Calypso is a real-time tumor tracking system used during radiation therapy to follow the exact position of a tumor while treatment is being delivered. Made by Varian Medical Systems, the system is FDA-approved for tracking soft tissue tumors and is most commonly used in prostate cancer treatment, though its applications have expanded to other cancers including pancreatic tumors. It works by detecting tiny transponders implanted near the tumor, allowing the radiation beam to stay locked on its target with submillimeter accuracy.
How the Calypso System Works
The core challenge in radiation therapy is that tumors move. You breathe, your organs shift, you might twitch slightly on the treatment table. Even small movements can cause the radiation beam to miss part of the tumor or hit healthy tissue instead. Calypso solves this by continuously tracking the tumor’s position 20 times per second in three dimensions.
Before treatment begins, a doctor implants two or three small transponders (called Beacon transponders) near the tumor. These are tiny, roughly the size of a grain of rice, measuring about 1.8 mm in diameter and 8.5 mm long. During each radiation session, an electromagnetic array positioned above the patient sends out a signal. The implanted transponders respond with their own electromagnetic signal, and the system calculates their precise 3D location in real time. That position data feeds directly into the treatment machine, which can adjust the radiation beam to follow the tumor wherever it moves.
Why Tumor Tracking Matters for Treatment
Without real-time tracking, radiation oncologists have to add a safety margin around the tumor to account for possible movement. Traditionally, prostate treatments used a 10 mm margin. Image-guided techniques brought that down to 5 mm. With Calypso’s continuous tracking, research shows that margins can safely shrink further to just 3 mm when the system’s tolerance level is also set at 3 mm.
Smaller margins translate directly into less radiation hitting the bladder, rectum, and other surrounding organs. Studies analyzing Calypso tracking data found that doses to nearby organs were consistently reduced compared to the original treatment plan across nearly all scenarios tested. The volumetric coverage of the prostate itself stayed within about 1.2% of the planned dose when using these tighter margins, meaning the tumor still receives its full treatment while healthy tissue gets better protection.
What the Transponder Implant Involves
The transponder placement is a minor outpatient procedure, typically performed by a specialist radiologist. For prostate cancer patients, the transponders are implanted through the rectum using ultrasound guidance, similar to a prostate biopsy. Patients generally take antibiotics starting the day before and use bowel preparation the night before and morning of the procedure. Those on blood-thinning medications typically stop them 7 to 10 days beforehand.
Most patients tolerate the implant well. In a study of 234 prostate cancer patients, the average pain score during the procedure was 1.1 out of 5, and 85% of patients reported the pain was manageable. About 90% of patients who had previously undergone a prostate biopsy said the transponder implant was equally painful or less so. That said, about 32% of patients experienced at least one new symptom afterward. The most common was increased urinary frequency, affecting 16% of patients. Blood in the urine, rectal bleeding, and discomfort during urination each affected 9 to 13% of patients, mostly at mild severity. For the vast majority, symptoms resolved within two weeks. Only 9% of patients had symptoms lasting longer than that. Serious complications like infection were rare, occurring in roughly 1% or fewer of cases.
How It Compares to Other Tracking Methods
Calypso is not the only way to monitor a tumor during treatment. Surface-guided radiation therapy (SGRT) is one alternative that tracks the external contour of a patient’s body using cameras, then infers the tumor’s internal position based on how the surface moves. The key advantage of SGRT is that it requires no implanted markers and no additional radiation exposure for monitoring. The trade-off is that it relies on the assumption that external body movement reliably predicts internal tumor movement, which is not always the case.
Calypso’s electromagnetic approach measures the tumor’s position directly rather than estimating it from surface motion. This makes it particularly valuable for tumors in areas where organ movement doesn’t correlate well with what’s happening on the skin’s surface, such as the prostate or pancreas. The system also avoids the radiation dose that comes with repeated X-ray based tracking methods, since electromagnetic signals carry no ionizing radiation.
Which Cancers It Treats
Prostate cancer remains the most established use for Calypso, where years of clinical data support its effectiveness in tightening treatment margins and protecting the bladder and rectum. More recently, researchers have begun applying the system to other soft tissue tumors. A study on locally advanced pancreatic cancer was the first reported use of Calypso for that disease, using the same transponder-based approach to track a notoriously difficult target. Pancreatic tumors move significantly with breathing, making real-time tracking especially valuable in that setting.
The system has also been integrated with advanced beam-shaping technology. In one clinical implementation, Calypso’s position data was used to drive real-time adjustments to the radiation beam’s shape (not just its position), allowing the treatment to continuously reshape itself around a moving target during delivery. This represents a more sophisticated level of motion compensation than simply pausing treatment when the tumor drifts out of range.