(Thursday, 8/2/2018) 10:00 AM - 12:00 PM
Room: Davidson Ballroom A
Purpose: GammaPod is the first stereotactic radiotherapy device for prone breast treatment. Non-coplanar Co-60 beams rotate and focus dose upwards at an isocenter that traverses throughout a breast target via continuous motion of the treatment table. The system was recently updated to eliminate the most upward beams to further reduce the doses to heart and lung, while maintaining the total source activity. In order to minimize unwanted dose to patient, extra tungsten has been added above the radiation sources to achieve a better shielding for the leakage emerging from the top of radiation unit. The objective of study is to investigate the impact of change of radioactive source configuration and to evaluate the dose to patient outside treated volume.
Methods: Radiation survey measurement was performed to investigate the impact of radiation leakage to peripheral dose during treatment. Gafchromic EBT3 film was used to measure the dose in tissue-equivalent material beyond isocenter to confirm falloff of the delivered dose. Anthropomorphic Alderson Rando phantom was set up with Optically Stimulated Luminescence Detectors (OSLDs) taped to evaluate the delivered dose to various anatomical locations outside treated area.
Results: The highest leakage dose during treatment through the shielding of radiation unit is 0.03% of treatment dose. Results of film measurement show that the dose falls off away from isocenter in an approximately inverse square manner as expected. The OSLD measurement with the Rando phantom indicates that the potential doses to contralateral breast and thyroid are less than 1% and 0.1% of the prescribed dose to target, respectively.
Conclusion: In GammaPod treatment, the contribution of radiation leakage to the peripheral dose of patient is small due to the well shielded radiation unit. In future clinical practice, the peripheral doses to sites of interest can be estimated according to the distance to target and empirical data.
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