Purpose/Objective(s): There is limited data correlating dosimetry with toxicity for intracavitary accelerated partial breast irradiation (APBI). We hypothesized that radiation dose to skin, dose heterogeneity, and PTV volume significantly correlated with physician assessed acute and chronic toxicities.

Materials/Methods: One hundred ninety one patients treated with balloon catheter high dose rate brachytherapy (BCHDRB) were included in this study. Median follow-up was 7 years. All patients were treated with 34Gy in 10 twice daily fractions. Acute (occurring within 6 months of radiotherapy) and chronic (occurring after 6 months of treatment) toxicities, including dermatitis, breast pain, breast infection, breast edema, hyperpigmentation, hypopigmentation, telangiectasia, volume reduction, fat necrosis, induration, chest wall pain, rib fractures, and cosmesis were assessed at each follow-up visit using the CTCAE (v3.0) criteria. Dosimetric parameters analyzed were maximum point dose to the skin, maximum point dose to a rind of skin 2 mm in thickness, dose to 1-5 cc of a rind of skin 2 mm in thickness, PTV volume, PTV V200%, PTV V150%, PTV V125%, and maximum rib dose. Statistical analyses were conducted using ROC curves, Cox regressions, Kaplan Meier curves, and log rank tests.

Results: Acute breast pain was associated with larger PTV volume (HR 5.0; P<0.019) and higher PTV V150% (HR 12.9; P=0.001). Acute induration was significantly correlated with skin maximum point dose (HR 3.8; P=0.025). For chronic toxicities, dose to the skin significantly correlated with chronic telangiectasia (HR 3.5), dermatitis (HR 2.8), breast pain (HR 1.9), hypopigmentation (HR 3.7), hyperpigmentation (HR 5.6), induration (HR 5.3), volume reduction (HR 45.2), and cosmesis (HR 1.7) when assessed as either maximum point dose or volumetrically to a rind of skin 2 mm in thickness (P<0.05 for all comparisons). No dosimetric parameters correlated with breast edema, chest wall pain, or rib fractures. Using maximum skin dose of 34Gy as cut off, rates of chronic toxicity below vs. above 34Gy were 7.1% vs. 17% (any telangiectasia), 36% vs. 51.8% (any breast pain), 9.1% vs. 20.2% (any hypopigmentation), 34.4% vs. 44.1% (any hyperpigmentation), and 32.4% vs. 43.8% (G2+ induration) (P<0.05 for all comparisons). The median time to first incident of chronic toxicities ranged from 8 to 40 months, but there were late first occurrences of chronic toxicities with 20% of patients experiencing a first chronic toxicity 30-137 months after completing radiotherapy.

Conclusion: Skin dose calculated as either a maximum point dose or volumetrically to a rind of skin 2 mm in thickness significantly correlated with chronic telangiectasia, dermatitis, breast pain, hypopigmentation, hyperpigmentation, induration, volume reduction, and cosmesis. Attempts should be made to limit maximum skin dose to less than 34Gy (100% Rx) to reduce chronic toxicity.