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SU-E-T-383: 3D Dose Reconstruction From Tomotherapy Detector Response of the Pre-Treatment QA.

PURPOSE
To evaluate the reconstructed 3D dose from the Tomotherapy detector response as a pre-treatment patient-specific dose verification tool by comparing the reconstructed dose with the 3D dose from the Tomotherapy treatment planning system (TPS).


METHODS
A simple dry-run of a patient plan was performed to capture the fluence sinogram with the detectors. Then, DosimetryCheck™ (DC) was used to calculate the 3D dose using the detector sinogram, CT images, and treatment delivery parameters. The couch attenuation was corrected mathematically. The reconstructed dose was compared with the Tomotherapy TPS 3D dose in the patient anatomy.


RESULTS
TPS dose and DC reconstructed dose were compared for 5 prostate plans, 1 head and neck plan, and 1 SRS plan. For the prostate plans, the average dose differences of the bladder, rectum, femur, and prostate were 3.0+-1.8%, 4.2+-1.1%, 3.0+-2.0%, and 0.1+-1.1% (+-S.D.). For the head and neck plan, the average differences of the spinal cord, parotid, mandible, and PTV were 6.9%, 8.6%, 1.2%, and 0.2%. For the SRS plan, the average differences of the brain stem, optical chiasm, left eye, and the PTV were -0.5%, 0.6%, 1.5%, and -3.4%. The overall gamma factor (3%, 3mm) passing rate was >90% for all the cases.


CONCLUSION
It is feasible to perform patient specific QA without Tomotherapy phantom DQA. The QA is more meaningful beca use the dose difference is visualized over patient anatomy instead of over a phantom. The statistics with prostate plans show a consistent match between the measured and planning dose. The head and neck plan suggests that DC Pencil Beam algorithm may need improvement in accuracy for areas with dramatic electron density changes. The SRS plan indicates DC accuracy is independent of jaw width and pitch. All the differences between the Tomotherapy TPS and the reconstructed doses were within clinical tolerance.