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

PURPOSE: 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. METHODS: The dose to PMMA (DPMMA) 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 (Dw) in broad-beam Co-60 reference geometry to DPMMA in the msr small field Co-60 (Qmsr) geometry, is derived using the Monte Carlo simulations and procedure described in TRS 483. RESULTS: The new conversion factor value for an Exradin A1SL chamber is 0.974. When combined with ND,w, DPMMA differs by 0.5% from the TG-21/Nx method and 0.2% from the IROC values. Uncertainty decreased from 2.2% to 1.6%. CONCLUSION: We successfully implemented 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 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.

Dosimetry evaluation of the GammaPod stereotactic radiosurgery device based on established AAPM and IAEA protocols.

PURPOSE: The GammaPod is a novel dedicated prone breast stereotactic radiosurgery (SRS) device recently developed at the University of Maryland Medical Center. This device utilizes multiple rotating Co-60 sources to create highly conformal dose distributions for breast treatments, including boosts, partial breast irradiation, or presurgery SRS. However, due to its small field sizes and nonstandard geometry, existing calibration protocols cannot be directly applied. In this study, we adapt and implement the American Association of Physicists in Medicine Task Group 21 (TG-21) and International Atomic Energy Agency (IAEA) Technical Report Series 483 (TRS 483) protocols for reference dose measurements for the GammaPod. This represents the first published dosimetric investigation GammaPod and is meant to serve as a reference to future users commissioning and calibrating these devices. METHODS: Reference dose measurements were performed following the TG-21/IAEA TRS 483 protocols using an ADCL-calibrated Exradin A1SL thimble chamber in a polymethyl methacrylate (PMMA) breast-mimicking phantom. Monte Carlo calculations and measurements were also performed in water to determine chamber-specific k PMMA Q m s r , Q 0 f msr , f ref quality conversion factor converting reference field size (fref ) to machine-specific field sizes (fmsr ) (25-mm) as well as k PMMA f clin , f msr , the conversion factor from the (fmsr ) to the clinical field size (fclin ) (15mm). Verification was performed using the thermoluminescent dosimeter remote monitoring service from the Imaging and Radiation Oncology Core (IROC) in Houston, TX. RESULTS: The (fref ) to (fmsr ) chamber-specific factor k PMMA Q m s r , Q 0 f msr , f ref was 0.992 while the (fmsr ) to (fclin ) chamber-specific k PMMA f clin , f msr factor was 1.014. The radiation absorbed dose to water measured in the PMMA phantom based on the TG-21/IAEA TRS 483 formalism agreed with IROC values to within 1% and 2% for the 25- and 15-mm collimators, respectively. CONCLUSION: We successfully implemented the TG-21 and TRS 483 reference dosimetry protocols for the GammaPod. These results show agreement between measurements performed with different reference dosimetry protocols and independent thermoluminescent measurements.