H Jiang*, N Demez, D Johnson, D Soultan, D Craft, H Li, R Badkul, The University of Kansas Cancer Center, Kansas City, KS

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  H Jiang

PO-GePV-T-405 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Brain tumor volumes in stereotactic radiosurgery (SRS) are usually smaller than in conventional radiation therapy. For small PTVs, the dose calculation resolution can affect the calculated monitor units (MUs) of a treatment plan. If a large resolution is chosen, a small PTV may be sampled without sufficient dose voxels to represent the dose distribution and DVHs. The calculated MU is consequently increased compared with a smaller resolution. This investigation is to study the dependence of plan MU on dose calculation resolution for small SRS PTVs.

Methods: MUs for fifty SRS PTVs were calculated using three calculation resolutions (2.5mm, 2mm, and 1mm) with Varian Eclipse. The prescribed dose ranged from 15Gy to 24Gy, dependent on the PTV volume. The volume of PTVs was from 0.08cc to 30cc. The coverage was to have 95% of PTV receiving the prescription dose. The plan for each PTV was identical, and the only variation was the calculation resolution. The collected data included the MU ratio of 2.5mm and 2mm resolution to 1mm resolution.

Results: The difference in plan MUs was discovered for PTV of all sizes. For tumors around 30 cc, the MU difference was at a negligible level of 1 to 2 percent. However, for smaller PTVs, the MU difference could be significant. For PTVs less than 0.1cc, when compared to 1mm resolution, an MU increase of more than 20% and 10% could be expected for 2.5mm and 2mm dose resolution, respectively.

Conclusion: Dose calculation resolution affects the calculated MUs for small SRS targets, and the selection of an appropriate resolution is clinically relevant. If a 3% MU difference is adopted as the threshold, PTV volume needs to be 15cc or above for using a 2.5mm resolution. For a 2mm resolution, PTV volume should be at least 5cc.

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