ESTRO 36 Abstract Book

S405 ESTRO 36 _______________________________________________________________________________________________

Conclusion The results of our study show that 3D printing technology can be used to fabricate patient-specific, large scale phantoms that could be used for a variety of research, dosimetric, and quality assurance purposes. PO-0766 The effect of air gaps on Magic Plate (MP512) for small field dosimetry K. Utitsarn 1 , N. Stansook 1 , Z. Alrowaiili 1 , M. Carolan 2 , M. Petasecca 1 , M. Lerch 1 , A. Rosenfeld 1 1 University of Wollongong, Center for Medical Radiation Physics, Wollongong, Australia 2 Wollongong Hospital, Illawara Cancer Care Centre, Wollongong, Australia Purpose or Objective We evaluate the impact of an air gap on the MP512 irradiation response at depth in a phantom and optimize this gap for accurate small field dosimetry in clinical photon and electron beams. Material and Methods MP512 is a 2 dimensional silicon monolithic detector manufactured on a p-type substrate. The array consists of 512 pixels with detector size 0.5x0.5 mm 2 and pixel pitch 2 mm. The overall area of the active part of the detector is 52x52 mm 2 . The output factor (OF) and the percentage depth dose (PDD) were measured with MP512 in 6MV and 10MV photon beams. The OF was measured at a depth of 10 cm in a solid water phantom for square field sizes ranging from 0.5 to 10 cm 2 . The PDD was measured for field sizes 2x2cm 2 , 5x5cm 2 and 10x10cm 2 by scanning the MP512 from the depth of 0.5 cm to 10 cm. Both the OF and PDD were measured at all field sizes with an air gap immediately above the detector of 0.5, 1.0, 1.2, 2.0 and 2.6 mm respectively. The PDD for 6, 12 and 20 MeV electron beams with a standard applicator providing 10x10 cm 2 field size, were measured using an air gap of 0.5mm and 2.6mm. Results The OF measured by the MP512 reduces with increasing air gap above the detector. The impact of the air gap is largest for the small fields of 0.5x0.5 and 1x1 cm 2 while negligible for field sizes larger than 4x4 cm 2 . The OF measured by the MP512 detector with an air gap of 0.5 and 1.2 mm show a good agreement with OF measured using EBT3 film and MO Skin for 6 and 10 MV, respectively. Similar results were observed for the PDD measurements in field sizes of 5x5 and 10x10 cm 2 . The PDD for a 2x2 cm 2 was within ±3% of the EBT3 for both photon energies. The PDD measured with MP512 is within ±1.6% and ±1.5% of that measured using a Markus ionization chamber (IC) for 6 and 10 MV fields respectively. The PDD measured by electron beams demonstrated no significant effect with increasing air gap above the MP512 for all energies. The results for both 0.5mm and 2.6mm gap are within ±3% of similar measurements made using the Markus IC. Conclusion The MP512 response with different air gaps immediately above the detector in solid water phantom have been investigated in clinical photon and electron fields. The results confirm that the MP512 monolithic diode array is suitable for QA of small fields in a phantom. The study shows that the air gap size has a significant effect on small field photon dosimetry performance of the MP512 consistent with a loss of electronic equilibrium. The small air gap of 0.5 mm and 1.2 mm is the best air gap for small field dosimetry in 6 and 10 MV photon beams respectively. The effect of air gap on electron beam dosimetry using the MP512 was demonstrated to be not significant due to the electronic equilibrium conditions always being fully established.

the inferior aspect of the patient on the printing surface. The slices were individually and collectively imaged and examined for printing accuracy. The original patient CT scan and the assembled phantom CT scan were registered together to assess the overall accuracy of the phantom construction. Results The slices took an average of 24 hours and 19 minutes to print, and the total material cost of the phantom was $524. Figure 1 shows images of the phantom with the left- most slices removed to show the interior anatomy (a), and the entire phantom assembled (b). As can be seen, the phantom fits together well, and has a high level of detail. Figure 2 shows a comparison of slices in the axial, sagittal and coronal orientations from the original patient CT image (a), and slices from the phantom CT image (b) in the same location and orientation. While material heterogeneity has been lost due to using only one material in the phantom, the anatomical and structural details agree very well between the printed phantom and the source image. The only disagreement is in the lungs, where unsupported nodules were removed prior to printing the phantom. Analysis of individual slices revealed that measurable dimensions were accurate within 0.5 mm, and the average volumetric discrepancy between printed slices and their models was 1.37%. Figure 1:

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