Clinical implementation of brass mesh bolus for chest wall postmastectomy radiotherapy and film dosimetry for surface dose estimates

Objective: This study presents the dosimetric data taken with radiochromic EBT3 film with brass mesh bolus using solid water and semi-breast phantoms, and its clinical implementation to analyze the surface dose estimates to the chest wall in postmastectomy radiotherapy (PMRT) patients. Materials and Methods: Water-equivalent thickness of brass bolus was estimated with solid water phantom under 6 megavoltage photon beam. Following measurements with film were taken with no bolus, 1, 2, and 3 layers of brass bolus: (a) surface doses on solid water phantom with normal incidence and on curved surface of a locally fabricated cylindrical semi-breast phantom for tangential field irradiation, (b) depth doses (in solid phantom), and (c) surface dose measurements around the scar area in six patients undergoing PMRT with prescribed dose of 50 Gy in 25 fractions. Results: Water-equivalent thickness (per layer) of brass bolus 2.09 ± 0.13 mm was calculated. Surface dose measured by film under the bolus with solid water phantom increased from 25.2% ±0.9% without bolus to 62.5% ± 3.1%, 80.1% ± 1.5%, and 104.4% ± 1.7% with 1, 2, and 3 layers of bolus, respectively. Corresponding observations with semi-breast phantom were 32.6% ± 5.3% without bolus to 96.7% ± 9.1%, 107.3% ± 9.0%, and 110.2% ± 8.7%, respectively. A film measurement shows that the dose at depths of 3, 5, and 10 cm is nearly same with or without brass bolus and the percentage difference is <1.5% at these depths. Mean surface doses from 6 patients treated with brass bolus ranged from 79.5% to 84.9%. The bolus application was discontinued between 18th and 23rd fractions on the development of Grade 2 skin toxicity for different patients. The total skin dose to chest wall for a patient was 3699 cGy from overall treatment with and without bolus. Conclusions: Brass mesh bolus does not significantly change dose at depths, and the surface dose is increased. This may be used as a substitute for tissue-equivalent bolus to improve surface conformity in PMRT.