Transrectal ultrasound (TRUS) is primarily used to examine the prostate gland, guide needle biopsies for suspected prostate cancer, and evaluate surrounding structures like the seminal vesicles and ejaculatory ducts. It works by inserting a small ultrasound probe into the rectum, which sits close enough to the prostate to produce detailed images that abdominal ultrasound can’t match. The procedure takes about 15 to 25 minutes, whether it’s a diagnostic scan alone or includes a biopsy.
Prostate Cancer Detection and Biopsy
The most common reason for a transrectal ultrasound is to investigate a possible prostate cancer. If a blood test shows persistently elevated PSA levels, or a doctor feels a hard or suspicious lump during a digital rectal exam, TRUS is the next step. The ultrasound can reveal areas in the prostate that appear darker than surrounding tissue (called hypoechoic lesions), particularly in the outer zone of the gland where most cancers develop. Current guidelines recommend that all such lesions be biopsied.
TRUS on its own has a sensitivity of roughly 50 to 63% for detecting prostate cancer, meaning it catches about half to two-thirds of cancers visible on imaging. Its specificity is higher, around 92 to 94%, so when it does flag something suspicious, it’s usually worth investigating. Because of that detection gap, TRUS is rarely used as a standalone diagnostic tool. Its real strength is guiding a biopsy needle precisely into the prostate so tissue samples can be collected for analysis under a microscope.
The standard biopsy protocol involves collecting 12 tissue cores from different zones of the prostate using thin needles. The ultrasound image updates in real time, letting the doctor see exactly where each sample is taken. If a suspicious nodule is visible, the doctor will target it directly in addition to sampling the standard locations. This combination of systematic and targeted sampling helps reduce the chance of missing a cancer.
MRI Fusion Biopsy
A newer approach overlays MRI images onto the live TRUS feed, creating a fused picture that combines the superior contrast of MRI with the real-time guidance of ultrasound. This technique detected cancer in about 68% of cases, compared to 76% for standard systematic biopsy alone. When both methods were combined, the detection rate jumped to roughly 88%, an absolute gain of about 12 to 20 percentage points over either method used individually. Fusion biopsy is increasingly used when MRI has already identified a suspicious area that needs precise targeting.
Measuring Prostate Size
TRUS is the go-to method for accurately measuring prostate volume, which matters in several clinical scenarios. Doctors measure the gland’s height, length, and width on the ultrasound screen, then plug those numbers into a standard formula to calculate volume. This measurement helps determine PSA density (how much PSA the gland produces relative to its size), which is a more reliable cancer indicator than PSA alone. Prostate volume also guides surgical planning for men with benign prostatic hyperplasia (BPH), the noncancerous enlargement that causes urinary symptoms in many older men.
Evaluating Male Infertility
For men with low sperm counts or absent sperm in their ejaculate, transrectal ultrasound can reveal structural problems in the reproductive plumbing. The probe can visualize the seminal vesicles, the ducts that carry sperm through the prostate, and the surrounding anatomy in fine detail. In one large study of men with obstructive causes of infertility, the most common findings were dilated ejaculatory ducts (about 30% of cases), abnormalities of the seminal vesicles (roughly 29%), and midline cysts within the prostate (about 22%).
TRUS can identify seminal vesicles that are too small (underdeveloped or absent, sometimes linked to the gene mutation that causes cystic fibrosis) or abnormally dilated from a downstream blockage. It can also spot cysts that press on the ejaculatory ducts, preventing sperm from reaching the semen. These findings often determine whether a blockage can be surgically corrected.
Staging Rectal Cancer
Though prostate evaluation is its primary role, transrectal ultrasound is also used to stage rectal tumors. Because the probe sits directly against the rectal wall, it can show how deeply a tumor has grown into the layers of tissue. This depth, called the T-stage, is critical for deciding whether a patient needs radiation or chemotherapy before surgery. TRUS correctly identified the T-stage in about 74% of cases overall, with the best accuracy for mid-stage tumors (around 80 to 81% for tumors that have grown into the muscle wall or just beyond it). It was also useful for detecting whether cancer had spread into the fat surrounding the rectum, with sensitivity and specificity both around 82 to 84%. Its ability to spot cancerous lymph nodes, however, was limited.
Guiding Cancer Treatment
Beyond diagnosis, TRUS plays a role in treating prostate cancer. It’s used to place tiny metal markers (fiducials) inside the prostate before external beam radiation therapy, giving the radiation machine a precise target to track during each session. During brachytherapy, where radioactive seeds are implanted directly into the gland, the ultrasound provides continuous visual guidance so seeds are positioned accurately throughout the prostate.
What the Procedure Feels Like
The ultrasound probe is roughly the diameter of a finger. You’ll lie on your side with your knees drawn toward your chest. The probe is lubricated and inserted a few inches into the rectum. Most people describe the sensation as uncomfortable pressure rather than sharp pain. A diagnostic scan alone involves no needles and is over in about 15 minutes.
If a biopsy is included, the doctor first numbs the area with a local anesthetic injected through the probe. You’ll hear a clicking sound each time a sample is taken, and you may feel a brief sting or pinch with each core. The entire biopsy typically wraps up within 25 minutes. You’ll be asked to stop blood thinners at least five days beforehand, and you’ll take antibiotics starting the day before to reduce infection risk. A cleansing enema on the morning of the procedure is standard.
Risks of TRUS-Guided Biopsy
A diagnostic TRUS without biopsy carries virtually no risks beyond temporary discomfort. When a biopsy is performed, complications are uncommon but possible. In a population-based study from Calgary tracking nearly 1,000 procedures, the most frequent infectious complication was sepsis at 2.2%, followed by urinary tract infection at 0.9% and prostatitis at 0.4%. Non-infectious complications included temporary urinary retention (1.2%), blood in the urine (0.9%), and rectal bleeding (0.8%). Minor blood in the semen is common for several weeks afterward and is not considered a complication. The transperineal approach, where the needle enters through the skin between the scrotum and rectum instead of through the rectal wall, is growing in popularity partly because it carries a lower infection risk.