Defining dose constraints for catheter insertion sites to minimize toxicity after interstitial breast brachytherapy

The purpose of this study was to define dose constraints for catheter insertion sites to minimize probability of prominent post-therapy skin marks after interstitial breast brachytherapy. Forty patients who had undergone interstitial breast brachytherapy were studied at followup at least 2 years after the procedure. Their implant marks were compared with background skin and areola and scored as Gr0 = invisible (same color as surrounding skin), Gr1 = darker than surrounding skin but lighter than areola or hypopigmentation, and Gr2 = same color as areola or darker. Highest point doses received in each plane of implant and their corresponding closest distances from clinical target volume, 85% isodose, and closest catheter dwell point were used for analysis. A logistic regression was performed to ascertain effects of various dosimetric parameters on the probability of Gr2 marks. Receiver operating characteristic curve was generated to derive cutoffs. A total of 280 dose points were studied. Median values for various parameters were 1.4 Gy (0.24–3.74) for Dose max and 1.6 cm, 1.5 cm, and 1.0 cm for Dist CTV min, Dist Iso min, and Dist Dwell min, respectively. On logistic regression, increasing Dose max alone was associated with an increased likelihood of developing Gr2 marks. Each unit increase of Dose max increased probability of development of Gr2 skin marks by 5.0% (2.391–10.328). Receiver operating characteristic analysis also showed greatest odd ratio (8.0), sensitivity (74.8%), and specificity (73%) for Dose max. It seems prudent to restrict dose to catheter insertion sites for better cosmesis.