Room: AAPM ePoster Library
GammaPod, a breast stereotactic radiosurgery (SRS) utilizes 25-rotating Co-60 sources to deliver conformal dose distributions. The non-standard nature of GammaPod dosimetry in terms of small field size (25mm), non-reference machine-specific geometry, and PMMA phantom prohibits to use of the TG-51 protocol. A recent paper demonstrated a method for reference dose measurements for the GammaPod using TG-21 and IAEA TRS 483. This method, however, contains inherently more uncertainties due to the use of TG-21 and Nx than TG-51 and ND,w. This study proposes to combine TRS 483 with a new measurement & Monte Carlo based kQ and ND,w to reduce uncertainties.
Dose_PMMA is determined under machine-specific reference (msr) conditions using TRS 483 with an Exradin A1SL chamber. Dose_PMMA is determined as the product of the reading, the chamber calibration factor, ND,w, and the kQ factor which converts from the water dose in broad beam Co-60 (Q0) reference (ref) geometry to DosePMMA in the msr small field Co-60 (Qmsr) geometrty. kQ has two parts; one is acquired from the measurements and the Monte Carlo calculations, and the other is determined from published data. The resulting DosePMMA is compared to the TG-21/Nx method and validated with the Imaging and Radiation Oncology Core (IROC) in Houston, TX.
The new kQ factor is 0.974. When combined with ND,w, Dose_PMMA differs by 0.3% from the TG-21/Nx method and 0.4 % to the IROC values. Uncertainty is decreased to 1.6% from 2.2%.
We successfully implemented the TRS 483 reference dosimetry protocols utilizing ND,w for the GammaPod in the PMMA phantom. These results show not only agreement between measurements performed with the previously published method and independent TLD measurements but also reduces the uncertainty noticeably.