The effect of VacfixTM on surface dose in radiation therapy

VacfixTM Bags are used for stabilising patients and increasing accuracy during the delivery of radiation therapy. At the WP Holman Clinic (WPHC), Hobart, it was observed that patients stabilised using this method developed acute erythema where the treatment beam traversed the VacfixTM bag. A study quantifying the changes in surface dose when varied thicknesses of VacfixTM bag were placed within the field was initiated. The surface dose was measured using a parallel plate chamber placed at 100cm SSD, and was irradiated using both 6MV and 18MV photons with 100 monitor units. The results showed marked increase in the surface dose with the varying thicknesses, and concurred with a similar study (Johnson et al., 1995). The thicknesses of VacfixTM bag with the greatest attenuation are rarely used clinically. Thus, monitor unit calculations were not altered to include a VacfixTM factor, nor was the planning computer programmed to account for the increased attenuation. The staff now ensure that minimal VacfixTM bag is in the treatment field at simulation, and increased reactions are identified and medically treated. Copyright (2003) Australian Institute of Radiography