(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: In external radiation therapy the impact of neutrons on late effects for the patients is still an open question when intensity modulation is used in combination with high photon energies. Therefore, we investigate the difference in whole-body dose equivalents between 6 MV and 15 MV image guided radiotherapy (IGRT) treatments of a rhabdomyosarcoma in the prostate.
Methods: An out-of-field dose model (photons and neutrons) for external radiotherapy was included into the Eclipse-TPS using the API-interface. For an anthropomorphic Alderson phantom the whole-body dose was calculated for three different treatment modalities (3DCRT, IMRT, VMAT) and two nominal beam energies (6 MV and 15 MV). The dose of daily kV cone beam CT (CBCT) was added. The calculated 3D dose distributions were compared to independent measurements conducted with thermoluminescent dosimeters (TLDs). To measure the neutron dose contribution, the photoneutron fluence was registered via the TLD600 (n,alpha)-reaction and converted to neutron dose equivalent (including fast neutrons) using Monte Carlo simulated neutron spectra from the literature.
Results: The calculated out-of-field whole-body dose equivalent for the IGRT treatments agreed within (9±10)% (mean and one standard deviation) compared to the measurements. The CBCT and neutron dose equivalent were minor contributions to the total out-of-field dose. For the same modality, there were small differences between the whole-body dose equivalent volume histograms (DEVHs) of the 6 MV and 15 MV treatments. However, the DEVHs between the three treatment modalities showed clear deviations.
Conclusion: Regarding patient protection for IGRT treatments, the choice of beam energy is not important whereas the therapy modality has a large influence on the out-of-field dose. If the patient is treated with intensity-modulated beams, VMAT should be used instead of dynamic IMRT. The general models for photon and neutron dose equivalent calculation can be used for any patient geometry, tumor location and linear accelerator.
Keywords
Photoneutrons, Intensity Modulation, Cone-beam CT
Taxonomy
TH- External beam- photons: out of field dosimetry/risk analysis
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