In 2009 prostate cancer caused 27,360 deaths in the United States, accounting for 9% of male cancer deaths and making it the second most fatal cancer among men. Given its prevalence and danger, early diagnosis of this disease is critical. One tool often used in looking for cancer in the prostate is Single Photon Emission Computed Tomography or SPECT, a technique of medical imaging which allows doctors to see any tumors in the body. However, current SPECT systems have limits in size of tumors they can detect and correspondingly on how early they can detect such cancers. The Compton Probe, a version of SPECT, proposes to break these limits by inserting a detector inside the body. Inserted endoscopically, the Compton Probe has the ability to detect prostate cancer clusters more clearly and up to 5 times smaller than is possible with current SPECT systems .
The basics of the Compton Probe are identical to those of mainline SPECT technology. SPECT works by detecting radioactivity from clusters of cancer cells using a system of detectors; the Compton Probe operates in essentially the same way. The system starts with the injection of a radioactive substance, called a radiotracer, which binds to cancer cells in the body. Since it binds more actively to cancer cells than healthy ones, the radiotracer concentrates in the clusters of cells that form tumors. These clusters of the radiotracer emit radioactivity which in turn is detected by the Compton Probe apparatus consisting of two parts: An external detector ring around the body and the actual probe which is inserted into the body through the rectum (in conventional SPECT this is just an external filter). These two parts work in tandem to detect radioactivity emitted by the radiotracer and triangulate the position of the tumor by registering the detection points on both parts.
What gives the Compton Probe advantages over current SPECT technology is the presence of a second detector, the probe, much closer to the imaging region than the filter on current machines. Current systems use a one detector system with a filter to detect the radiotracer’s emitted radioactivity, but for these systems the detector is external. The probe, inserted into the body, is another detector, added to the conventional SPECT system to replace the filter. Although it would not work for images of large areas of the body, it is large enough to see the entire localized region of the prostate. The advantage of having the detector close to the object being imaged is twofold. First, having it close to the prostate increases the sensitivity of the detector since there is less material that the radioactivity which it detects must pass through. This proximity also makes discriminating between distinct clusters of cancer cells easier, just as it is easier to pick out two people in a crowd from 20 meters than 200 meters. Second, by virtue of being in the body, the probe is separated from the second detector by a greater distance than the filter and detector in a conventional SPECT system. This distance also improves the ability to see distinct and smaller tumors because it makes triangulating the position of the cancer cells easier.
The enhanced sensitivity of the Compton Probe and its improved triangulation quality produces impressive gains in detection ability over conventional SPECT systems. The use of the endorectal probe enhances spatial resolution (the smallest diameter of a tumor that can be seen) by a factor of 4-5 over external detector SPECT and provides efficiency (amount of radioactivity detected) gains of a factor of 16-40 . Not only does this mean that smaller tumors can be detected, but also because of the efficiency gains that they can be pin-pointed more clearly. As most prostate cancers have doubling times (time needed for tumor to double in size) in their early stages of 24 months, this decrease in the size of cancer that can be seen translates into the possibility of detection 6 months to a year earlier (taking into account the frequency of check-ups) than would be possible using conventional SPECT .
Conventional SPECT has been inherently limited in its ability to detect prostate cancer by its location outside the body and its use of only one detector. The Compton Probe overcomes these obstacles of conventional SPECT by inserting a second detector in contact with the prostate and in so doing allows the opportunity to see much smaller tumors, bringing earlier diagnoses that can save lives.