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Delivery Accuracy of Stereotactic Radiosurgery with Tomotherapy Using Treatment Planning System Version V4.0 and the Recent Upgrade V4.2

V Thakur¹*, R Ruo², J Seuntjens³, W Parker⁴, R Doucet⁵, E Soisson⁶, (1) McGill University, Montreal, QC, (2) Montreal General Hospital, Montreal, QC, (3) McGill University, Montreal, QC, (4) McGill Univ Health Center, Montreal, QC, (5) CHUM-Hopital Notre-Dame, Montreal, Qc, (6) McGill University, Montreal, QC

SU-E-T-429 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To compare the delivery accuracy of helical tomotherapy for stereotactic radiosurgery using two treatment planning system (TPS) versions, v4.0 and the recent upgrade that includes increased sampling in the dose calculation algorithm v4.2.

Methods: Two spherical targets of diameter 6 mm and 10 mm were contoured on the CT scan of a Lucite phantom. Three sets of plans (targets positioned at the machine isocenter and 10 cm to both anterior and lateral to the machine isocenter) were generated, with both TPS v4.0 and v4.2 using Fine dose grid resolution of 2x2x1 mm3. Radiochromic film was used to measure the dose profiles and two ionization chambers (A1SL and MicroLion) were used to measure the point dose.

Results: The agreement between delivered and calculated dose was inferior for v4.0 compared to v4.2. For both planning versions, the measured dose for the targets placed at the isocenter was higher than the planned dose whereas for off-axis targets, v4.2 showed higher measured dose and v4.0 showed lower measured dose compared to the planned dose. For v4.0, the percentage dose difference varies 13%to 24%(depending upon the detector) when the targets were placed at 10 cm away from the machine isocenter instead of at the isocenter, whereas for v4.2 this range was 3% to 6%.

Conclusion: The agreement between delivered and calculated dose depends upon the position of the target inside the treatment bore. The higher dose delivered at isocenter with both planning versions could be due to the relatively large size of smallest dose grid available at tomotherapy TPS. A further investigation to determine the cause of higher dose at isocenter is required and is ongoing.

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