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Validation of Treatment Planning Dose Calculations: Experience Working with MPPG 5.a

J Xue

J Xue1*, J Park1, L Kim1, P Balter2, J Ohrt2, S Kirsner2, C Wang1, G Ibbott2, (1) MD Anderson Cancer Center at Cooper, Camden, NJ, (2) UT MD Anderson Cancer Center, Houston, TX


TU-D-201-5 (Tuesday, August 2, 2016) 11:00 AM - 12:15 PM Room: 201

Newly published medical physics practice guideline (MPPG 5.a.) has set the minimum requirements for commissioning and QA of treatment planning dose calculations. We present our experience in the validation of a commercial treatment planning system based on MPPG 5.a.

In addition to tests traditionally performed to commission a model-based dose calculation algorithm, extensive tests were carried out at short and extended SSDs, various depths, oblique gantry angles and off-axis conditions to verify the robustness and limitations of a dose calculation algorithm.

A comparison between measured and calculated dose was performed based on validation tests and evaluation criteria recommended by MPPG 5.a. An ion chamber was used for the measurement of dose at points of interest, and diodes were used for photon IMRT/VMAT validations. Dose profiles were measured with a three-dimensional scanning system and calculated in the TPS using a virtual water phantom.

Calculated and measured absolute dose profiles were compared at each specified SSD and depth for open fields. The disagreement is easily identifiable with the difference curve. Subtle discrepancy has revealed the limitation of the measurement, e.g., a spike at the high dose region and an asymmetrical penumbra observed on the tests with an oblique MLC beam.

The excellent results we had (> 98% pass rate on 3%/3mm gamma index) on the end-to-end tests for both IMRT and VMAT are attributed to the quality beam data and the good understanding of the modeling. The limitation of the model and the uncertainty of measurement were considered when comparing the results.

The extensive tests recommended by the MPPG encourage us to understand the accuracy and limitations of a dose algorithm as well as the uncertainty of measurement. Our experience has shown how the suggested tests can be performed effectively to validate dose calculation models.

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