2968 - Evaluation of Direct Electron Density Reconstruction Using Spectral Information on Iodine Contrast-Enhanced CT From Photon-Counting CT for Radiotherapy Treatment Planning

Purpose/Objective(s):

Radiotherapy planning often requires a plain CT for dose calculation and a separate contrast-enhanced CT (CECT) for target and OAR delineation. This dual-scan approach increases radiation exposure and risks inter-scan motion artifacts. We evaluated whether direct electron density reconstruction using spectral information (?_direct) from photon-counting CT (PCCT) allows accurate dose calculation directly on iodine-enhanced images, potentially eliminating the need for plain CT.

Materials/Methods:

Iodine plugs with various concentrations (0–10 mg/mL) were scanned at 120 kVp / 204 mAs on a PCCT. ?_direct images were reconstructed. Energy-integrated images were also converted to electron density using conventional stoichiometric calibration (?_{stoichiometric}) for comparison. Accuracy of estimation of electron density was compared to theoretical values.

Three abdominal VMAT cases were included. Dose was recalculated on CECT using both ?_{stoichiometric} and ?_direct, then compared with reference plans calculated on plain CT. Gamma analysis (1%/1 mm) in high-dose regions, PTV D_95% and PTV D_mean deviations were evaluated.

Results:

?_{stoichiometric} consistently overestimated the electron density, with errors increasing with iodine concentration (ranging from -0.28% to 9.65%). ?_direct reconstruction significantly reduced error, achieving <1% deviation for iodine concentrations = 3 mg/mL (Table 1). ?_direct values were closer to unity (equivalent to water), aligning better with treatment conditions absent contrast.

In clinical plans, ?_direct resulted in higher gamma passing rates (mean 80.53%) than ?_{stoichiometric} on CECT (mean 77.73%). Compared with plain CT reference plans, ?_direct also demonstrated smaller PTV dose deviations than ?_{stoichiometric} (D_95%: 0.3% vs 1.1%; D_mean: 0.6% vs 1.3%).

Conclusion:

Direct spectral electron density reconstruction on photon-counting CT substantially improves dose calculation accuracy on iodine contrast-enhanced images. This approach may allow safe omission of plain CT, reducing radiation dose and registration uncertainty in radiotherapy planning.

Table 1. Electron Density Errors by Iodine Concentration

Iodine Concentration (mg/mL)	Error (?stoichiometric)	Error (?direct)
0	-0.28%	0.08%
0.2	0.06%	0.06%
0.4	0.22%	0.13%
0.6	0.42%	0.21%
0.8	0.59%	0.20%
1	0.86%	0.49%
2	1.84%	0.48%
3	2.62%	0.84%
4	3.72%	1.33%
5	4.66%	1.51%
8	7.51%	2.16%
10	9.65%	2.77%