2475 - Feasibility of Dose Calculation for Pelvic Treatment Plans Using Electron Density Map from Novel Spectral CT Simulator

Q. Zhu; Department of Radiation Oncology, Peking Union Medical College Hospital, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing, China

Purpose/Objective(s): To assess the feasibility of utilizing electron density (ED) maps from a novel spectral CT simulator for dose calculation in the treatment plan. Materials/

Methods: 10 patients from pelvic treatment sites were selected retrospectively, and all of them underwent spectral CT simulation. Treatment plans based on conventional CT images were transplanted to ED maps with the same structure set of planning target volume(PTV) and organ at risk (OARs), and the dose was then recalculated. The differences in dosimetric metrics of the PTV and OARs of the two types of plans were analyzed and compared. Besides, gamma analysis between these plans was performed by using MEPHYSTO Navigator software.
Results: In terms of PTV dosimetric metrics, the homogeneity index (HI), conformal index (CI), gradient index (GI), D_2%, D_98%, and D_mean showed no significant difference between conventional plans and ED plans (0.921 vs. 0.929, 3.73 vs. 3.71, 0.06 vs. 0.06, 47.46 vs. 47.50 Gy, 44.53 vs. 44.53Gy, 46.33 vs. 46.34Gy, P>0.05). For OARs, a significant difference was observed in rectal D_50% between these two treatment plans (40.45±3.65 vs. 40.55±3.72Gy, P=0.014), although the variance remained minor. For the rest, the dosimetric metrics exhibited no significant difference between conventional plans and ED plans. The mean gamma passing rate (GPR) of gamma analysis was 96.89%, 99.57%, and 99.71% under 1%/1mm, 2%/2mm, and 3%/2mm criteria, respectively.
Conclusion: Compared with conventional treatment plans based on CT images, the plans based on ED maps had similar dosimetric quality. However, the latter can be directly used in dose calculation without establishing and selecting a specific HU-ED calibration curve, providing an advantage in clinical applications.

PTV	Conventional plan	ED plan	P value
CI	0.921±0.01	0.929±0.01	0.168
GI	3.73±0.26	3.71±0.26	0.071
HI	0.06±0.00	0.06±0.00	0.343
D2%(Gy)	47.46±0.19	47.50±0.19	0.059
D98%(Gy)	44.53±0.06	44.53±0.05	0.703
Dmean(Gy)	46.33±0.07	46.34±0.08	0.212

OAR	Metric	Conventional plan	ED plan	P value
Small intestine	D2cc(Gy)	46.50±0.38	46.47±0.41	0.222
	D50%(Gy)	20.07±6.20	20.03±6.17	0.106
Rectum	Dmean(Gy)	35.95±2.56	36.00±2.59	0.085
	D50%(Gy)	40.45±3.65	40.55±3.72	0.014
Bladder	Dmean(Gy)	35.81±3.10	35.75±3.11	0.607
	D50%(Gy)	36.73±4.15	36.75±4.20	0.306
Spinal cord	D0.1cc(Gy)	20.48±3.18	20.48±3.25	0.989