Room: Exhibit Hall
Purpose: To evaluate the dose build up characteristics of a 2.5MV imaging photon beam
Methods: A Varian True beam linear accelerator is capable of producing a 2.5MV x-ray beam along the same beam path as its high energy clinical treatment beams. This functionality, allows a portal image to be taken where an increased contrast and resolution compared to traditional MV images is performed. Skin and Build up dose characteristics were evaluated for a 2.5 MV beam to assess variations in dose delivery caused by this beam type. Irradiations were performed in a RMI 257 solid water stack phantom which was 30cm x 30cm x 30cm in size. Measurements were made with an Attix Parallel plate ionisation chamber at depths ranging from 0mm to 10mm in 1 millimetre intervals until the depth of maximum dose was measured and at specified depths after wards. Field sizes ranging from 5cm x 5cm up to 40cm x 40cm were evaluated.
Results: Results showed that surface dose at central axis varied from 39% to 69% of maximum dose for field sizes of 5cm x 5cm to 40cm x 40cm respectively. This is compared to 10% to 42% for a 6MV flattening filter photon beam at central axis. The depth of maximum dose was found to be 5mm at a 5cm x 5cm and reached 4mm for a 40cm x 40cm field. Interestingly the peripheral surface dose was in general smaller for a 2.5MV beam compared to a 6MV beam and this is assumed to be due to the enhanced electron contamination produced in the accelerator head with the 6MV x-ray beam which spreads surface dose over a wider region than the 2.5MV beam.
Conclusion: Dose build up occurs for 2.5MV beams where a skin sparing effect still occurs to some extent.