Simplified method for determining dose to a non-water phantom through the use of N D,w and IAEA TRS 483 for the GammaPod

GammaPod, a breast stereotactic radiosurgery device, utilizes 25 rotating Co-60 sources to deliver highly conformal dose distributions. The GammaPod system requires that reference dosimetry be performed in a specific vendor-supplied poly-methylmethacrylate (PMMA) phantom. The nonstandard nature of GammaPod dosimetry, in both the phantom material and machine-specific reference (msr), prohibits use of the American Association of Physicists in Medicine Task Group 51 (TG-51) protocol. This study proposes a practical method using TRS 483 to make the reference dosimetry procedure simpler and to reduce overall uncertainties. The dose to PMMA (D ) is determined under msr conditions using TRS 483 with an Exradin A1SL chamber placed in a PMMA phantom. The conversion factor, which converts from the dose-to-water (D ) in broad-beam Co-60 reference geometry to D in the msr small field Co-60 (Q ) geometry, is derived using the Monte Carlo simulations and procedure described in TRS 483. The new conversion factor value for an Exradin A1SL chamber is 0.974. When combined with N , D differs by 0.5% from the TG-21/N method and 0.2% from the IROC values. Uncertainty decreased from 2.2% to 1.6%. We successfully implemented TRS 483 reference dosimetry protocols utilizing N for the GammaPod in the PMMA phantom. These results show not only agreement between measurements performed with the previously published method and independent thermoluminescent dosimetry measurements but also reductions in uncertainty. This also provides readers with a pathway to develop their own IAEA TRS 483 factor for any new small field machine that may be developed.