Dose and volume effects on fibrosis after breast conservation therapy

To analyze factors involved in the development of fibrosis in the boost area after breast conservation therapy (BCT) in patients treated with continuous low dose rate iridium implants following 50 Gy whole breast irradiation.

Fibrosis was estimated by palpation in 404 patients by four physicians. The median follow-up (FUP) duration was 70 months (range 30-133 months). Original implant data were used for reconstruction and dose-volume calculations. The total dose of the external whole breast irradiation and iridium implants was expressed in Normalized Total Dose (NTD): the total dose given in fractions of 2 Gy, which is biologically equivalent to the actual dose given according to the linear-quadratic model, using an alpha/beta value of 2 Gy, and 1.5 h for the recovery half-life of sublethal damage repair. To identify predictors of fibrosis we used a proportional odds model in a polychotomous logistic regression analysis.

Seven independent factors were identified that were related to the severity of fibrosis: age, duration of FUP, clinical T-size, photon beam energy, NTD level, implant volume, and adjuvant chemotherapy. From the proportional odds model, a volume exponent could be estimated (0.16 +/- 0.04) that enabled us to determine dose-effect relations for different volumes. A 10-fold higher risk of fibrosis was seen when the total dose was above 79 Gy as compared with doses lower than 70 Gy. A fourfold increase in risk of fibrosis was seen for each 100 cm3 increase in irradiated boost volume. The use of adjuvant chemotherapy resulted in a twofold increase in the risk of fibrosis (dose modifying factor approximately 1.08). The application of Co-60 beams had a similar effect. The relative odds for the other factors were smaller (1.4 for each 10 years of older age, and 1.2 for clinical T-size over 20 mm). The FUP-period had a nonlinear effect: relative odds 2.2 at 6 years, 3.6 at 7-8 years, and 2.8 at 9-11 years. The dose rate (mean 0.57, range 0.26-0.89 Gy/h) had no influence on the development of fibrosis and there was no correlation between dose rate and irradiated volume.

To optimize cosmetic results after BCT, both the total dose and the irradiated volume should be kept as low as possible. Minimum effective dose levels still have to be established. The boost volume can be minimized by more conformal brachytherapy techniques and optimal localization. It may be worthwhile to take adjuvant chemotherapy into account in decisions on boost dose levels.