ESTRO 2024 - Abstract Book

S4991

Physics - Radiomics, functional and biological imaging and outcome prediction

ESTRO 2024

imaging (DWI). However, for reliable response monitoring on an MR-linac, the repeatability of DWI-derived apparent diffusion coefficient (ADC) needs to be known to discriminate treatment response from measurement variations. Therefore, this study aimed to determine ADC repeatability on a 1.5 T MR-linac during radiotherapy treatment.

Material/Methods:

Twenty-two rectal cancer patients – cT3c-d(MRF-)N0M0 or cT1-3(MRF-)N1M0 – were treated consecutively on a 1.5 T MR-linac (Elekta AB, Stockholm, SE) with five to nine radiotherapy fractions of 5 Gy delivered within two weeks. The planning MRI on a 3 T Ingenia MRI scanner (Philips, Best, NL) included one transverse T2w-, and one DWI sequence with b-values (averages): 0(1), 200(2), 700(4), 1000(5). Each fraction included two transverse T2w-, and two DWI sequences (Figure 1) with b-values (averages): 0(1), 30(2), 150(4), 500(16). The rectal tumor and normal prostate tissue volumes were delineated using b=500 by a radiation-oncologist with >15y of experience. Within these delineations, the median ADC per scan was used for analyses and was calculated with a mono-exponential model of all b-values. The ADC repeatability was evaluated using Bland-Altman analysis and its repeatability coefficient was calculated as 1.96 times the standard deviation (SD) of ADC differences of two sequential DWI sequences. The Pearson correlation analysis was conducted to investigate a relationship between ADCs derived from the diagnostic MRI and MR-linac.

Figure 1.

Results:

Patients had an average age of 63 years (ranging 43 to 81) and were primarily male (77%). Their tumor volume was on average 19.5 (ranging 3.5 to 55.3) cm3. There were 109 treatment sessions that included two sequential DWI sequences. The mean tumor ADC with SD on the Unity was 1.26 (0.11) x 10-3 mm2/s. The mean tumor ADC difference with 95% confidence interval between two sequential DWI sequences of the tumor and prostate volume was -0.01 (- 0.22 to 0.21), and 0.01 (-0.15 to 0.16) x 10-3 mm2/s, respectively (Figure 2). The repeatability coefficient was 21.5% change in ADC for rectal tumors and 15.6% for normal prostate tissue. The mean tumor ADC with SD on the diagnostic MRI was 0.91 (0.14) x 10-3 mm2/s . A significant correlation between median ADCs derived from the diagnostic MRI and the MR-linac was found with R = 0.75, and p < 0.001.