The Cancer That's Sometimes 'Invisible' On An MRI

Cancer is diagnosed with a wide range of tests, including a complete blood count, urinalysis, and urine cytology, to name a few. To accurately diagnose solid tumors as possible cancer though, oncologists often rely on imaging tests for confirmation. A magnetic resonance imaging (MRI) test is one such option considered highly effective in detecting various types of cancers. This includes prostate cancer, which is one of the most common sites of cancer in males. Surprisingly, it is possible for prostate cancer to not be detected on a prostate MRI scan, but researchers also have valid explanations for this phenomenon.

Widely regarded as one of the greatest medical discoveries of all time, an MRI takes pictures of the inside of your body with the help of a large magnet. These images are taken in angles (or "slices"), which are combined together to help detect possible abnormalities such as malignant tumors. MRI is a popular tool used in diagnosing prostate cancer because it can help provide clearer images of soft tissues compared with other types of imaging tests. These images can confirm a suspected prostate tumor to help guide next steps in further testing and treatment. Any areas of concern may appear on an MRI scan as a darker part of the prostate gland compared with the rest of the surrounding tissues.

MRIs are also common tools used to detect possible prostate cancer recurrence. MRI scans may even provide more details about the possible severity of prostate cancer, and inform next steps regarding the need for a biopsy. Nevertheless, MRIs are not foolproof, and the only definitive way to know whether a prostate tumor is cancerous is by getting a biopsy.

Despite their widespread use and general accuracy in providing images of areas of concern, it is sometimes possible for prostate gland tumors to not show up during an MRI scan. This can seem counterintuitive, given the fact that MRIs are designed to detect soft tissues like tumors that other imaging tests could easily miss, yet such scenarios are still possible, and are referred to as false-negative results. Unlike a false-positive, which reveals a possible tumor that a biopsy reveals is actually non-malignant, a false-negative completely misses the cancerous area entirely.

Another term for this false-negative result is an "invisible" tumor, and this can even occur within targeted types of scans called multiparametric MRIs as outlined in a study published in the journal Cancers in 2023. Also known as unidentified cancers on the MRI scans, the authors noted that these types of results were most often associated with smaller lesions, as well as those with lower Gleason scores (a measure of prostate cancer progression).

Such results are more common in multiparametric MRIs used for prostate cancer than one might think. Research published in Prostate Cancer and Prostatic Diseases estimates a 10% to 20% false-negative rate. Previous analyses published in European Urology also reported that 4.4% of prostate cancer cases were missed.

While multiparametric MRIs are often used for prostate cancer diagnosis, the rate of false-negative results underscores the fact that this cancer shouldn't be diagnosed with MRI scans alone. Like other types of cancer, there isn't one single test that can confirm the presence of malignant cells or tumors. A prostate MRI may be recommended before a biopsy, or in conjunction with one. Each scan is also accompanied with a prostate imaging reporting and data system (PI-RADS) score, which helps guide next steps. PI-RADS scores range from 1 to 5, with 1 being the lowest possibility that an area of concern may be cancerous. Additional MRIs may be recommended to see whether prostate cancer has spread, including in cases where patients are under active surveillance before any official treatment.

It's also worth noting that an MRI of the prostate gland isn't exclusively used to diagnose cancer. Aside from cancer, MRIs may be used to help diagnose other conditions affecting the prostate, such as prostatitis and benign prostatic hyperplasia. Despite the possibility of false-negative results, MRIs are still powerful tools in cancer diagnosis –- an MRI can even help detect certain "hidden" conditions, too.