Dose calculation deviations induced by fractional image-guided-couch-shifts for Varian Halcyon MV cone beam CT

To assess the criticality of calculation deviations induced by fractional image-guided-couch-shifts for Halcyon MV cone beam CT (CBCT) dose, which is incorporated as part of total treatment dose.

Eclipse-calculated imaging dose was first validated in 'Cheese Phantom' by measurement. Then, the actual imaging dose (D_act) for 18 historical patients of various sites were recalculated based on 513 MV CBCT-guided-couch-shift data, and compared with reference computations based on treatment isocentre (D_ref). Patient- and plan-specific dose from treatment fields was integrated with D_act and D_ref respectively for comparison.

The average absolute relative disagreements between the measured and calculated dose were less than 1.23%. The mean ± 1SD of gamma passing rates of the accumulated imaging dose and total dose were 80.71 ± 6.22% and 99.81 ± 0.32% respectively based on 3 mm/3%/local/10% threshold criteria. The accumulated errors of minimum imaging dose to PTV were no larger than -14.38 cGy, which were reduced to -0.82 cGy after the heterogeneous treatment dose was overlaid. The mean relative discrepancies of PTV minimum dose were -0.61 cGy (-0.71%) and -0.00 (0.00%), before and after incorporating the treatment dose respectively.

The Eclipse-calculated Halcyon MV CBCT dose was validated. Although the isocentre displacement-induced imaging dose calculation errors for Halcyon MV CBCT were partially cancelled out by couch shifts of various directions and distances, especially after the incorporation of heterogeneous treatment dose, it was still advisable to monitor the accumulated deviations and replan when unacceptable target under-dose or organ over-dose were observed.