One of the most exciting features of radiopharmaceutical therapy is that, for many treatments, we can image the therapy after it has been given.
That may sound simple, but it is not routinely performed.
With many cancer treatments, we administer therapy and wait to see what happens. We follow symptoms, blood tests, tumour markers, or later imaging. In radiopharmaceutical therapy, the medicine itself carries a radionuclide. When its emissions are suitable for imaging, we can see where that treatment travelled inside the body.
This gives us a remarkable opportunity to ask three increasingly useful questions:
Where did it go? Did the treatment reach known disease? Did it localize in the expected organs? Did it reveal areas we had not appreciated before?
How much got there? Can we quantify activity in tumours and normal organs, and understand how long it remained localized there?
Was it enough? Did the tumour receive a radiation dose likely to matter? Were normal tissues kept within an acceptable tolerated radiation dose range?
That last question is where dosimetry begins.
Administered activity is not absorbed dose
Many radiopharmaceutical therapies are still prescribed using a standard administered activity. This approach has been important: it made treatment simpler to deliver, easier to scale, and available to more patients.
But administered activity describes what enters the patient. Absorbed dose describes the energy deposited in tissue. They are related, but they are not interchangeable.
Two patients can receive the same administered activity while experiencing very different biodistribution and clearance. One patient’s tumours may take up and retain substantial activity, while for another one it may not. Kidneys, bone marrow, salivary glands, and other normal tissues can also receive very different absorbed doses from one patient to the next.
Dosimetry helps us estimate those absorbed doses, which are typically reported in units of Gray (Gy), by combining quantitative measurements with information about how activity changes over time.
Think of dosimetry as a staircase
People sometimes hear dosimetry and imagine an all-or-nothing proposition: multiple scans, complex calculations, specialized software, and a workflow that is impossible to sustain in routine care.
That framing is not helpful.
Dosimetry is not an on/off switch. It is a staircase.
We need to start where the clinical workflow can support it and build toward adaptive therapy.
A centre may begin without routine post-treatment imaging. The next step might be visual SPECT/CT after therapy. From there, it should add quantitative imaging, simplified or single-time-point approaches where appropriate, more complete patient-specific dosimetry, and eventually adaptive treatment planning in which information from one cycle helps guide the next.
The message is not that every clinic must jump to the top tomorrow, but that we need to start climbing. Each step can create value while the people, processes, equipment, and evidence needed for the next step are built.
This view respects clinical reality. Different therapies, patients, and decisions may require different levels of measurement. Each step can create value while the people, processes, equipment, and evidence needed for the next step are built. In fact, this will help us generate the evidence needed to justify and support the next step.
The vicious cycle of dosimetry
There is another reason the staircase matters. Dosimetry has long faced a self-reinforcing implementation problem, described very nicely by my colleagues in Europe Journal article Rethinking Dosimetry: A European Perspective Read the paper :
We do not use dosimetry routinely because we believe there is not enough evidence, and we struggle to generate evidence because dosimetry is not used routinely.
This cycle affects more than publication counts. Without routine, standardized measurements, it is harder to pool data across centres, establish dose–response and dose–toxicity relationships, compare treatment strategies, or identify which dosimetry approach is sufficient for a particular decision.
Waiting for definitive evidence before collecting routine dosimetry data can therefore preserve the very evidence gap we need to close.
The staircase offers a way out. A centre does not need to implement the most complex workflow before it can contribute useful information. Consistent post-treatment imaging, quantitative calibration, harmonized reporting, and carefully chosen simplified methods can all build the datasets and experience needed for the next step.
In that sense, climbing the staircase is not only an implementation strategy. It is also an evidence-generation strategy.
Ask a better question
The debate is often framed as:
Should we do dosimetry or not?
A more useful question is:
What level of dosimetry does this decision need?
For some routine cases, visual imaging and quantitative trends may already add useful information. When toxicity risk or cumulative exposure is a concern, organ dosimetry may matter more. For retreatment, dose escalation, clinical trials, combination therapies, or emerging alpha-emitter treatments, more detailed patient-specific dosimetry may become especially valuable.
The appropriate level of dosimetry depends on the clinical question and the consequence of the decision.
This is a positive, practical way forward. It treats dosimetry not as a burden to apply identically in every situation, but as a tool whose depth should match the decision it needs to support.
Why this matters for patients
The goal is straightforward: safer, more effective, and more personalized treatment.
Dosimetry may help us:
- understand why response varies between patients;
- identify patients who might safely receive more treatment;
- recognize when normal tissues may be approaching a relevant tolerance;
- make better-informed retreatment decisions; and
- design stronger clinical trials and radiopharmaceuticals.
None of this means that every treatment decision can (or should) immediately be based on dosimetry. Methods, evidence, access, and standardization continue to evolve. Dosimetry is an estimate, and its uncertainties need to be understood and communicated.
But the direction is compelling:
- Radiopharmaceutical therapy is often imageable.
- Its therapeutic mechanism is radiation.
- Absorbed dose should be part of the language of theranostics.
A practical path forward
Theranostics has already made extraordinary progress. Treatments once available only in highly specialized settings are now helping patients around the world.
The next opportunity is to learn more from every treatment we give.
Post-treatment imaging is how we stop being blind.
Quantitative SPECT imaging is how we stop being purely descriptive.
Dosimetry is how we start adapting treatment.
Absorbed dose should become part of the language of theranostics.
The future does not depend on making dosimetry perfect before we use it. It depends on building the staircase together one useful step at a time.
This article is for education and discussion. It is not a substitute for patient-specific medical advice or local clinical protocols.
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