Room: ePoster Forums
Purpose: To describe an accurate TMR measurement technique in a limited physical space for the Zap-X (Zap Surgical Systems Inc., San Carlos, CA) self-shielded SRS machine.
Methods: A PTW MP3-XS water tank top was set to 5mm below the collimator to allow for 218mm max TMR depth. PTW 60016 photon diode was set at 450mm SAD using a front pointer at gantry G=0Â° and positioned at isocenter using scanning software centering function. Water quantities ranging from 100cc to 1500cc were measured and poured into the tank via a tube running through a conduit on the machineâ€™s outer shell. Measurements began after adding water to be flush with the diode surface. Variable amounts of water were added (smaller amounts for accuracy, larger for time-savings) depending on the measurement depth of the TMR curve. 100MU was delivered for each data point and repeated for all collimator sizes (4mm-25mm). Water depth change was evaluated by initial and final markers drawn on tape placed on the tank wall, and verified by water depth/volume ratio tested separately. Couch sag was estimated by comparing known tank dimensions with top-of-tank-line shift on an in-room monitor at the beginning and end of the measurements. TMR data was corrected for couch sag. Measurements near dmax (7mm) were repeated at the end. TMR data was compared to calculated TMR values using measured PDD and derived PSF. Max depth doses on TMR curves were analyzed extensively for accuracy.
Results: Couch sag was 8.3mm by adding 18,900cc (or 218mm depth) of water during measurements, and can cause up to 3.7% TMR correction. Measured TMRs were within 1.4% (-0.25%Â±0.53%) agreement at all depths and collimator sizes with calculated values.
Conclusion: We successfully measured TMR at 450mm SAD for a 2.7MV beam used in the new ZAP-X self-shielded SRS system.
Not Applicable / None Entered.