Exhibit Hall | Forum 4
Purpose: Patients with resectable recurrent head and neck cancer can potentially be treated using Cs-131 seeds placed intraoperatively. For treatments in the head and neck, however, which are far less commonly treated using brachytherapy, commonly used TG-43 based calculations may not be adequate for the accurate determination of the absorbed dose. This study compares Monte Carlo simulated doses to doses calculated TG-43 based on post-implant CT scans for five head and neck treatments.
Methods: The EGSnrc user code egs_brachy was used for all Monte Carlo simulations. Five post implant CT scans were converted to a format compatible with EGSnrc. DICOM-RT plan files from the clinical plans used to treat the patients were converted into egs_brachy compatible input files. The number of histories run were chosen to achieve uncertainties of less than 1% within the tumor volume. Monte Carlo dose distributions were then compared to TG-43 calculations from the treatment planning system.
Results: Proximal bony anatomy to the implanted seeds was up to 350% hotter in the Monte Carlo calculations than the respective TG-43 calculation. An implant in the maxillary sinus had prescription isodose lines that extended up to 1 cm further than the TG-43 calculated doses and also involved less uniform coverage than the TG-43 calculations indicated.
Conclusion: The discrepancies between the Monte Carlo calculated dose distribution and TG-43 calculated doses for Cesium-131 implants in the head and neck could be substantial, especially in the presence of proximal bone and airy sinuses. The observed discrepancies are mainly due to low energy photon mass attenuation coefficient ratios of bone and air to that of water. Cs-131 implants are gaining popularity for treating recurrent head and neck cancers and centers should be aware of the limitations of TG-43 based planning, especially since TG-186 does not provide guidance for head and neck anatomy.