Author: Carla D. Bradford, Linda Ding, Yansong Geng, Alan Hartford, I-Lin Kuo, Salvatore LaRosa, Joshua N Wancura 👨🔬
Affiliation: University of Massachusetts Chan Medical School 🌍
Purpose: When treating nasal skin cancers, electron beam radiotherapy dose distributions can be improved by using custom bolus to compensate for uneven surfaces. Here we describe our experience commissioning such a bolus using a silicone material (FlexiBol, .decimal).
Methods: Dose calculations performed with Eclipse eMC 16.1 on a planning CT (pCT) were used to design a tissue-compensating flat-top bolus. Upon receiving the manufactured bolus from the vendor, a verification CT (vCT) was acquired and assessed for volume accuracy, uniformity, air gap, and dose coverage. Radiochromic film was used to measure dose underneath the bolus across a 1-cm2 region. On both the pCT and vCT, the bolus Hounsfield unit (HU) was assigned to match vendor-specified mass density of 1.03. This HU assignment was verified by finding the distance-to-agreement (DTA) between planned and measured dose beneath a 1-cm-thick sample slab, using a chamber positioned near the R50 of a 6 MeV beam.
Results: The manufactured bolus volume was 73% of the designed volume (19.7cc vs 27.0cc), corresponding to a change in thickness of -1.5mm when registered flush against the patient’s nose. For uniformity, the HU coefficient of variation was 9.3%. The vCT maximum air gap between bolus and nose was 3 mm, which was in part due to the absence of nose plugs on the pCT. The percentages of target volume enclosed by the prescribed isodose line on the pCT and vCT were 96.1% and 94.6% respectively. For the film measurement, the dose difference between planned and measured dose was 1.2%. Beneath the 1-cm slab, DTA was 0.3mm.
Conclusion: Silicone is a suitable material for tissue-compensating bolus in electron treatments, provided that bolus volume shrinkage is anticipated during the design stage. Quality control using a CT scan of the manufactured bolus is advised.