Development of an Orthotopic Murine Model of Rectal Cancer in Conjunction With Targeted Short-Course Radiation Therapy

Abstract

Purpose: Orthotopic tumors more closely recapitulate human cancers than do ectopic models; however, precision targeting of such internal tumors for radiation therapy (RT) without inducing systemic toxicity remains a barrier. We developed an innovative murine orthotopic rectal tumor model where the insertion of clinical grade titanium fiducial clips on opposing sides of the rectal tumor allowed for targeted administration of short-course radiation therapy (SCRT). With this novel approach, clinically relevant RT regimens can be administered to orthotopic tumors to explore the biology and efficacy of radiation alone or as a combination therapy in a murine model that closely recapitulates human disease. Methods and Materials: Murine Colon 38-luciferase tumor cells were injected into the rectal wall of syngeneic mice, and fiducial clips were applied to demarcate the tumor. An SCRT regimen consisting of 5 consecutive daily doses of 5 Gy delivered by an image-guided conformal small animal irradiator was administered 9 days after implantation. Tumor burden and survival were monitored along with histological and flow cytometric analyses on irradiated versus untreated tumors at various time points. Results: SCRT administered to orthotopic rectal tumors resulted in a reduction in tumor burden and enhanced overall survival with no apparent signs of systemic toxicity. This treatment paradigm resulted in significant reductions in tumor cellularity and increases in fibrosis and hyaluronic acid production, recapitulating the SCRT-induced effects observed in human cancers.

Shear wave elastography can stratify rectal cancer response to short-course radiation therapy

“…One of our long-term goals is to understand why some tumors in this model respond to radiation therapy while others don't. The results of our ongoing research suggest that the induction of type I interferon after SCRT is one of the factors responsible for the varying tumor responses 16 .…”

“…Colon 38-luciferase-expressing tumor cells (2.5 × 10 4 /5 μL) were mixed in a 1:1 ratio of Evans blue to Matrigel matrix (BD Bioscience, Mississauga, Canada). The tumor cells were injected orthotopically into the rectal wall as described by Uccello et al 16 . Table 1 includes the number of tumors in each group, and the subsets of tumors excised on days 20 and 25 post-injection.…”

“…Before treatment, two 4 mm titanium fiducial markers (Horizon, Teleflex, Morrisville, NC, USA) were placed on opposing sides of the rectal tumor to aid in CT-guided SCRT targeting as described in Uccello et al . 16 . A small animal radiation research platform (SARRP, Xstrahl Inc., Suwanee, GA, USA) with a 5 mm collimator was used to irradiate mice in the treated group with 5 Gy (Gy) from day 9 to 13 post-tumor cell injection.…”

“…This research tests the hypothesis that shear wave elastography (SWE) can assess the response of rectal cancer to short-course radiation therapy (SCRT). To corroborate this, SWE imaging was performed on murine rectal tumors undergoing SCRT; this murine model was designed to replicate the SCRT response rate observed in the clinical setting 16 . Healthy rectums were used as positive controls, untreated tumors were used as negative controls, and radiation-responsive and radiation-non-responsive tumors were evaluated on days 6, 10, 12, 16, 20, 23, and 25 post-injections.…”

Recent advances in the development of transplanted colorectal cancer mouse models

“…Enquist et al 83 used the prolapse technique to suture human CRC cell lines to the rectal mucosa to reveal the origin of liver metastases. In a recent mouse model of rectal cancer, clinical-grade titanium fiducial clips were placed on opposite sides of a rectal tumour to enable targeted delivery of short-term radiation therapy 84 . Interestingly, to avoid laparotomy and reduce the damage caused by surgery, some non-surgical orthotopic CRC mouse models have also been established using mouse colonoscopy or other instruments 85 .…”