Health Briefing · The Hill Report

A Prostate's Size Comes From Three Measurements and a Constant

Gland volume is calculated from length, width and height on ultrasound or MRI, and that single number now sets device trial eligibility, frames procedure choices in the urology guideline and adjusts how a PSA result is read.

Connor Hill · InsightfulWord · October 15

Key points

Red flags come first: the National Institute of Diabetes and Digestive and Kidney Diseases says to contact a clinician right away for inability to urinate at all, blood in the urine, fever and chills with painful urination, or severe lower abdominal or urinary tract pain.

Prostate volume is usually estimated as length × width × height × 0.52, the ellipsoid formula, on ultrasound or MRI.

A U.S. trial of a new prostate implant, whose maker won European certification this week, enrolled only men with glands of 30 to 90 mL.

At a PSA density cutoff of 0.15, one 2025 study found sensitivity of 83.1% in small glands and 33.3% in large ones.

An enlarged prostate is described in conversation by its effects: the stream, the night waking, the urgency. In a clinic it is also described by a number. That number is the gland's volume, expressed in milliliters, cubic centimeters or grams, and it is calculated rather than observed directly.

Device news this week shows how much rides on that figure. One implant's first U.S. commercial procedure was reported on Wednesday, and another implant's European certification was reported a day earlier, after a pivotal trial that admitted men only within a set volume range.

On the tape

Oct 14

Urology Times reported the first U.S. commercial procedure with the Urocross Expander System, a temporary implant cleared by the Food and Drug Administration (FDA) in March; its pivotal trial randomized 240 men

Why it matters: The report lists age and symptom score as entry criteria but no gland size, so each trial's eligibility table is worth reading.

Oct 14

A check of FDA press announcements from Oct 1 to Oct 14 found none on prostate enlargement

Why it matters: The week's device news came through companies and trade coverage, not agency releases.

Oct 13

Urology Times reported European certification for the Butterfly Prostatic Retraction Device; its U.S. pivotal trial of 244 men required a volume of 30 to 90 mL and a length of 25 to 45 mm

Why it matters: Volume and length worked as gates for who could enroll.

Oct 9

Urology Times reported FDA fast track designation for an investigational therapy in late-line metastatic prostate cancer

Why it matters: Cancer and benign enlargement are separate conditions that share one gland, one blood test and often one scanner.

Oct 8

A release on a European Radiology study of 787 men said radiologist reads plus artificial intelligence could have avoided 18.9% of biopsies

Why it matters: The software described outlines the gland automatically and calculates PSA density from it.

Before anything else

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) tells men with prostate enlargement to contact a health care professional right away if they cannot urinate at all, have painful, frequent and urgent urination with fever and chills, see blood in the urine, or have great discomfort or pain in the lower abdomen or urinary tract. Inability to urinate is treated as an emergency. A volume figure on a report does not override any of these symptoms. No prescribed medication should be started, stopped or changed except with the prescriber.

Three Diameters and Pi Over Six

The standard calculation treats the prostate as an ellipsoid, a stretched sphere. On transrectal ultrasound, length is measured in a side view and width and height in a cross-section, according to an online urology textbook. The three diameters are multiplied together and then by pi divided by six, about 0.52. A gland of 5 cm by 4 cm by 3 cm works out to roughly 31 mL.

MRI uses the same arithmetic. A 2021 study in Diagnostic and Interventional Radiology measured 114 patients' glands with the formula. Agreement between two readers was high, with an intraclass correlation of 0.93 under version 2 of the Prostate Imaging Reporting and Data System (PI-RADS) and 0.96 under version 2.1.

Reliable is not the same as exact. A McGill University Health Center study in BMC Urology on July 24, 2019, weighed and measured removed prostates from 153 patients. The ellipsoid formula explained 55% of the variation in true volume, and a new coefficient of 0.66 explained 64%. The authors wrote that the ellipsoid formula suits smaller prostates, under 30 cc, better.

The National Library of Medicine's StatPearls review, updated October 20, 2024, adds that size before surgery is often estimated at cystoscopy, or by ultrasound, CT or MRI.

By the numbers

0.52

Approximate value of pi divided by six in the ellipsoid formula

0.93 and 0.96

Agreement between readers measuring volume on MRI under two PI-RADS versions

55%

Share of true volume variation explained by the ellipsoid formula in 153 removed glands

30 to 90 mL

Volume range required for one implant's 244-man U.S. pivotal trial

29% to 33%

NIDDK estimate of benign prostatic hyperplasia among men 65 and older

5% to 6%

NIDDK estimate among men aged 40 to 64

Volume Changes What a PSA Number Means

Prostate-specific antigen, or PSA, comes from benign and enlarged tissue alike. StatPearls calls PSA somewhat predictive of volume and says a benign 35 cc prostate typically produces about 1.5 ng/mL.

PSA density adjusts for that: the PSA level divided by gland volume. The review's example gives about 0.04. Higher densities draw closer attention because more antigen comes from each milliliter.

A cutoff of 0.15 is common. A November 2022 Urologic Oncology study of 349 men with equivocal PI-RADS 3 findings found significant cancer in 20.0% overall and 13.6% of those below 0.15.

