Technical Note: Comparison of peripheral patient dose from MR-guided 60 Co therapy and 6 MV linear accelerator IGRT

Abstract

Purpose: The use of X-ray imaging in radiation therapy can give a substantial dose to the patient. A Cobalt machine combined with an magnetic resonance imaging (MRI) was recently introduced to clinical work. One positive aspect of using non-ionizing imaging devices is the reduction of the patient exposure. The purpose of this study was to quantify the difference in out-of-field dose to the patient between the image guided radiation therapy (IGRT) treatment applied with a linear accelerator with cone beam CT (CBCT) equipment and a Cobalt machine combined with an MRI.

Methods: The treatment of a rhabdomyosarcoma in the prostate was planned and irradiated using different modalities and radiation therapy machines. The whole-body dose was measured for a 3D-conformal radiation therapy (3DCRT), an intensity-modulated radiation therapy (IMRT), and a volumetric-modulated arc therapy plan applied with a conventional linear accelerator operated at 6 MV beam energy. Additionally, the dose of an IMRT plan applied with a ⁶⁰ Co machine combined with an MRI was measured. Furthermore, the dose of one CBCT scan using the linear accelerator's on-board imaging system was determined. The 3D dose measurements were performed in an anthropomorphic phantom containing 168 slots for thermoluminescence dosimeters (TLDs). A combination of LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD100H) was used to accurately determine the in- and out-of-field dose. The plans were rescaled to different fractionation schemes (2 Gy, 3 Gy, and 5 Gy fraction dose) and the dose of one CBCT scan was additionally added to the treatment dose per fraction applied with the linear accelerator. The resulting absorbed doses per fraction of the two machines were compared.

Results: In the target region, all measured treatment plans presented the same magnitude of dose, while the CBCT dose was a factor of 100 smaller. Close to the planned target volume (PTV), the dose from the ⁶⁰ Co machine was a factor of two higher compared with the 3DCRT + CBCT dose. Up to 45 cm from the PTV, the treatment applied with the ⁶⁰ Co-sources showed an increased out-of-field dose compared to the linear accelerator + CBCT IGRT treatments. Further away from the PTV in the region where leakage from the gantry head is dominating, the out-of-field dose of the Cobalt machine was smaller compared to the linear accelerator + CBCT.

Conclusion: The peripheral dose of the ⁶⁰ Co machine combined with an MRI is larger up to 45 cm from the PTV and further away, it is lower than the dose from a linear accelerator + CBCT treatment. The presented fractionation schemes had a marginal impact on the results.

Keywords: CBCT; IGRT; MRI; cobalt; dosimetry.