Abstract
PurposeTo 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).MethodsWinston‐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.ResultsArcCHECK 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.ConclusionMechanical 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.
Highlights
Conventional linear accelerators, unlike dedicated machines such as the Cyberknife, were originally not considered suitable for stereotactic radiosurgery
We demonstrate in the treatment planning system (TPS) how misalignment of the main rotational axes and a mismatch between treatment and imaging isocenters can affect planning target volume (PTV) coverage in stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments of cerebral metastases
Performing the SRT quality assurance (QA) check results in a mean absolute pass rate of (85.0 ± 0.85)% at Gamma 2%/2 mm averaged over three measurements
Summary
Conventional linear accelerators, unlike dedicated machines such as the Cyberknife, were originally not considered suitable for stereotactic radiosurgery. Even with all these advances, there are several effects that may adversely affect treatment delivery, such as mechanical uncertainties of the treatment machine. These include: gravity‐induced gantry and leaf bank sag, misalignment of rotational axes, as well as positioning errors of individual multi‐leaf collimator (MLC) leafs, as they affect the mechanical stability and location of the radiation central beam axis. Though flex maps are commonly used to account for the movement of the imager components, it may lead to an offset between the mechanical and the imaging isocenters.[5,6] Rigid quality control is of utmost importance, and quality standards for stereotactic machines have been defined that exceed those required for conventional therapy according to TG‐142.7
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