Volumetric and dosimetric impact of MRI in delineation of gross tumor volume of non-spinal vertebral metastases treated with stereotactic ablative radiation therapy.

Purpose: To investigate the Gross Tumor Volumes (GTV) and its dosimetric impact of magnetic resonance imaging (MRI) assisted contouring for non-spinal metastasis treated with stereotactic ablative body radiotherapy (SABR).

Material And Methods: Five observer contours on CT (GTV) and CT+MR (GTV) were evaluated against expert team contours (GTV) for 14 selected cases. Dice Similarity Index (DSC) and Geographical Miss Index (GMI) quantify observer variation. We also analyze the maximum dose (D) and dose received by 0.35cc (D) of the spinal cord (SC) for GTV and GTV, where optimization parameters and priorities were unchanged. Percent rank function is also evaluated for SC doses.

Results: The mean DSC and GMI scores for the CT-only dataset are 0.6974 and 0.2851 and for CT+MR dataset is 0.7764 and 0.1907 respectively. Statistically, significant results were found for mean GTV volumes between GTV versus GTV and GTV versus GTV (P<0.001). Dosimetric analysis of D and D exceeded 84.2% and 88.5% of times its respective threshold doses for CT-only dataset, whereas for the CT+MR dataset, it exceeded only by 18% and 15.7% times. 'Percent rank' function analysis for SC doses also indicates the same.

Conclusion: This study supports MRI fusion for GTV and OAR delineation for non-spinal metastasis. Our study showed that the dosimetric analysis is vital for observer variation studies and the addition of the MR data set is significant to improve the confidence of Stereotactic treatments.