Quantifying superficial tissue doses in Gamma Knife radiosurgeryDilini Pinnaduwage1, Andrew Hwang2, Sebastien Gros3, Martina Descovich3, Penny K Sneed, MD4, Michael W McDermott5, LIJUN MA51San Francisco, United States 2Alta Bates Medical Center, 450-30th Street, Oakland, CA 3Department of Radiation Oncology, University of California San Francisco 4Dept. of Radiation Oncology, University of California San Francisco 5UCSF Keywords: gamma knife, dose planning, dose delivery, skin, radiosurgery
Many groups have reported on radiation-induced meningioma following cranial irradiation. In radiosurgery, the dose to superficial tissues is typically low but can vary significantly depending on target location.
The objective of this study is to validate the accuracy of superficial tissue doses in Gamma Knife (GK) radiosurgery, calculated by the Gamma Plan treatment planning system.
A CT scan was acquired of an anthropomorphic head phantom immobilized in a stereotactic frame. A patient treatment plan was projected onto the phantom CT scan by placing all shots in the patient plan in corresponding locations within the phantom using a single transformation matrix. MOSFET detectors, previously calibrated in the GK unit using a spherical dosimetry phantom, were placed on several locations on the head phantom under 3-mm thick pieces of bolus. The projected plan was then delivered to the phantom. Similarly, superficial dose measurements were obtained from EBT2 Gafchromic films. The films were placed on the same measurement sites of the MOSFETS. The EBT2 measurements were analyzed using a dose calibration curve for a 6 MV beam, as EBT2 film dose measurements are independent of beam energy. The measured superficial doses were compared with those predicted by Gamma Plan.
The MOSFET and film dose measurements ranged from 4 to 53 cGy for a plan scaled to deliver 6 Gy to the 50% isodose line (approximately half the clinical dose). The surface dose varied by location and was sufficiently high (100-200 cGy) for accurate measurement with both MOSFET detectors and EBT2 films. The mean difference between absolute values of the MOSFET and Gamma Plan measurements was 8 ± 12 cGy. The EBT2 measurements showed a better agreement with the Gamma Plan doses with a mean difference of 4 ± 6 cGy. Maximum differences from the Gamma Plan measurements of 29 cGy and 14 cGy were observed for the MOSFETS and EBT2 film, respectively.
The data obtained were from a limited data set.
We have demonstrated a viable technique for quantifying superficial tissue doses in Gamma Knife radiosurgery, utilizing MOSFETS and EBT2 Gafchromic films.
Superficial tissue doses may be relevant for dose planning. Clinical research is required. Project Roles:
D. Pinnaduwage (), A. Hwang (), S. Gros (), M. Descovich (), P. Sneed (), M. McDermott (), L. MA ()