Room: Exhibit Hall
Purpose: To evaluate the impact of brass mesh bolus on surface dose in 6MV photon volumetric arc therapy (VMAT) of the chest wall.
Methods: A VMAT treatment plan of a simulated chest wall structure located in a quadrant of a cylindrical phantom without brass mesh was designed with a Varian Eclipse v13.6 RTPS, using the AAA algorithm. The surface dose of the phantom with and without the presence of a brass mesh bolus was later measured with Ashland EBT3 Gafchromic films and Landauer nanoDot OSLDs. Individual dose calibration curves were created for each film. Individual calibration factors for nanoDot detectors were previously established. Similar experiments were performed for tangent treatment plan and for enface 6MV beam projected on solid phantom.
Results: Surface doses for the VMAT plan were 60 to 85% of prescription dose. The brass mesh bolus systematically increased the surface dose by a factor of 1.4. The brass bolus effect for the VMAT plan was 17% lower than the observed in tangent and normal beam setups. This is probably due to the multiple angles of incidence in the VMAT plan. Film analysis shows a wave â€“ like textured dose profile caused by the mesh interlinking design, with an average 10% peak to peak dose variation. However, the absolute, averaged Gafchromic dose measurements and point OSLD dosimetry agree within 5%. Overall measurement uncertainties for Gafchromic and OSLD dosimetry at 200cGy dose level were 3% (1Ïƒ).
Conclusion: Brass mesh is an appropriate choice for 6MV VMAT therapy of the chest wall. Surface dose can be optimized in the plan by constraining skin dose to an isodose lower than 80% and using the brass bolus for matching prescription. Gafchromic film and OSLDs provide comparable results for in-vivo surface dosimetry for this treatment modality.