LET-Dependent Rbe In the Skin of Breast Patients Treated with IMPT Vs 3DCRT
Abstract
Purpose
Breast patients treated with intensity-modulated proton therapy (IMPT) often experience more severe skin dose toxicity compared to those with photon 3DCRT. Here we quantitatively estimated the LET-dependent RBE (LET-RBE) vs clinically used constant RBE=1.1 for the IMPT patients.
Methods
33 and 25 Caucasian patients treated with IMPT and 3DCRT in a single institute without chemotherapy were retrospectively analyzed in this IRB approved study. Scores for radiation dermatitis (RD) were recorded weekly. For each patient, the skin was contoured with 3 mm inside from the patient’s surface, within 50% isodose line. The EQD2 of D10 of the skin was studied due to different prescriptions. Particularly, we analyzed the correlation between the RD and D10 of the course and the accumulated D10 (D10ac) when the new toxicity grades were recorded. LETd was calculated in a Monte-Carlo dose engine and 3 RBE models were employed to estimate LET-RBE. The average RBE dose from the 3 models was used for our analysis.
Results
For both proton and photon groups, no correlation between RD and the course D10 was found, however, a strong correlation was found between RD and D10ac. The mean D10ac (RBE=1.1) was 26.69 Gy, 43.71 Gy and 49.11 Gy for RD=1, 2 and 3 respectively. When LET was considered, the mean D10ac was 28.62 Gy, 46.52 Gy and 51.94 Gy for RD=1, 2 and 3 respectively. For the photon group, the mean D10ac was 28.11 Gy and 46.29 Gy for RD=1 and 2 (no photon patient had RD=3). Thus, the mean D10ac (LET-RBE) excellently agreed with photon D10 and was 7.2%, 6.4% and 5.8% higher than the mean D10ac (RBE=1.1) for RD=1, 2 and 3 respectively.
Conclusion
Breast skin toxicities show evidence of LET-RBE effect for breast patients. Our results indicate RBE=1.165 (=1.1*(1+(7.2%+6.4%+5.8%)/3) may be appropriate for breast skin.