Dosimetric Accuracy of TomoTherapy Based Stereotactic Radiotherapy

Abstract

Helical Tomotherapy (HT) has gained wide acceptance as a delivery technique for intra- and extra-cranial stereotactic radiotherapy using the on-board MVCT for stereotactic localization. Verifying the dosimetric accuracy of the delivery becomes more difficult for small targets. Of 70 patients treated on our HT unit since January 1, 2010, routine delivery quality assurance (DQA) has only exceeded the expected dose by more than 2.5%, for 5 patients, all with stereotactic targets in the lung and brain. The largest dose difference (+5%) was seen for the smallest target (8.4 cm3). Measurements were performed with the standard DQA procedure using small volume A1SL ion chamber and radiochromic film. Film measurements showed excellent localization accuracy, but confirmed the dose discrepancy for the 5 patients. The goal of this work is to investigate these discrepancies and to quantify the dosimetric accuracy of the HT stereotactic delivery. HT plans were run using a cylindrical water-filled phantom with contoured spherical targets of variable diameter (5 mm-40 mm, volume 0.05 cm3 to 32.4cm3). The phantom was scanned with a 48 cm field of view and 1 mm slice width. Images were imported into the TomoTherapy TPS resulting in a “Fine” dose grid voxel size of 1.9 mm x 1.9 mm x 1.0 mm, which is consistent with the smallest we use clinically for HT stereotactic procedures. Plans utilized the 1.0cm field width and a pitch of 0.287. Dose was measured using a very small volume (0.002cm3) liquid ion chamber. The phantom and chamber were positioned using MVCT imaging for lateral and vertical positioning and to approximate a longitudinal position. The final longitudinal position was determined by searching for the maximum signal for the 5 mm diameter target in 1 mm increments. Measured dose was found to be higher than that predicted by the TPS in all cases with the discrepancy between the planned and measured dose increasing with decreasing target volume. For the 5 mm, 10 mm, 15 mm, 20 mm, 30 mm and 40 mm diameter targets, measured doses were 7.9% ± 0.2%, 4.6 ± 0.3%, 2.1% ± 0.4%, 1.4% ± 0.1%, 0.7% ± 0.3%, and 0.6 ± 0.1% higher than expected from the TPS. The results confirm that the planning system underestimates the dose delivered for these very small targets, resulting in a delivered dose that is higher than predicted by the TPS. The HT system delivers acceptable doses (within 5%) down to targets 10 mm in diameter, but smaller fields incurred discrepancies of up to 8% as measured with a fine resolution dosimeter. To ensure accurate dosimetry, care must be exercised when choosing planning and verification techniques for systems not designed for stereotactic dose calculations.