Evaluation of skin dose associated with different frequencies of bolus applications in post-mastectomy three-dimensional conformal radiotherapy

Background: The study aimed to calculate chest-wall skin dose associated with different frequencies of bolus applications in post-mastectomy three-dimensional conformal radiotherapy (3D-CRT) and to provide detailed information in the selection of an appropriate bolus regimen in this clinical setting. Methods: CT-Simulation scans of 22 post-mastectomy patients were used. Chest wall for clinical target volume (CTV) and a volume including 2-mm surface thickness of the chest wall for skin structures were delineated. Precise PLAN 2.11 treatment planning system (TPS) was used for 3D-CRT planning. 50 Gy in 25 fractions were prescribed using tangential fields and 6-MV photons. Six different frequencies of bolus applications (0, 5, 10, 15, 20, and 25) were administered. Cumulative dose-volume histograms were generated for each bolus regimen. The minimum, maximum and mean skin doses associated with the bolus regimens were compared. To test the accuracy of TPS dose calculations, experimental measurements were performed using EBT gafchromic films. Results: The mean, minimum and maximum skin doses were significantly increased with increasing days of bolus applications (p < 0.001). The minimum skin doses for 0, 5, 10, 15, 20, and 25 days of bolus applications were 73.0% +/- 2.0%, 78.2% +/- 2.0%, 83.3% +/- 1.7%, 88.3% +/- 1.6%, 92.2% +/- 1.7%, and 93.8% +/- 1.8%, respectively. The minimum skin dose increments between 20 and 25 (1.6% +/- 1.0%), and 15 and 20 (4.0% +/- 1.0%) days of bolus applications were significantly lower than the dose increments between 0 and 5 (5.2% +/- 0.6%), 5 and 10 (5.1% +/- 0.8%), and 10 and 15 (4.9% +/- 0.8%) days of bolus applications (p < 0.001). The maximum skin doses for 0, 5, 10, 15, 20, and 25 days of bolus applications were 110.1% +/- 1.1%, 110.3% +/- 1.1%, 110.5% +/- 1.2%, 110.8% +/- 1.3%, 111.2% +/- 1.5%, and 112.2% +/- 1.7%, respectively. The maximum skin dose increments between 20 and 25 (1.0% +/- 0.6%), and 15 and 20 (0.4% +/- 0.3%) days of bolus applications were significantly higher than the dose increments between 0 and 5 (0.2% +/- 0.2%), 5 and 10 (0.2% +/- 0.2%), and 10 and 15 (0.2% +/- 0.2%) days of bolus applications (p <= 0.003). The TPS overestimated the near-surface dose 10.8% at 2-mm below the skin surface. Conclusion: In post-mastectomy 3D-CRT, using a 1-cm thick bolus in up to 15 of the total 25 fractions increased minimum skin doses with a tolerable increase in maximum doses.