The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets—Quantification and possible corrections

Abstract

Purpose To assess the impact of isocenter shifts due to linac gantry and table rotation during cranial stereotactic radiosurgery on D98, target volume coverage (TVC), conformity (CI), and gradient index (GI). Methods Winston‐Lutz (WL) checks were performed on two Elekta Synergy linacs. A stereotactic quality assurance (QA) plan was applied to the ArcCHECK phantom to assess the impact of isocenter shift corrections on Gamma pass rates. These corrections included gantry sag, distance of collimator and couch axes to the gantry axis, and distance between cone‐beam computed tomography (CBCT) isocenter and treatment beam (MV) isocenter. We applied the shifts via script to the treatment plan in Pinnacle 16.2. In a planning study, isocenter and mechanical rotation axis shifts of 0.25 to 2 mm were applied to stereotactic plans of spherical planning target volumes (PTVs) of various volumes. The shifts determined via WL measurements were applied to 16 patient plans with PTV sizes between 0.22 and 10.4 cm3. Results ArcCHECK measurements of a stereotactic treatment showed significant increases in Gamma pass rate for all three measurements (up to 3.8 percentage points) after correction of measured isocenter deviations. For spherical targets of 1 cm3, CI was most severely affected by increasing the distance of the CBCT isocenter (1.22 to 1.62). Gradient index increased with an isocenter‐collimator axis distance of 1.5 mm (3.84 vs 4.62). D98 (normalized to reference) dropped to 0.85 (CBCT), 0.92 (table axis), 0.95 (collimator axis), and 0.98 (gantry sag), with similar but smaller changes for larger targets. Applying measured shifts to patient plans lead to relevant drops in D98 and TVC (7%) for targets below 2 cm3 treated on linac 1. Conclusion Mechanical deviations during gantry, collimator, and table rotation may adversely affect the treatment of small stereotactic lesions. Adjustments of beam isocenters in the treatment planning system (TPS) can be used to both quantify their impact and for prospective correction of treatment plans.

Implementing corrections of isocentric shifts for the stereotactic irradiation of cerebral targets: Clinical validation

“…Combining the corrections is straight forward. A script generated using Pinnacle (Philips Radiation Oncology Systems, Fitchburg, WI, version 16.2) presented by Wack et al 4 creates the true isocenter per beam direction. This is the nominal isocenter shifted due to gantry sag and wobble, the distance between CBCT and MV isocenter, the distance of the collimator axis and couch axis to the gantry axis.…”

“…All planned dose distributions were calculated and exported using (1 mm) 3 voxel size. While Low and Dempsey 15 suggest using a spatial criterion as three times the calculation grid size or more for accurate calculation of gamma values in steep dose gradient regions, here, for the calculation grid size of 1 mm a stricter criterion of 2%/2 mm was chosen, as in Wack et al 4 (see this reference for a further discussion of the choice). The common clinical choice of a gamma pass rate was also evaluated for the 1%/1 mm criterion.…”

“…Despite all efforts to improve patient positioning and delivery accuracy, mechanical imperfections of these machines remain. [3][4][5][6][7][8] With the increasing consideration of irradiation of multiple metastases as a treatment option, 9 and the increasing use of singleisocenter multi-target irradiations, 10 geometric problems come even more into the focus.…”

“…12,13 Different geometric deviations affect a dose distribution by different degrees. Wack et al 4 analyzed the impact of different geometric imperfections on small circular cerebral lesions in a planning study, finding that the dose to the target volume can change to a clinically relevant degree due to realistic offsets between MV and kV isocenters, missing alignment of the rotation axes and due to gantry sag. It was further shown that a measured dose distribution was more similar to a plan in which the anticipated geometric shifts were included than to the original plan.…”

Technical Note: Effects of rotating gantry on magnetic field and eddy currents in 0.35 T MRI‐guided radiotherapy (MR‐IGRT) system

“…RT nominally requires a geometric accuracy of ≤2 mm or 0.5-1 mm for stereotactic radiosurgery (SRS). [21][22][23] The peak-to-peak deviation from the average inhomogeneity reported for the 1.5 T Elekta Unity was measured for a single slice in a 40 cm diameter cylindrical phantom at a repetition rate of 0.7 Hz using field maps. 6 The measured field homogeneity was not affected by gantry rotation speeds of 1 or 2 rpm.…”