Room: AAPM ePoster Library
Purpose: The purpose of this project is to evaluate the differences of IMRT whole brain plans with hippocampal, lacrimal and parotid gland sparing against the clinical plans based on clinical dosimetric metrics and radiobiological indices.
Methods: Five patients previously treated with 3DCRT were selected to be re-planned in Raystation (v.8A RaySearch Laboratories) using IMRT prescribed to 30 Gy over 10 fractions. The plans were optimized following the standards set by the NRG-CC001 protocol. In this study, additional constraints were used for the parotid and lacrimal glands. The planning PTV was generated by taking the whole brain minus the hippocampi with 5mm expansion. The plans were determined to be acceptable when the PTV received 90-95% of the prescription dose and the OARs were spared per protocol. The plan comparison was based on the clinical dosimetric goals. For the parotid and lacrimal glands, the normal tissue complication probabilities (NTCP) were calculated using in-house dose-response parameters.
Results: The IMRT plans provided large reductions in doses to lacrimal and parotid glands when compared the 3DCRT plans, and met the minimum goals of hippocampal sparing and overall PTV coverage. More specifically, the largest average differences are observed in the reduction in the V15Gy (82.5% and 89.5%) and V20Gy (42.9% and 53.0%) of the lacrimal and parotid glands, respectively. The corresponding reductions in the values of NTCP are 18.2% and 25.0%, respectively. A considerable reduction in the D100% and D0.03cc of about 14.2 – 20.6 Gy is observed for hippocampus. This sparing comes with the cost of a small reduction (3.7%) in the target coverage (V95%).
Conclusion: When compared to 3DCRT, whole brain IMRT plans can provide clinically acceptable plans while significantly sparing the hippocampi, lacrimal and parotid glands. Such plans are expected to reduce the clinically observed complications of dry eyes and xerostomia.