Recommendations for using analytical anisotropic algorithm and AcurosXB for epidermal dose calculations in breast radiotherapy from an in vivo Gafchromic film study.

BACKGROUND AND PURPOSE: This study recommends clinical epidermal dose calculation methods based on in-vivo film measurements and registered skin dose distributions with the Eclipse (Varian Medical Systems) treatment planning system's Analytical Anisotropic Algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms. MATERIALS AND METHODS: Eighteen AAA V13.6 breast plans were recalculated using AXB (dose to medium) V13.5 with the same beam parameters and monitor units as in the original plans. These are compared against in-vivo Gafchromic film measurements from the lateral and inferior breast regions. Three skin structures in the treatment planning system are evaluated: a surface layer of voxels of the body contour, a 0.2 cm internal skin rind, and a 0.5 cm internal skin rind. RESULTS: Systematic shifts are demonstrated between the film measurements of skin dose and the Eclipse dose calculations. On average, the dose to the surface layer of pixels is underestimated by AAA by 8% and overestimated by AXB by 3%. A 5 mm skin rind extended into the body can increase epidermal dose calculations on average by 8% for AAA and 4% for AXB. CONCLUSION: This is the first study to register in-vivo skin dose distributions in the breast to the treatment planning system for comparison. Based on the results from this study it is recommended that epidermal dose is calculated with a 0.5 cm skin rind for the AAA algorithm and with rind thickness up to 0.2 cm for the AXB algorithm.

Toward a Skin Dose-Area Metric Predictive of Moist Desquamation Using In Vivo Skin Dosimetry and Skin Assessments.

Purpose: Moist desquamation (MD) is a concerning acute side effect of radiation therapy for breast cancer, often seen in skin folds for patients having large or pendulous breasts. In vivo skin dosimetry, clinical assessments, and patient-reported skin reactions were used to determine a relationship between dose-area metrics and the development of MD, to lend insight into skin tolerances and possibly guide future treatment planning dose constraints. Methods and Materials: Skin dose was measured using GafChromic film on the inner surface of an early prototype carbon-fiber accessory for breast support to remove the inframammary fold in 20 patients at high risk of developing MD undergoing adjuvant whole breast radiation therapy. Prescribed doses were 42.5 Gray (Gy) in 16 fractions or 50 Gy in 25 fractions using 6 to 15 MV x-rays. To account for fraction size differences, analysis was performed using the equivalent dose in 2 Gy fractions using α/ß = 11 (EQD211). MD was assessed out to 2 weeks post radiation therapy by trained therapists and by a patient-reported outcome questionnaire. Results: Statistically significant differences in areas receiving 30 to 48 Gy (EQD211) were observed between patients who did and did not develop MD in the inframammary area. Patients receiving EQD211 maximum dose ≤ 46 Gy and ≥ 38 Gy to ≤ 50 cm2 of their breast skin did not develop MD. Conclusions: The findings of this study offer insight into the relationship between skin toxicity and areas of skin irradiated to doses up to 50 Gy. Potential skin dose constraints to test in future studies to prevent MD are suggested.