SU-F-T-555: Accurate Stereotactic Cone TMRs Converted from PDDs Scanned with Ray Trace.

PURPOSE: To investigate whether the accuracy of TMRs for stereotactic cones converted from PDDs scanned with Ray Trace can be improved, when compared against the TMRs converted from the traditional PDDs.

METHODS: Ray Trace measurement in Sun Nuclear 3D Scanner is for accurate scan of small field PDDs. The system detects the center of field at two depths, for example, at 3 and 20 cm in our study, and then performs scan along the line passing the two centers. With both Ray Trace and the traditional method, PDDs for conical cones of 4, 5, 7.5, 10, 12.5, 15, and 17.5 mm diameter (jaws set to 5×5 cm) were obtained for 6X FFF and 10X FFF energies on a Varian Edge linac, using Edge detectors. The formalism of converting PDD to TMR given in Khan's book (4th Edition, p.161) was applied. Sp values at dmax were obtained by measuring cone Scp and Sc. Continuous direct measurement of TMR by filling/draining water to/from the tank and spot measurement by moving the tank and detector were also performed with the same equipment, using 100 cm SDD.

RESULTS: For 6XFFF energy and all the cones, TMRs converted from Ray Trace were very close to the continuous and spot measurement, while TMRs converted from traditional PDDs had larger deviation. Along the central axis beyond dmax, 1.7% of TMR data points calculated from Ray Trace had more 3% deviation from measurement, with maximal deviation of 5.2%. Whereas, 34% of TMR points calculated from traditional PDDs had more than 3% deviation, with maximum of 5.7%. In this initial study, Ray Trace scans for 10XFFF beam were noisy, further measurement is warranted.

CONCLUSION: The Ray Trace could improve the accuracy of PDDs measurement and the calculated TMRs for stereotactic cones, which was within 3% of the measured TMRs.