ESTRO 2021 Abstract Book

S1304

ESTRO 2021

Hospital and Health Service, Herston Biofabrication Institute, Brisbane, Australia

Purpose or Objective This study aims to look at the effect of air gaps when using 3D printed bolus for wide field VMAT treatments. These wide field VMAT treatments are used to treat the skin for whole limbs such as arms and legs. 3D printed bolus is used for build up however, this is quite rigid and as patient swelling occurs as treatment progresses, air gaps become an issue. Currently there has been no investigation conducted into the effect of an air gap on a small narrow field and the unique geometry delivering these treatments. Materials and Methods Surface dose measurements were measured with solid water and film, using foam as an air gap. A 1cm slab of jelly bolus was placed on top to simulate the bolus. Measurements were made with varying air gap from 0- 1cm, depth of measurement at surface and 1cm depth, field sizes ranging from 10x10cm 2 to 1x1cm 2 and slit fields of 3x20cm 2 and 0.5x20cm 2 and gantry angles from 0 to 30 0 using a 6MV beam energy. Results Figure 1 and 2 shows results of the effect gantry angle and varying air gap and the dose difference between field sizes with 1cm air gap for surface dose and dose at 1cm depth, respectively.

Conclusion After analysing the results, it was found that for field smaller than 3x3cm 2 having an air gap larger than 0.5cm will cause differences greater than 5% in surface dose measurements. However, having an air gap will cause little effect to doses at a depth of 1cm. Increasing the gantry angle will also cause a decrease in surface dose compared to gantry 0 0 for a 1x1cm 2 field size. There was little difference for a 3x3cm 2 field size and varying gantry angle. In conclusion, when presented with an air gap of greater than 0.5cm, it would be recommended to check the positioning of the bolus to try and decrease the air gap, particularly in cases where there is varying gantry angles and small field sizes. PO-1580 Absolute gel dosimetry without dose renormalization employing fractionated calibration A. Elter 1 , P. Mann 1 , E. Hellwich 1 , S. Dorsch 1 , C.P. Karger 1 1 German Cancer Research Center (DKFZ), Medical Physics in Radiation Oncology, Heidelberg, Germany Purpose or Objective Polymer gel (PG) dosimetry allows for 3D dose measurements of complex dose distributions with high geometric and dosimetric accuracy. In modern radiotherapy, these dose distributions are typically applied using multiple beams and different fractionation schemes. Although PGs are generally considered to accumulate the applied dose [1], first studies have shown an impact of fractionation on the gel response,