297 - Bridging the Gap between the Treatment Plan and In Vivo Dosimetry through Clinical Scintillation Imaging

Purpose/Objective(s): Current methods for on-patient dosimetry cannot precisely map measured doses to the patient anatomy or treatment planning systems (TPS), limiting the clinical interpretation. Cherenkov imaging provides visualization of spatial dose deposition across the patient surface, and in vivo dosimetry (IVD) can be achieved through concurrent imaging of a scintillator placed in the treatment field. The utility of Cherenkov imaging can be further extended using the known system-inherent properties to relate 2D images to the 3D world scene, such that the treatment plan can be projected to the camera point-of-view for comparison of planned and measured doses. This work demonstrates the first optical correlation of IVD measurements with the treatment plan and implements this technique in an ongoing clinical trial to quantify contralateral breast dose (CBD).

Materials/Methods: The intrinsic and extrinsic characteristics of a Cherenkov imaging system were harnessed to correlate the 2D images acquired during treatment with 3D world coordinates. Treatment plan dose predictions were exported from a treatment planning system, sampled and averaged from 0-5 mm, and transformed into camera point-of-view 2D image coordinates. Thermoluminescent dosimeters (TLDs) and scintillation imaging were used concurrently to measure surface dose and validated against the 2D projected treatment plan using phantom measurements. This technique was then tested on images collected in an ongoing clinical trial to identify and quantify CBD, where a scintillator and TLD were placed on the contralateral breast for dose measurement.

Results: For phantom validation measurements, imaging agreed within 4 ± 2 cGy (2±1%) of TLDs and 2 ± 1 cGy (1.0±0.5%) of the projected TPS. For patients treated with tangents, average CBD was measured per fraction to be 157 ± 43 cGy and 123 ± 20 cGy, with TLDs and scintillation imaging respectively, agreeing with each other within 2±2% of total planned dose. These IVD measurements were higher compared to the 65 ± 62 cGy predicted by the treatment plan at the same point. Patients receiving accelerated partial breast irradiation saw lower average CBD of 15 ± 7cGy and 8.4 ± 5.7 cGy from TLDs and the plan, respectively, agreeing with each other within 0.2±0.2% total planned dose; however, visualization of the planned and delivered dose highlighted low dose spill outside of the target volume.

Conclusion: The ability to directly compare measured with planned doses has the potential to change the field of IVD. Demonstrated here through the implementation of this technique to quantify CBD, measured CBD was attributed to the treatment plan, though higher doses are measured than originally predicted. Here we show that spatially-calibrated Cherenkov imaging can be combined with scintillator placement for error detection, treatment optimization, dosimetry, and direct comparison to planned dose estimates.