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Dose Calculation in Metallic Prosthesis for Pinnacle and RayStation

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L Archambault

Laurie Archambault/Hospital Maisonneuve-Rosemont


SU-I-GPD-T-416 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The number of radiotherapy patients with metallic prostheses is increasing. In this context, this work compares dose calculations in Pinnacle and RayStation to Gafchromic film measurements in the presence of metallic prostheses.

Methods: Aluminum and steel cylindrical rods of 3 cm diameter were placed in solid water fantoms and irradiated with 10x10 cm² 6 MV photon beams at SSD 90 cm. Gafchromic films were inserted in the rods and downstream from the rods. Calculations were performed in Pinnacle and RayStation. The dose measured inside the rods was converted into dose to medium and point doses on the beam central axis were compared to the calculations.

Results: The aluminum rod has a density of 2.7 g/cm³, which is below the saturation density of our CT scanner. In this situation, both TPS calculate a downstream dose within 3% of the measured dose. The steel rod has a density of 8.1 g/cm³, which greatly exceeds the CT saturation density of 3.2 g/cm³. This leads to an overestimation of the transmitted dose in both TPS calculations by up to 35%. If the rod is assigned its real density in the TPS, the dose calculation error drops to a maximum of 10% for both TPS. Finally, inside the aluminum prosthesis, we observed that Pinnacle’s dose was within 2% of the measured dose to medium while RayStation reported a dose 10% greater. This is due to the choice of RayStation to display dose to water.

Conclusion: This work evaluates the dose calculation overestimation downstream from a prosthesis in a worst-case clinical scenario. This error can be reduced if the prosthesis geometry and density are considered in the TPS. We also highlight the dose reporting to water in RayStation and to medium in Pinnacle, suggesting that prescription points should be placed outside prostheses for consistency.

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