Abstracts Biological Sciences

To investigate the effect of bolus on skin dose in VMAT/IMRT for head and neck cancer

by Chitra J Shenoy

External beam radiotherapy is used to treat the postsurgical bed of primary tumours and regional lymph nodes in head and neck cancer treatment. As megavoltage photons have a skin sparing effect, bolus is used to provide dose build-up and deliver high doses to scar tissue on the skin. At the Radiation Oncology Department of Auckland City Hospital (ACH) IMRT and VMAT are used to treat these patients. This work investigates whether bolus material is necessary to ensure that the scars on the skin of head and neck radiotherapy patients receive an adequate dose of radiation. A retrospective planning study was performed to investigate whether clinically acceptable IMRT and VMAT plans could be developed to deliver full dose to the scar tissues without bolus. It was possible to generate excellent PTV coverage in all cases without bolus, but doses to the scar tissue were on average 12% lower than those produced in the original plans. The results of this study were verified with a phantom study and film dosimetry. Surface dose in the original plan with bolus measured with film was in good agreement with the dose predicted by Pinnacle3. However, there was considerable discrepancy in measured and predicted surface doses for the no bolus plans. Pinnacle3 overestimated the surface doses in no bolus plans. If the goal is to adequately treat the scar tissue whilst maintaining a good quality dose distribution within the target, this study demonstrates that bolus is a necessary component of the treatment delivery process.