Author: Alyssa Gadsby, William J. Godwin, Daniel G. McDonald, Jean L. Peng, Alek K. Rapchak, Sean A Roles, Austin M. Skinner, Stephanie Tan 👨🔬
Affiliation: Medical University of South Carolina 🌍
Purpose: The Sun Nuclear MapCheck3 device is used for patient-specific QA. While it is typically employed in perpendicular field-by-field delivery using the Isocentric Mounting Fixture, fields not fully encompassed by the active area (e.g., off-axis or half-beam-blocked fields) require positioning the device on the treatment couch. This study explores the response of MapCheck3 for off-axis shifts in these clinical situations.
Methods: Relative diode response was evaluated by measuring off-axis square fields for five photon energies. Evaluated fields were 10x10cm2 and created to extend off axis by 10cm in the superior, inferior, right, or left directions. Fields were initially delivered in their native off-axis positions, and central diode dose was recorded. Fields were then delivered with the center of the beam aperture at the central axis (CAX) of the MapCheck3. Central diode readings were compared. Dose distributions of each measurement were compared to the treatment planning system using a 3%/2mm gamma criteria.
Results: Square field central diode readings agreed within 1.25% for native off-axis and centered CAX measurements for all locations and beam energies. Gamma pass rates for 6X, 6FFF, 10X, and 10FFF exceeded 96% at 3%/2mm with a 10% threshold for superior, right, and left native offset deliveries. 16X showed a reduced pass rate for superior (85%) and inferior (81%) offset deliveries. All energies exhibited relatively lower pass rates in the native inferior position, between 89 and 94%.
Conclusion: Off-axis fields can be accurately measured at the CAX of MapCheck3 without significant impact on dose or gamma pass rate, increasing the number and type of fields that can be measured successfully with this device. More research is needed to examine systematic lower pass rates in the inferior position, which may be related to the presence of the device electronics.