Patient Education — Basics of Dosimetry

Basics of Dosimetry

Dosimetry in Theranostics: A Simple Overview

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What dosimetry is

Dosimetry is measuring how much radiation each organ and tumor actually absorbs from a radioactive drug. The result is the absorbed dose, reported in gray (Gy).

In theranostics, a targeted radioactive drug (for example, Lu-177 PSMA for prostate cancer) travels through the body and sticks to cancer cells. Some of it also collects in healthy organs such as the kidneys, salivary glands and bone marrow. Dosimetry answers two questions: how much radiation reached the tumor, and how much reached the healthy organs we need to protect.

MBq / GBq

Activity

The amount of drug given is measured in activity (MBq or GBq). Activity is what goes into the IV. In the United States, units of Curies may sometimes replace the units of Becquerels (MBq or GBq).

Absorbed dose

Absorbed dose (Gy) is what the tissue actually receives.

Two patients given the same activity can absorb very different organ doses.

Step by step

How it is done

Dosimetry uses repeated scans after each treatment to track where the drug goes and how long it stays.

1
Give the treatment. The patient receives the radioactive drug by IV.
2
Scan several times. SPECT/CT images are taken at set times after the dose, often around 4, 24 and 48–168 hours. Lu-177 gives off gamma rays that the scanner can see.
3
Measure uptake over time. The team outlines each organ and tumor and measures how much drug is there at each time point.
4
Calculate absorbed dose. Software converts that uptake curve into an absorbed dose (Gy) for each organ, usually with the MIRD method and standard body models.
5
Add up across cycles. Each cycle's dose is added to a running total and compared against the organ limits.
6
Predict the next cycle. The team estimates what the totals will be after the next planned dose, so the physician can decide whether it is safe to continue.

Approved treatments

Standard of care today

In routine US practice, approved theranostic drugs use the same fixed dose for every patient, without dosimetry. Safety is monitored with blood counts, kidney function and symptoms instead of measured organ doses.

Drug	Cancer	Approved dosing	Dosimetry required?
Pluvicto (Lu-177 PSMA-617)	PSMA-positive metastatic castration-resistant prostate cancer	7.4 GBq (200 mCi) every 6 weeks, up to 6 doses	No
Lutathera (Lu-177 dotatate)	SSTR-positive gastroenteropancreatic neuroendocrine tumors	7.4 GBq (200 mCi) every 8 weeks, 4 doses	No

Pluvicto was first approved in 2022 after chemotherapy, and in March 2025 the FDA expanded it to patients who have not yet had chemotherapy.

Outside the US

Some centers, especially in Europe, still do post-treatment imaging or dosimetry to guide care. It is not required by the US labels.

In clinical trials

How clinical trials use it differently

Many theranostic trials, especially early-phase ones, make dosimetry a required safety step after every cycle.

	Standard of care	Clinical trials
Dosing	Fixed dose for everyone	Often varies by cohort; may step up or down by cycle
Dosimetry	Not required	Often required after each cycle
Organ limits	None used	Protocol sets normal and sometimes absolute limits
Go/no-go decision	Labs, symptoms, response	Labs, symptoms, and projected cumulative organ dose
Physician paperwork	Usual clinical notes	Review, sign and document each dosimetry report in source

Trials use dosimetry to keep patients safe with new drugs, to test higher or more frequent doses, and to learn how much radiation the tumor needs to respond. Some trials also add rules such as re-consent if a patient passes a normal limit but stays below the absolute limit.

Protecting healthy organs

Organ dose limits

Kidneys and bone marrow are the organs most trials watch closely, because they are the usual dose-limiting organs in radioligand therapy. Typical ranges below are approximate and drawn from common external-beam tolerance tables; each protocol sets its own values.

Organ	Typical limit range (Gy)	BET-PSMA-121 limit (Gy)	BET-PSMA-121 absolute limit (Gy)
Kidneys	23 (some trials allow 28–40 using BED)	23	35
Bone marrow	2–3	2	2.5
Salivary glands	Often none; 24–32 when set	24	—
Lungs	17.5–20	20	—
Heart	26–40	26	—
Liver	30–32	30	—
Intestine	40–50	45	—
Stomach wall	45–50	45	—
Brain	45–60	60	—
Bladder wall	65	65	—
Spleen	Usually none	None	—
Lacrimal glands	Usually none	None	—

Normal limit

A normal limit is the level the protocol aims to stay under.

Absolute limit

An absolute limit, when one exists, is a hard stop: once reached, treatment ends.

Behind the numbers

Where the limits come from

Most organ limits are borrowed from external-beam radiation therapy. The main sources are the Emami tolerance tables (1991) and the later QUANTEC reviews (2010), which estimated the doses that cause a small risk of serious organ damage. The 23 Gy kidney limit is the classic Emami whole-kidney value. The 2 Gy bone marrow limit comes from older radioiodine safety studies.

These numbers are probably conservative for radioligand therapy. Radioligands deliver radiation slowly over days, which healthy tissue tolerates better than the fast doses of external beam. That is why some trials add higher absolute limits or use biologically effective dose (BED). Regulators such as the FDA do not set fixed organ limits; each sponsor proposes its own in the protocol, which is reviewed by the FDA and the IRB.

Sources

FDA: Pluvicto approval (2022)
Novartis: Pluvicto pre-chemotherapy approval (March 2025)
FDA: Lutathera pediatric approval and dosing (2024)
BET-PSMA-121 dosimetry report, subject 84006-2003 (protocol limits)
Typical limit ranges, Emami and QUANTEC: general knowledge, approximate, not looked up

Keep going

Where to next

Basics of Radiation How Theranostics Work Interpreting Your Scans What to Expect Theranostic Components

Educational content, not medical advice. Dosing and dose limits are set by each drug's label or trial protocol. Always discuss your specific treatment with your Theranostics care team.