A non-invasive ultrasound imaging method to measure acute radiation-induced bladder wall thickening in rats
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RESEARCH
A non‑invasive ultrasound imaging method to measure acute radiation‑induced bladder wall thickening in rats Antonello E. Spinelli1*† , Andrea Bresolin2,3†, Stefania Zuppone4, Laura Perani1, Giuseppe Fallara4, Nadia Di Muzio5,6, Riccardo Vago4,6, Claudio Fiorino2 and Cesare Cozzarini5
Abstract Background: Methods for the non-invasive quantification of changes in bladder wall thickness as potential predictors of radiation cystitis in pre-clinical research would be desirable. The use of ultrasound for this aim seems promising, but is still relatively unexplored. A method using ultrasound for bladder wall thickness quantification in rats was developed and applied to measure early radiation-induced bladder wall thickness changes. Methods: Two groups (n = 9 each) of female Fischer rats were treated with a single radiation dose of 25–30 and 35–40 Gy respectively, using an image-guided micro-irradiator; six untreated rats were monitored as a control group. Empty, half-filled and fully-filled bladder volumes were determined for four non-irradiated rats by measuring axes from ultrasound 3D-images and applying the ellipsoid formula. Mean bladder wall thickness was estimated for both ventral and dorsal bladder sides through the measurement of the bladder wall area along a segment of 4 mm in the central sagittal scan, in order to minimize operator-dependence on the measurement position. Ultrasound acquisitions of all fully-filled rat bladders were also acquired immediately before, and 4 and 28 days after irradiation. Mean bladder wall thickness normalized to the baseline value and corrected for filling were then used to evaluate acute bladder wall thickening and to quantify the dose–effect. Results: The relationship between mean bladder wall thickness and volume in unirradiated rats showed that for a bladder volume > 1.5 mL the bladder wall thickness is almost constant and equal to 0.30 mm with variations within ± 15%. The average ratios between post and pre irradiation showed a dose–effect relationship. Bladder wall thickening was observed for the 25–30 Gy and 35–40 Gy groups in 2/9 (22%) and 5/9 (56%) cases at day 4 and in 4/9 (44%) and 8/9 (89%) cases at day 28, respectively. The two groups showed significantly different bladder wall thickness both relative to the control group (p 1.5 mL: the mean value in the plateau region was 0.30 mm with maximum variations of about 15%. Interestingly, the measurements on the ventral side were nearly always found to be higher than those on the dorsal side, with a decreasing difference as the bladder volume increased, likely due to the difference in gravity effects between the two sides.
Acute radiation‑induced bladder toxicity
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Table 1 and Fig. 4 summarize the resulting BWTratio for all animals according to group and the timing of the observations. The variations in the control group ranged between − 22% and + 11% relative to baseline, close to the expected inter-rat variations of BWT with full bladder previously investigated. BWT above the control gr
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