). Patients treated on the 8 MV accelerator we re older, had a higher percentage of clinical T2 tumors, a higher perc entage of pathologically positive nodes, and a lower incidence of exte nsive intraductal component (EIC). Median follow-up times were 130, 15 3, and 102 months, respectively, for survivors treated on the 4, 6, an d 8 MV machines. Results: We analyzed the site and 5-year crude incide nce of first failure by machine energy and found the pattern of first failure site (local, nodal, or distant) to be virtually identical for each energy group. Of the local failures, It were in the skin of the t reated breast, and these failures were evenly distributed by machine e nergy. We performed a multivariate analysis to adjust for factors know n to predict for treatment failure. When adjusted for these other vari ables, machine energy was not associated with an increased (or decreas ed) risk of recurrence (RR for 8 MV vs. 4 MV = 0.94, p = 0.7; RR for 6 MV vs. 4 MV = 1.0, p = 0.9). We also analyzed the nature and incidenc e of treatment complications (rib fracture, radiation pneumonitis, sof t tissue necrosis, and brachial plexopathy) and found no significant d ifferences among the three treatment groups when stratified by treatme nt technique (tangents only vs. three-field). There was also no signif icant difference in cosmetic outcome at 5 years among the three groups . Conclusions: We conclude that machine energy over the range of 4 to 8 MV does not significantly affect treatment outcome. Specifically, it was feasible to treat patients with large chest wall separations usin g an 8 MV machine without an increase in skin recurrences and with the improved dose homogeneity afforded by 8 MV machines as compared with those of lower energies. (C) 1997 Elsevier Science Inc.