There are various genetically engineered mouse models to study pancreatic cancer. The most commonly used model is the Kras, p53 mouse model that harbors an activating mutation in the Kras oncogene, and an inactivating mutation in the p53 tumor suppressor gene. Both mutations are conditionally activated by the Pdx-Cre promoter.
This Kras, p53, Pdx-Cre mouse model, in short KPC model, develops endogenous pancreatic tumors between the age of two to six month. In preparation of your ultrasound procedure, the following equipment should be prepared and ready to use. For high resolution ultrasound, a stationary ultrasound device is used.
The ultrasound procedure is conducted in general anesthesia using isoflurane evaporation. Once the mouse is under deep sedation, it is transferred to the green working stage. After the ultrasound procedure, the mouse should be placed in a pre-heated cage to accelerate recovery.
Tumor development is highly variable in the KPC mouse model. To avoid unnecessary ultrasound screening procedures, we usually perform abdominal palpation weekly starting from the age of two months. Gently, palpate the mouse abdomen up and down to detect any pancreatic tumors.
Keep in mind that hard feces may mimic abdominal tumor masses. After initiation of general anesthesia, in a separate induction chamber, the mouse is carefully placed onto the green and pre-heated working stage. During the ultrasound procedure, isoflurane is continuously supplied via the nose cone.
Fix the trandsucer head at the top of your working stage by using the mounting system. Perform the foot-pinch test to make sure the mouse is under deep anesthesia. Afterwards, the mouse is fixed onto the working platform using medical strips.
It is crucial to completely remove the abdominal fur prior to the ultrasound procedure, to avoid any artifacts. First, carefully shave off the mouse fur. Great care should be taken to avoid any superficial skin lacerations using the pet clipper.
Second, apply a thin layer of the depilatory cream onto the shaved area. Completely remove the cream using moist tissues. Apply abundant ultrasound gel onto the mouse abdomen, and a thin layer of gel directly onto the ultrasound probe.
Adjust the transducer onto the abdomen using soft pressure. Heavy compression of the mouse abdomen may lead to respiratory or circulatory impairment, and should be avoided. Scan up and down and adjust the frame of the working platform and the wheels for the x and y axis.
Some tumors may only be visible by changing the initial position of the mouse. To correctly interpret B-mode ultrasound images, basic anatomical knowledge is required. In the KPC mouse model, pancreatic tumors can be detected in the head, body, or tail of the pancreas.
The tumors usually demarcate as hypoechoic masses. Due to their desmoplastic features, these tumors are usually hard to compress by the ultrasound probe. Following the ultrasound, completely remove the ultrasound gel and gently place the mouse in the pre-heated recovery cage.
Once you identified a tumor, scan up and down to get an impression of the total size, and measure the largest diameter by using the ultrasound caliper. For the exact volume calculation, we use the formula of an ellipsoid. After finishing the measurement of the first plane, either rotate the scan-head by 90 degree, in a longitudinal position, or turn the mouse to its side, to detect the tumor's longitudinal expansion.
For the evaluation of novel therapeutic approaches, it is important to determine the initial tumor volume and the tumor growth kinetics during treatment. During pre-clinical trials, abdominal ultrasound can be repeated every three to four days to monitor tumor growth.