Gland size changes how a single cutoff performs. A Royal Marsden NHS Foundation Trust study in The Prostate, published online March 3, 2025, grouped 548 men into glands of 30 mL or less, above 30 and below 50 mL, and 50 mL or more.

At a density threshold of 0.15, sensitivity for grade group 2 or higher cancer was 83.1% in small glands, 63.6% in medium glands and 33.3% in large glands. Specificity ran the other way, at 48.5%, 67.5% and 79.3%. The authors wrote that defining thresholds by volume category could improve the measure as a risk predictor.

These are associations in men already referred for evaluation, describing how a test behaves rather than what one result means.

🩺 Clinical Signal

83.1% vs 33.3%

In the 2025 Royal Marsden analysis, the same PSA density cutoff of 0.15 caught 83.1% of significant cancers in glands of 30 mL or less and 33.3% in glands of 50 mL or more. The volume entered in the denominator is the ellipsoid estimate described above, which a 2019 specimen study found less precise in larger glands. Two measurement steps therefore stack: how the gland is measured, and how the resulting density is read for that size of gland. A report that lists the volume, the method and the density together gives a clinician all three pieces.

Size Ranges Sit Inside the Guideline and the Trials

The American Urological Association (AUA) published a full rewrite of its guideline on symptoms attributed to benign prostatic hyperplasia in The Journal of Urology on May 7, 2026. Its release counted 62 recommendations in three parts: evaluation, medical management and procedures.

A May 14, 2026, manufacturer release said the update strengthened the recommendation for waterjet resection in glands of 30 to 80 mL and added that it may be offered at 80 to 150 mL. The 2020 amendment, as summarized by Urology Times, tied the prostatic urethral lift to glands of 30 to 80 grams, water vapor therapy to glands under 80 grams and transurethral incision to glands under 30 grams.

These ranges mark where evidence was gathered and sit alongside symptom scores, anatomy and patient preference. They are reported as written, without a view on any procedure.

NIDDK estimates that benign prostatic hyperplasia affects 5% to 6% of men aged 40 to 64 and 29% to 33% of men 65 and older. Autopsy studies cited by StatPearls find microscopic changes in as many as 50% to 60% of men in their 60s. NIDDK links age, family history, certain heart, metabolic or kidney conditions and inactivity to a higher likelihood. Those are associations; an association alone does not establish cause.

A study of 1,106 Spanish men aged 40 and older, indexed in PubMed, reported average glands above 30 grams from age 60 on.

Where prostate volume is written

Ultrasound or MRI report

Three diameters and the ellipsoid volumeLength × width × height × 0.52

MRI report under PI-RADS

Gland volume and, where provided, PSA densityVersion 2.1 measurement rules

AUA guideline in The Journal of Urology

Volume ranges tied to procedural optionsMay 7, 2026

Device trial eligibility table

Minimum and maximum gland volume and length30 to 90 mL; 25 to 45 mm

One question for the desk

When a prostate volume appears on a report, does the report also say how it was measured and which formula was used?

Hit reply — one line is enough.

Worth stating plainly

Prostate volume is an estimate built from three diameters and a constant of about 0.52, taken on ultrasound or MRI. Readers agree closely on it, though specimen studies show it is less precise in larger glands. The figure feeds PSA density, whose performance at a fixed cutoff varies with gland size, and it frames the size ranges written into the 2026 AUA guideline and into device trial eligibility. Benign enlargement becomes more common with age, and the factors linked to it are associations rather than established causes.

The short checklist

Treat inability to urinate, blood in the urine, fever with painful urination or severe lower abdominal pain as reasons for prompt care.

Look for the three diameters, the formula and the imaging method next to any volume figure.

Read PSA density together with the gland volume used to calculate it.

Check the volume range in a device trial's eligibility criteria before comparing results.

Note the date and version of any guideline cited, since the AUA guideline was rewritten on May 7, 2026.

Keep medication decisions with the prescriber.

Replies to this briefing reach a person, not a queue.

Connor Hill reads every reply.

Sources checked

Verified October 14, 2026

Urology Times, Urocross Expander first U.S. commercial procedure, Oct 14, 2026; Butterfly Prostatic Retraction Device EU MDR certification, Oct 13, 2026; VIR-5500 fast track designation, Oct 9, 2026; 2020 AUA BPH guideline summary, March 13, 2020

Food and Drug Administration, press announcements page, checked Oct 14, 2026; GlobeNewswire release on a European Radiology prostate MRI study, Oct 8, 2026

National Institute of Diabetes and Digestive and Kidney Diseases, Prostate Enlargement (Benign Prostatic Hyperplasia); National Library of Medicine StatPearls, Benign Prostatic Hyperplasia, updated Oct 20, 2024

Urology Textbook, transrectal ultrasound chapter; Gündoğdu and Emekli, Diagnostic and Interventional Radiology, January 2021; Aprikian et al., BMC Urology, July 24, 2019

Robinson et al., The Prostate, March 3, 2025; PI-RADS 3 PSA density study, Urologic Oncology, November 2022; Andalusia prevalence study, PubMed record 9474174

American Urological Association release on the 2026 BPH guideline, May 7, 2026; The Journal of Urology, guideline part on presentation and evaluation, May 7, 2026; manufacturer release on the guideline update, May 14, 2026

Connor Hill writes The Hill Report for InsightfulWord, a daily briefing on the machinery behind the numbers.