Highlights
- •Dosimetry using TRS 483 for GammaPod Breast SRS device.
- •Dosimetry for non-standard phantom, PMMA.
- •Simple method, ready to use for clinic.
- •Improved uncertainty.
Abstract
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.
Keywords
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Article info
Publication history
Published online: July 06, 2021
Accepted:
May 16,
2021
Received in revised form:
May 12,
2021
Received:
November 25,
2020
Identification
Copyright
© 2021 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
