2340 - Improving the Clinical Workflow of Lung Cancer Patients Treated at MR-Linac by Synthetic CT Validation

M. Liu, B. Tang, X. Liao Sr, J. Li, F. Wu, X. Xin, F. Yang, and L. C. Orlandini; Sichuan Cancer Hospital and Research Institute, University of Electronic Science and Technology of China, Chengdu, China

Purpose/Objective(s): In magnetic resonance (MR) based adaptive workflows for an MR-linac, the contours of the structures are adjusted with the patient anatomy visualised on the daily MR images; the adapted plan is optimised and recalculated on the MRI-based synthetic computed tomography (sCT). At Unity MR-Linac the sCT are generated by bulk density assignment; nevertheless, for sites where there is a high electronic density gradient between the target and surrounding tissues, such as in lung cancer, the assignment of an average electronic density (ED) may not be able to reproduce an exact dose calculation. This study aims to evaluate the accuracy of the sCT calculation for lung targets in different ED gradient areas. Materials/

Methods: The CT simulation images of thirty-six patients with lung cancer treated in our hospital with stereotactic boby radiotherapy were used in this retrospective study. Patients were divided into two groups G1 and G2 according to the location of the target at the edge (18 patients), or in the middle (18 patients) of the lung, respectively. The contours of the patient’s target and organs at risk (OARs) were imported into a treatment planning system and a reference treatment plan (TP_ref) was optimized to deliver 50 Gy (10Gy x 5 fractions) at Unity MR-Linac. TP_ref contains all the information including the contours average EDs and layering to generate the MR-based sCT. To assess the accuracy of the dosimetry of the MRI-based sCT plans (TP_sy), each TP_ref plan was recalculated on the ideal sCT obtained from the reference CT by forcing the drawn contours to the average EDs as defined by TP_ref. Targets and OARs dose-volume histograms of TP_ref and TP_sy were compared within each group. Statistical analysis was performed by using statistical software to test the consistency of the dosimetric results obtained within each group; paired t-test or Wilkinson signed-rank tests were used after the normality test; a value of p < 0.05 was considered a statistically significant difference.
Results: The mean value and standard deviation of the target and OARs dosimetry obtained with TP_ref and TP_sy for G1 and G2 are reported in the Table below. For patients of G2 the mean percentage of the target volume receiving the prescribed dose (V_Dpre) with TP_ref is significantly different (p < 0.05) from TP_sy, with a mean absolute difference of 5.98 %. No significant differences were highlighted in both groups for the OARs.
Conclusion: The sCT generated by density assignment should be carefully used in the adaptive workflow for lung cancer treatments. The treatment plan calculated on the sCT when the target is located in the middle of the lung may lead to a dosimetric inaccuracy. Abstract 2340 – Table 1

	G1			G2
	TPref	TPsy	p	TPref	TPsy	p
PTV VDpres (%)	81.75 ± 3.28	82.06 ± 3.16	0.33	82.23 ± 3.15	88.03 ± 7.93	0.00
Spinal Cord Dmax (Gy)	14.26 ± 5.43	14.23 ± 5.39	0.16	9.80 ± 3.49	9.76 ± 3.48	0.37
Heart Dmax (Gy)	22.58 ± 18.66	22.63 ± 18.74	0.32	13.49 ± 9.42	13.36 ± 9.44	0.13
Esophagus D5cc (Gy)	8.23 ± 5.99.	8.22 ± 5.97	0.91	4.06 ± 2.48	4.07 ± 2.52	0.90