Accelerated partial-breast irradiation with interstitial implants - The clinical relevance of the calculation of skin doses

Purpose: To describe relative skin dose estimations and their impact on cosmetic outcome in interstitial multicatheter accelerated partial- breast irradiation ( APBI). Patients and Methods: Between April 2001 and January 2005, 105 consecutive patients with early breast cancer were recruited in Erlangen, Germany, for this substudy of the German- Austrian APBI phase II trial. 51% ( 54/ 105) received pulsed- dose- rate ( PDR), and 49% ( 51/ 105) high- dose- rate ( HDR) brachytherapy. Prescribed reference dose for HDR brachytherapy was 32 Gy in eight fractions of 4 Gy, twice daily. Prescribed reference dose in PDR brachytherapy was 49.8 Gy in 83 consecutive fractions of 0.6 Gy every hour. Total treatment time was 3 - 4 days. With a wire cross on the skin surface during the brachytherapy- planning procedure the minimal, mean and maximal relative skin doses ( SD(min%), SD(max%), SD(mean%)) were recorded. Endpoint of this evaluation was the cosmetic outcome in relation to the relative skin doses. Results: Median follow- up time was 38 months ( range, 19 - 65 months). Cosmetic results for all patients were excellent in 57% ( 60/ 105), good in 36% ( 38/ 105), and fair in 7% ( 7/ 105). The SD(min%) ( 27.0% vs. 31.7%; p = 0.032), SD(mean%) ( 34.2% vs. 38.1%; p = 0.008), and SD(max%) ( 38.2% vs. 46.4%; p = 0.003) were significantly lower for patients with excellent cosmetic outcome compared to patients with a suboptimal outcome. SD(mean%) ( 37.6% vs. 34.2%; p = 0.026) and SD(max%) ( 45.4% vs. 38.2%; p = 0.008) were significantly higher for patients with good cosmetic outcome compared with the patients with excellent results. Conclusion: The appraisal of skin doses has been shown to be relevant to the achievement of excellent cosmetic outcome. Further investigations are necessary, especially on the basis of CT- based brachytherapy planning, to further improve the treatment results of multicatheter APBI.