Purpose: Halcyon O-ring Linac is equipped with large field of view (FOV) kV-CBCT imager for image-guided RT. 3D reconstruction potentially enables patient’s CT simulation on Halcyon. We report the feasibility of using Halcyon’s kV-iCBCT imaging for simulation in extenuation circumstances, e.g. CT simulator downtime.
Methods: Halcyon’s kV-iCBCT reconstruction algorithm was used to image RANDO body phantom for different palliative treatment sites and beam arrangements (whole brain RT, single-field c-spine, AP/PA thorax, 4-fields pelvis and 2-fields thigh). Pelvis iCBCT protocol was used regardless of treatment site due the available maximal FOV of 25cm in superior/inferior (S/I) and 50cm in right/left lateral (R/L) directions. Three-point BB’s were placed on each phantom to replicate standard clinic workflow for isocenter-localization in treatment planning system (TPS). Palliative 3D-CRT treatment plans were created without heterogeneity correction in Eclipse TPS via AAA algorithm for a TrueBeam Linac for 6/18MV beams. Overall phantom set up and scanning time was <5min. Total MU/fraction and qualifiers of minimum/maximum point doses and V100 were used for plan evaluation.
Results: In palliative care setting, all five RT plans for various treatment sites produced expected outcomes for total MU with 3D-CRT planning regardless of homogenous calculation. AP/PA thorax/mediastinum plan demonstrated slight increase in expected MU due to tissue-to-air heterogeneity within lung volumes that was not being accounted for in dose calculation, although it was clinically acceptable in the palliative setting.
Conclusion: Halcyon’s kV-iCBCT was tested for simulation of palliative patients. FOV enables full viewing to permit the use of treatment fields. Dose calculation without heterogeneity correction is reasonable in most or all palliative cases, with caution needed for beams passing through the lung. Generating an electron density curve for Halcyon kV-iCBCT is ongoing. Stitching multiple scans together (if needed) for >25 cm S/I anatomy is possible, and work in this area is underway.