2403 - Initial Validation of Cone Beam Computed Tomography (CBCT) Only Treatment Planning Using a C-Arm Linac CBCT Solution

A. Rezaei¹, A. Y. Jia², B. Guevara¹, G. C. Pereira¹, R. Kashani¹, D. E. Spratt³, A. T. Price⁴, and L. E. Henke⁴; ¹University Hospitals, Seidman Cancer Center, Case Western Reserve University, Cleveland, OH, ²Weill Cornell Medical College/New York Presbyterian Hospital, New York, NY, ³Case Western, Cleveland, OH, ⁴Department of Radiation Oncology, University Hospitals Seidman Cancer Center, Case Western Reserve University, Cleveland, OH

Purpose/Objective(s): We sought to test the utility of an advanced cone beam computed tomography (CBCT) solution on a C-arm linac to precisely formulate prostate treatment plans directly from CBCT scans, thereby obviating the necessity for initial simulation or any subsequent resimulation. Materials/

Methods: Five patients undergoing prostate radiotherapy were imaged on a HyperSight CBCT (hCBCT) on a c-arm linear accelerator as part of a prospective imaging clinical trial (NCT05975619). Each patient had a treatment planning CT (TPCT) and a study hCBCT protocol scan at 140 kVp, followed by the reconstruction of each CBCT using three methods: FDK (Feldkamp-Davis-Kress), iCBCT (iterative CBCT), and iCBCT-MAR (Metal Artifact Reduction) for comparative analyses. Subsequent steps involved registering the CBCT images to TPCTs for alignment, and contouring targets and critical, in-field organs-at-risk (OARS; rectum, bladder) upon the iCBCT-MAR. Contours were edited exclusively on the iCBCT-MAR and then copied to registered FKS, iCBCT, and TPCT images without adjustment so there would be no difference in contour structures across all registered image sets. This was followed by plan reoptimization such that each CBCT reconstruction type had its own plan. Each of the three CBCT-derived plans per patient were then recalculated on respective TPCTs without modifications, and without reoptimization, to evaluate dose differences. Mean and standard deviation for the percent differences for the CTV, PTV, maximum, and mean doses to the OARs were calculated.

Results: Three CBCT-based plans that met all clinical goals were successfully created for all 5 patients using each reconstruction type as a primary dataset for each patient. Dose comparisons (in % difference from TPCT recalculation) are shown in Table 1 for each of the reconstructions. Target coverage and bladder metrics had lower differences between CBCT and TPCT compared to the rectum structure. 16 of the 18 (88.9%) metrics were within 5% of dose difference. A single OAR objective exceeded constraint on an iCBCT-MAR plan after recalculation on the TPCT (>98% of all OAR objectives met in all scenarios).

Conclusion: Our preliminary findings suggest that dose calculation relying solely on CBCT images as the primary dataset results in acceptable and dosimetrically minimal dose differences for the majority of evaluable structures, thus potentially enabling CBCT only treatment planning across all reconstruction types.

	CTV V100	PTV V100	Rectum Max	Rectum Mean	Bladder Max	Bladder Mean
FDK % Avg	1.1%	0.7%	-1.1%	4.6%	0.0%	1.2%
iCBCT % Avg	0.2%	0.9%	-6.8%	4.7%	-0.3%	0.4%
iCBCT-MAR % Avg	0.2%	0.3%	-1.3%	5.1%	-0.3%	0.8%
FDK % Std Dev	1.0%	1.6%	3.0%	11.7%	1.3%	4.9%
iCBCT % Std Dev	0.9%	0.9%	11.1%	11.2%	0.9%	5.5%
iCBCT-MAR % Std Dev	0.7%	0.4%	3.2%	11.4%	0.9%	5.6%