Purpose: Optically stimulated luminescence dosimeters (OSLDs) are commonly used for in-vivo dosimetry for in- and out-of-field applications. When kV-CBCT is part of IGRT, it is recommended to place OSLDs after imaging to avoid the dosimeter over-response to diagnostic x-rays. However, entering the treatment room between imaging and treatment is disruptive to IGRT clinical workflows. We quantify herein the OSLD dose reading overestimation due to kV-CBCT so that imaging and therapeutic doses can be separated.
Methods: A thorax CBCT protocol (125kV, 270mAs) was utilized as the diagnostic beam quality. The OSLD kV dose linearity with number of scans was confirmed (predictive model). Standard and patient-specific setups were considered. For the standard setup (100cm SSD, 10x10cm2 field size, d=1.5cm), in-field OSLDs were placed at the central axis, and out-of-field OSLDs were placed 10cm off-axis; outside the primary photon field but within the imaging field. Three irradiations were delivered with individual OSLD sets per irradiation: 200MU 6MV only, kV-CBCT only (12 scans), 200MU 6MV combined with kV-CBCT. For the patient-specific setup, a patient’s head-and-neck VMAT plan was recalculated on a solid-water phantom with the OSLD placed under 1cm of bolus. The same three irradiation scenarios listed above were delivered for the patient plan, placing in-field OSLDs only. Only screened OSLDs were used and read according to manufacturer recommendations utilizing a 6MV calibration curve.
Results: Per scan, and relative to MV only measurement, doses were overestimated on average by 1.9±0.02% (in-field) and 60±0.3% (out-of-field) when using the standard setup, and 4.6±0.05% for the patient-specific setup. The added uncertainty was consistent with the prediction model.
Conclusion: We quantified the OSLD readout uncertainty associated with the effect of kV-CBCT-based IGRT on MV readings. With overresponses as high as 60% for out-of-field applications, it is recommended to place OSLDs after IGRT or to create appropriate correction factors.