Evaluating Skin Dose From The MRI Linac
Evaluating skin dose from the MRI-Linac
2017-10-25 17:28:35
External-beam radiotherapy systems that that employ real-time or near real-time MRI guidance show much promise as a new technique to treat cancer. Prototype hybrid MRI-Linac systems are being evaluated as modalities to deliver high precision ablative radiotherapy.
Researchers at Sunnybrook Health Sciences Centre in Toronto are investigating the feasibility of using an MRI-Linac to treat breast cancer patients, using hypofractionated partial breast irradiation (HPBI). Sunnybrook's Odette Cancer Centre is evaluating a clinical prototype of Elekta's MRI-Linac. Its clinical staff believe that the system's online visualization and tumour contouring capabilities, combined with the ability to reduce internal motion margins using multileaf collimator tracking or exception gating from real-time MR images, will be advantageous for treating intact breast tumours. However, one concern is that treatment with an MRI-Linac can cause elevated radiation doses to the skin. The ever-present magnetic field can create electron return effects (ERE), in which electrons liberated at tissue "air and tissue "lung interfaces curl back on themselves and deposit larger radiation doses in tissue at these interfaces. Medical physicist Anthony Kim and colleagues conducted a simulation study to determine the impact of the magnetic field on HPBI dose distributions. After evaluating a tangential beam arrangement (TAN), 5-beam intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT), the researchers confirmed their hypothesis that the magnetic field increases the skin dose. The magnetic field had clinically negligible effects on radiation dose to the heart and the lung (J. Appl. Clin. Med. Phys. doi: 10.1002/acm2.12182).
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