Abstract

Henriette Klitgaard Mortensen¹, Malene Pilunguaq Rahbek Mathisen², Janni Mortensen², Henriette Lindberg³, Lene Sonne Mouritsen⁴, Line Hammer Dohn³, Jens M. Edmund⁵

¹Herlev and Gentofte Hospital, University of Copenhagen, Department of Oncology, Radiotherapy Unit, 2730 Herlev, Denmark; ²Gentofte and Herlev Hospital, University of Copenhagen, Department of Oncology, Radiotherapy Unit, 2730 Herlev, Denmark; ³Gentofte and Herlev Hospital, University of Copenhagen, Department of Oncology, 2730 Herlev, Denmark; ⁴Herlev and Gentofte Hospital, University of Copenhagen, Department of Oncology, 2730 Herlev, Denmark; ⁵Gentofte and Herlev Hospital, University of Copenhagen, Radiotherapy Research Unit, Department of Oncology, 2730 Herlev, Denmark

To explore whether selected dual energy CT (DECT) reconstructions (rec) in addition to standard CT imaging, would add meaningful support to the delineation process in the radiotherapy (RT) planning of prostate cancer patients, specifically to benefit those with contraindication to MR

We performed a standard T2-weighted MR imaging on 10 RT patients with prostate cancer. DECT images were acquired using a dedicated contrast enhanced protocol with a fast 80/140kV switching DECT technique (GE Revolution CT), with a standard 120kVp-like rec (74keV) with CT numbers corresponding to 120kV. 2 additional DECT rec were further investigated: A 40 keV monochromatic rec (40keV) and an iodine material density with water suppression rec (MDIod). All the patients were subject to a combined standard clinical 74keV/MR delineation process performed by varying oncologists on clinical duty.

4 structures; prostate, seminal vesicle (vesicle), rectum and penile bulb delineated as standard combined 74keV/MR imaging, were chosen. These structures were blinded. Three oncologists repeated the delineation of the structures using only the MDIod, 40keV and 74keV images. We compared the structure volumes (vol) as measured in Eclipse (Varian Medical Systems) for prostate and vesicle obtained from 74keV/MR images and the selected DECT rec. After the delineation, each oncologist evaluated the clinical usefulness of the 40keV and MDIod rec in an evaluation scheme graded from 0-3 with 0=unusable, 1=slightly usable, 2=usable and 3= very useful. Student-t tests were carried out to identify significant differences.

Figures 1 and 2 show boxplots of the 74keV/MR and DECT based structure vol from respectively all patients and oncologists, of the prostate and vesicles. The mean and standard deviation (std) of DECT and 74keV/MR based prostate vol were respectively 39,9 +/- 17.7 and 37,6 +/- 15.1 cm3 and non-significantly different (p=0.55). The outliers in figure 1 were ascribed to one single patient which otherwise indicate a DECT vol with a smaller interquartile range (IQR) of 16.5 vs. 19.0 for the 74keV/MR vol.

In figure 2, mean and std of the vesicles were respectively 9,4 +/- 3,9 and 10,8 +/- 3.2 cm3 for the DECT and 74keV/MR and non-significantly different (p=0.60). DECT vol were consistently smaller but with a larger IQR of 5.4 vs. 3.2 for the 74keV/MR.

All 3 oncologists preferred to delineate the structures on the MDIod rec. Grading mean value and std of MDIod were 2.5 (between usable and very useful) +/- 0.65 pooled over all patients and structures. In contrast, the 40KeV grading mean and std was 1.3 (slightly usable) +/- 0.92. There was a significant difference between the usefulness of the two DECT rec (p<1e-16).

DECT based delineation can be a useful alternative to support the delineation process for prostate patients undergoing RT planning who cannot participate or have access to MR imaging.

MDIod reconstruction seems especially useful