Room: Karl Dean Ballroom A2
Purpose: To measure the in-room radiation levels for an MR-linac.
Methods: An ADCL-calibrated large volume spherical ionization chamber was used to measure in-room radiation level in the isocenter plane from a 6FFF, 0.35-T MR-Linac with no magnetic field present. A custom lead enclosure with minimum thickness of 5.1 cm and a 10 cm2 opening shielded the chamber from low energy scatter radiation. Two radiation conditions were accessed: 1) head leakage with the double-stacked MLC fully closed; and 2) scatter radiation with MLC open to maximum field size and a 15 (T) Ã— 30 (W) Ã— 60 (L) cm water-equivalent phantom placed at the isocenter. The chamber and lead shield were positioned in turn at 14 points around the room, and at each point measurement was taken with the linac at 8 different positions around the gantry in 45-deg increments. For the effective TVL measurement, the gantry was rotated to 270 deg and the chamber was placed at 2.18 m from the center of the linac at the height of isocenter, resulting in the maximum shielding from the magnet and linac components. Concrete (29.9Ã—29.9Ã—4 cmÂ³) slabs with a density of 2.16 g/cmÂ³ were placed between gantry and chamber, and measurements taken with increasing number of slabs.
Results: Radiation levels were measured for fourteen points surrounding the machine. The measured radiation levels are comparable to previous measured values on a TomoTherapy unit. Transmission measured indicated the effective TVL is 18.9 cm, indicating that the surrounding split magnet and other components have a shielding effect on the leakage radiation from the linac perpendicular to the direction of the beam.
Conclusion: We established a radiation map to be used for future vault designs for this MR-linac. The readings indicated that the leakage and scatter radiation is less than reported for 6MV C-arm linacs.