Histotripsy of the Prostate in a Canine Model: Characterization of Post-Therapy Inflammation and Fibrosis

Abstract

Introduction: Histotripsy is a nonthermal, noninvasive, pulsed ultrasound technology that homogenizes tissue within the targeted volume. From previous experiments, it appeared that the resultant fibrotic response from histotripsy was limited compared with the typical tissue response seen after thermoablation. The objective of this study was to characterize the inflammatory response and quantify patterns of collagen deposition 6 weeks after in vivo canine prostate histotripsy.

Methods: Histotripsy was applied to the left half of eight canine prostates to produce an intraparenchymal zone of tissue homogenization. Six weeks after treatment, prostates were harvested, sectioned, and stained with hematoxylin and eosin for histologic evaluation, CD3, CD20, and Mac387 immunohistochemistry to characterize the inflammatory components, and picrosirius red staining to identify collagen.

Results: Seven of eight treated prostates exhibited only minimal residual inflammation. Visual microscopic analysis of picrosirius red slides revealed a band of dense collagen (0.5 mm wide) immediately adjacent to the cavity produced by histotripsy. This was surrounded by a second band (1 mm wide) of less dense collagen interspersed among glandular architecture. A lobar distribution of epithelial atrophy and basal cell hyperplasia reminiscent of periurethral glands and ducts was apparent surrounding the margin of the treatment cavities. Tissue loss (-31%) was apparent on the treated side of all prostates while four demonstrated a net decrease in collagen content.

Conclusions: In vivo histotripsy of canine prostate produced a decrease in prostate volume coupled with a limited inflammatory and fibrotic response. A narrow (1.5 mm) band of fibrosis around the empty, reepithelialized treatment cavity was observed 6 weeks after treatment. In four cases, an overall reduction in collagen content was measured. Further studies are planned to correlate these histologic findings with alteration in mechanical tissue properties and to explore histotripsy strategies for treatment of benign prostatic hyperplasia that optimize tissue volume removal with minimization of fibrosis.

FIG. 1.

Apparatus for canine prostate histotripsy treatment. An anesthetized canine is positioned supine with transrectal ultrasonography probe inserted in the rectum. The histotripsy therapy transducer suspended over the shaved abdomen is positioned within the degassed water bath for acoustic coupling when delivering energy to the prostate.

FIG. 2.

A 2400 dots per inch scan of a prostate section from subject 2 stained with picrosirius red (left) for collagen identification. The histotripsy treatment cavity is evident in the left hemiprostate. The red dots define the region of interest used to calculate the tissue collagen density and tissue area. The empty treatment lesion was excluded from the region of interest. A binary, black and white rendering (right) of the treated hemiprostate is shown with collagen containing pixels displayed as white.

FIG. 3.

Representative portion of a 20× magnified image displaying the histotripsy treatment cavity as well as extent of the surrounding inner and outer fibrotic bands, designated on the image.

FIG. 4.

Representative histopathology of histotripsy-treated canine prostate. (A) A composite image of a histotripsy-treated canine prostate (subject 7). Anterior (Ant.) and posterior (Post.) are indicated, along with the treated and untreated sides. The urethra (U) and cavitation cavity (C) are noted. Minimal interstitial inflammation was noted in the treated and untreated sides in this subject. Indicated areas shown in dashed black and green boxes are shown as insets in B and C. (B) The reepithelialized cavitation cavity is surrounded by fibromuscular stroma similar in appearance to that surrounding the urethra. (C) Foci of atrophic glands and basal cell hyperplasia surround the cavitation cavity, reminiscent of periurethral ducts/glands. In addition, areas of unremarkable prostate glands were also adjacent to the cavitation cavity. Higher power view of atrophy and basal cell hyperplasia with surrounding minimal interstitial chronic inflammation is shown in the inset. Original magnifications 1.25× (A), 4× (B), 10× (C), and 20× (inset of C).

FIG. 5.

This 2400 dots per inch scan of a picrosirius red stained prostate section demonstrates a partially collapsed histotripsy treatment cavity and tissue volume loss on the left side of the prostate compared with the untreated right side of the prostate. The measured cross-sectional tissue area of the left hemiprostate was 2.4 cm² less than the right.

FIG. 6.

2400 dots per inch image of a picrosirius red stained prostate section from subject 3. The histotripsy treatment cavity is seen to communicate with the urethral lumen with collagen deposition apparent surrounding the treatment zone.