Exhibit Hall | Forum 4
Purpose: The purpose of this study was to report single institution experience of TBI commissioning of four Linacs using parallel opposed single large field.
Methods: In our institution, we have commissioned four Linacs for TBI, with one Varian Novalis(N) and three TrueBeams(three vaults,T1,T2,and one new Truebeam T3 replacing Novalis N). The extended SAD were configured as 365,400 and 475cm based on vault size, respectively. The commissioning measurements consists of output factor, TMR, profiles, tray and spoiler factor, phantom scatter factor(Sp), brass and Cerrobend transmission factor, and end-to-end test. The measurements across different machines were compared for consistency and reproducibility.
Results: The output of four machines(N,T1,T2,T3)was 0.0498,0.0816,0.0682 and 0.0490cGy/MU, respectively. TMR measured at extended SSD with tray/spoiler was consistently lower ~1.4% than databook which was measured in a water tank for all machines at depth of 10,15 and 20cm. The spoiler distance was determined to be 154,100,118,135cm from patient midline for N,T1,T2 and T3 to provide over 90% skin dose from single beam; the spoiler factor was measured 0.956,0.958,0.954 and 0.955 with difference within 0.4%. The tray factor was 0.970 except for linac-T3 ~0.968. The single profile measured with tray and spoiler agreed well with measurement on all machines within 1% at 5cm depth. Sp measurement for Linac-T3 agreed with databook value within 0.6%; the variation of Sp for all four machines as a function of depth 5 to 20cm was within 2%. The brass and Cerrobend transmission factors for Linac-T3 agreed with other machines measured data within 3.5%. In-house OSLDs and TLDs from ADCL measurements agree with calculated dose via end-to-end test within 5%.
Conclusion: Our experience showed consistent dosimetric endpoint measurement in TBI commissioning across different vaults/geometries and machine types over ten years. Customized dosimetric measurement such as TMR in extended SSD setting could potentially improve the overall dosimetry.