ESTRO 2024 - Abstract Book

S3415

Physics - Dose calculation algorithms

ESTRO 2024

current bunker designs have not been properly optimised to reduce cost or space usage. Rijken et al compared the shielding recommendations provided by the NCRP 151, IAEA SRS 47 and IPEM 75 to radiation survey data taken from 75 bunkers and found that a 43% ($340,000 AUD) cost increase can be incurred by employing all overly conservative shielding estimates 1 .

Material/Methods:

A Varian TruBeam linac head has been modelled in Geant4 using the GATE extension. The surrounding linac bunker has been modelled based off the floor plans at Sir Charles Gairdner Hospital and using a standard shielding concrete material. A beam has been simulated using a monoenergetic electron beam of 6, 10 or 18 MeV for the testing of improvements. A number of new shielding concrete materials have been tested, including both high and low density concretes The width of the primary and secondary barriers has been changed by ± 37 cm (1 tenth value layer). Changes to the width of the maze (± 50 cm) were made. Modifications were applied to the angle of each of the triple bend angle mazes, adjusting the first, second and third by ± 2°,± 4° and ± 6° respectively. More novel changes to the bunker have been made regarding the inclusion of additional shielding layers to the bunker walls. A layer of lead has been added to the floor and walls of the maze with thicknesses of 2 and 5 mm simulated. A layer of steel was added to the internal room of the bunker, with thicknesses of 20 and 40. A new dual layer addition of 37 cm of shielding concrete and 1 mm of has also been implemented. Higher density concretes were found to more effectively attenuate photons, reducing the energy deposition at a depth of 90-100 cm within the primary barrier by nearly an order of magnitude as compared to standard shielding concrete. Equivalent shielding can be achieved with a high-density concrete of 83% the thickness of a standard shielding concrete. Simulations showed 1 tenth value layer could be safely removed from the linac bunker, resulting in a saving of approximately $50,000 USD for new constructions. A similar result was observed for the secondary barriers, providing a potential cost saving of $130,000 USD for new constructions. Reducing the width of the maze by 0.5 m allowed for a 45% dose reduction at the maze entrance, while modification of the triple bend maze angles reduced the dose at the maze entrance by 35%. 2 mm lead sheeting applied to the maze walls and floor reduced the dose at the maze entrance by 85%. The 40 cm of steel added to the internal walls of the bunker shielded almost the entirety of the underlying secondary barrier from receiving any dose, with the primary barriers dose dropping by five orders of magnitude. The 20 cm steel layering also substantially dropped the dose to the underlying concrete primary and secondary barriers but didn’t reduce the secondary barriers dose to negligible levels. The 1 mm of lead incorporated after the first 37 cm of shielding concrete was found to act as another full tenth value layer, allowing for approximately $100,000 USD in potential shielding concrete savings in future constructions. Results:

Conclusion:

The Varian TrueBeam linac bunker and linac head within it was modelled in GATE to produce Monte Carlo simulations in which the shielding capabilities of the bunker can be tested. Changes to the bunker design were then made in terms of the use of different shielding materials used in the bunker construction, adjustments to the wall thicknesses, changes to the geometry of the maze and the addition of further shielding layers. In making these changes, recommendations have been made for changes that could work to increase the space and cost efficiency of the linac bunker, with some features resulting in a substantial cost reduction for future constructions.