ESTRO 2021 Abstract Book

S1292

ESTRO 2021

patients. Conclusion

Recalculation of dose distribution using up-to-date CT scans and estimation of the accumulated dose is a tool for comprehensive RT monitoring for head and neck patients. It allows to account for anatomy changes during adaptive radiotherapy and to keep the original objectives of the primary treatment plan at an initially accepted level. PO-1569 Density calibrated cone-beam CT as a tool for adaptive radiotherapy T.B. Nyeng 1 , A.I. Holm 1 , D.S. Møller 1 , M.S. Assenholt 1 , R. Hansen 1 , L. Nyvang 1 , T. Ravkilde 1 , M.S. Thomsen 1 , L. Hoffmann 1 1 Aarhus University Hospital, Department of Oncology, Section for Medical Physics, Aarhus, Denmark Purpose or Objective In adaptive radiotherapy (ART), anatomical changes observed on daily cone-beam CT (CBCT) setup images are often used as a trigger for treatment plan adaptation to avoid unacceptable dose levels to the target or organs at risk (OARs). This study aims to investigate the accuracy of dose calculations on CBCTs for phantom and patient data, working towards automatic daily dose surveillance. Materials and Methods The HU to mass density calibration curves for eight Varian TrueBeam CBCT imagers were measured, using stoichiometric calibration. CBCT scans were acquired for a Gammex (Sun Nuclear) phantom simulating the head and neck (HN) or the pelvic region. Different density inserts used as centre insert for the phantom ranged from 0.28 g/cm3 to 1.69 g/cm3. For each insert the mean HU was calculated and mean calibration curves for either HN or pelvis generated across all CBCT imagers. The uncertainty in dose calculated on CBCT images was quantified by comparing the dose at the planning CT (pCT) and the CBCT for maximum dose (D1cc), dose coverage (D90%) and mean dose of a modulated two arc dose plan. The mean HN and mean pelvic calibration curves were used to calculate the daily CBCT dose for ten HN, and ten lung/ten pelvic patients respectively. Patient doses were also calculated using a corrected calibration curve with increased mass density of three points between -850 HU and zero (3-point curve), as the HU values at CBCT in this range was lower than at pCT, hereby empirically correcting for this. Patient doses were compared (pCT minus CBCT) in terms of mean dose and D98% for targets, and D1cc for OARs. All dose calculations were performed in MIM v.7.1.0 (MIM Software Inc.) using the Monte Carlo dose algorithm SureCalc (Extra Precise, 2mm resolution). Results For phantom dose, the differences between the pCT and the CBCT ranged from -1.0% to 0.7% for HN and from -0.9% to 0.5% for pelvis for delineations simulating both target and OARs (Fig 1). For patient scans, the differences in target coverage and mean dose were largest in dense bone. Using the mean calibration curve, the range of dose difference for all structures were within a few percent, with both the mean and median value being below zero. However, using the 3-point corrected calibration curve decreased the range of observed differences and increased the mean and median values with ~0.5% on average. The dose differences between the pCT and the CBCT are shown in Fig 2 for ten pelvic patients. The same trends were observed for HN- and lung patients. For all three patient groups, the median difference in target mean and D98% were within ± 0.5% and the maximum OAR doses were within ±1% using the 3-point corrected curve. For all patients except for one HN and one lung patient, the differences in target mean and target coverage were within ±2%. Conclusion Dose calculation on the calibrated phantom and patient CBCT images results in target coverage and target mean doses, within an accuracy on the order of 1%, which is suitable for clinical use.