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Commissioning of a Treatment Plan Couch Model Using a 3-Dimensional Dose Measurement System

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F Zhang

F Zhang1*, R Schurr2 , J Park3 , N Krishnamurthy4 , C Huerta Hernandez5 , S Kim6 , A Morrow7 , L Vazquez Quino8 , (1) BSWH, Temple, Texas, (2) Baylor Scott & White Health, Temple, TX, (3) Baylor Scott & White Health, Temple, TX, (4) Scott and White Medical Center, Temple, TX, (5) ,Little Rock, AR, (6) Baylor Scott & White Health, Temple, TX, (7) Scott & White Hospital , Temple, AA, (8) Baylor Scott & White Health, Temple, TX


SU-I-GPD-T-475 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To commission a treatment plan couch model using a 3-dimensional (3D) dose measurement system and to evaluate the dosimetric effect of the couch model.

Methods: The BrainLAB ExacTrac couch model of Eclipse treatment planning system (TPS) was commissioned for a Novalis Tx linac using a Delta4 Phantom+, ion chambers with solid water and CIRS IMRT QA phantom. Multiple open-field treatment plans with different beam angles were generated in 20x20x20cm solid water phantom. A Farmer ion chamber was placed at 11cm depth with SAD technique. 100MU were delivered with 10x10cm field size per each gantry angle. Various combinations of couch interior and exterior HU were tested as described in previous studies. Per each combination, the same solid water phantom with various beam angles were used to assess the dose differences. Meanwhile, doses measured with PinPoint ion chamber in the CIRS phantom was compared with those in TPS. In addition, four-field, IMRT and Rapid Arc treatment plans in Delta4 Phantom+ were also employed to verify the dose distributions using Gamma Index.

Results: The ideal combination of couch interior and surface HU were found to be -875 and -300 respectively. The dose discrepancies between TPS and ion chamber measurements was within 1% with both solid water and CIRS phantom with this setting. Dose distribution test using Delta4 Phantom+ was found to be 100% Gamma Index passing rate for all four field, IMRT and Rapid Arc plans with a 3%, 3mm gamma criteria.

Conclusion: We successfully commissioned the BrainLAB couch model using Delta4 Phantom+ and ion chamber measurements. This work demonstrates that default TPS couch HU may not produce accurate dose distributions and consequently significant dose deviations may occur when incorrect couch HU are used. The 3D dose measurements using Delta4 Phantom+ was found to be essential for the commissioning task.

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