Y-90 DOSIMETRY:
HOW RADIATION DOSE IS CALCULATED
A patient-facing guide to the most technically important step in Y-90 radioembolization planning โ how the radiation dose is calculated, why getting it right matters, and what distinguishes personalised dosimetry from simpler empirical approaches.
analyticsAt a Glance
- check_circleDosimetry determines how many gigabecquerels (GBq) of Y-90 are injected โ the most consequential calculation in TARE planning
- check_circleUnderdosing leads to poor tumour response; overdosing causes radiation-induced liver disease (REILD)
- check_circleThe partition model is the most accurate dosimetric method โ using MAA scan data to calculate tumour and normal liver dose separately
- check_circlePersonalised dosimetry targeting >200 Gy to tumour is associated with significantly improved HCC response rates
Why Dosimetry Is the Most Important Step in TARE Planning
In Y-90 radioembolization, the prescribed activity (measured in gigabecquerels, GBq) determines how much radiation the liver and tumour receive. Too little โ and the tumour does not receive enough radiation to respond. Too much โ and the normal liver is damaged, potentially causing radiation-induced liver disease (REILD), which can be severe and life-threatening. Dosimetry is the calculation that navigates this therapeutic window.
โIn TARE, the dose is the therapy. Getting the dose right is not an administrative step โ it is the clinical decision that determines whether the patient benefits.โ
The Therapeutic Window
Effective TARE requires delivering a radiation dose high enough to kill the tumour (generally >100 Gy, ideally >200 Gy for HCC) while keeping the absorbed dose to non-tumorous liver below the threshold for REILD (typically <70โ80 Gy for non-cirrhotic liver). Personalised dosimetry is the tool for achieving this balance.
Units: Gy vs GBq
GBq (gigabecquerels) describes the activity โ the rate of Y-90 radioactive decay โ and is what is ordered and injected. Gy (Gray) describes the absorbed dose โ the energy deposited per kilogram of tissue โ and is what determines biological effect. Dosimetry converts the GBq activity to estimated Gy dose in each tissue compartment.
Y-90 Dosimetry Methods โ Comparison
Three dosimetry methods are used in clinical practice for TARE activity calculation. They vary in complexity, accuracy, and the degree to which they individualise the dose to the patient's specific tumour biology.
| Method | Inputs Required | Output | Accuracy | Best Use Case |
|---|---|---|---|---|
| BSA (Body Surface Area) Method | Patient body surface area, liver volume, lung shunt fraction | Total GBq to inject based on BSA formula | Low โ does not account for tumour vs normal liver distribution | Historical standard; now largely superseded by partition model for HCC |
| Empirical Method | Tumour extent, lesion category (uninvolved/minimal/moderate/extensive) | GBq based on lesion category lookup table | Low-moderate โ categorical rather than continuous | Simplified clinical estimation; SIR-Spheres historical standard |
| Partition Model | MAA SPECT/CT tumour-to-normal ratio, tumour and normal liver volumes, LSF, prescribed absorbed dose | Gy dose to tumour, normal liver, and lung; GBq activity to achieve target | High โ individualised to patient anatomy and vascularity | Recommended for all HCC patients; especially critical for radiation segmentectomy |
| MIRD (Medical Internal Radiation Dosimetry) Framework | Organ uptake from SPECT/CT, mass of target organs | Absorbed dose (Gy) per organ based on radionuclide decay data | High โ physics-based compartmental calculation | Used in academic centres; forms the theoretical basis for partition model |
The Partition Model โ How It Works Step by Step
The partition model is the most clinically meaningful dosimetric approach for TARE. It calculates the absorbed dose separately for three compartments: tumour, non-tumorous liver (NTL), and lungs.
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Step 1 โ MAA SPECT/CT Segmentation
The SPECT/CT images from the MAA scan are imported into dosimetry software. A nuclear medicine physicist manually or semi-automatically segments the images into three regions of interest: the tumour region (T), the non-tumorous liver (NTL), and the lungs.
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Step 2 โ Calculate Tumour-to-Normal Liver Ratio (T:N Ratio)
The count density (counts per voxel) in the tumour vs the non-tumorous liver is measured from the SPECT data. This T:N ratio reflects the preferential arterial blood supply to the tumour โ the higher the T:N ratio, the more selectively the tumour takes up injected particles vs normal liver.
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Step 3 โ Measure Compartment Volumes
The volumes of the tumour (Vt), non-tumorous liver (Vntl), and total liver are measured from the CT component of the SPECT/CT โ typically using semi-automated volumetric segmentation software.
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Step 4 โ Set Prescribed Tumour Dose (Target Gy)
The treating team specifies the target absorbed dose to the tumour โ typically โฅ120 Gy for palliative intent and โฅ200 Gy for ablative intent (radiation segmentectomy). The normal liver dose constraint is set simultaneously โ typically <70โ80 Gy for non-cirrhotic and <50โ70 Gy for cirrhotic liver.
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Step 5 โ Calculate Required Activity (GBq)
Using the T:N ratio, compartment volumes, LSF, and prescribed dose targets, the partition model equation solves for the required injected activity A (in GBq) that simultaneously achieves the tumour dose target while keeping normal liver and lung doses within safe limits.
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Step 6 โ Activity Ordering and Verification
The calculated activity (GBq) is ordered from the manufacturer (typically on a specific day, as Y-90 activity is time-decay adjusted for the treatment date). On treatment day, the activity is verified by the medical physics team before injection.
Y-90 Absorbed Dose Reference Values
Clinically established dose thresholds and targets that guide dosimetry prescription for TARE โ based on published radiobiological data and clinical outcomes studies.
| Compartment / Endpoint | Dose Reference | Clinical Implication |
|---|---|---|
| Tumour โ minimum effective dose (HCC) | โฅ 100โ120 Gy | Threshold for meaningful tumour response โ below this, partial response is unlikely |
| Tumour โ ablative dose (HCC, RS) | โฅ 200 Gy | Associated with complete pathological response and improved survival in radiation segmentectomy studies |
| Normal liver โ REILD threshold (non-cirrhotic) | < 70โ80 Gy | Mean whole-liver dose above this level associated with radiation-induced liver disease risk |
| Normal liver โ REILD threshold (cirrhotic) | < 50โ70 Gy | Cirrhotic liver is more radiation-sensitive โ lower tolerance than non-cirrhotic normal liver |
| Lung dose limit | < 30 Gy per session (< 50 Gy cumulative) | Exceeding this threshold associated with radiation pneumonitis โ linked to lung shunt fraction |
Personalised Dosimetry vs Empirical Dosing โ Clinical Impact
Multiple studies have demonstrated that personalised dosimetry targeting โฅ200 Gy to HCC tumours significantly improves response rates and survival compared to empirical BSA-based dosing.
Tumour Response Rate: Personalised (โฅ200 Gy) vs Empirical Dosing โ HCC
Complete response and objective response rates comparing dose-escalated (โฅ200 Gy) vs standard empirical dosing in HCC TARE. Source: Garin et al., Hepatology 2015; multiple retrospective cohorts.
- Complete response rate (โฅ200 Gy tumour dose)~25โ35%
- Complete response rate (empirical BSA dosing)~5โ10%
- Objective response rate (โฅ200 Gy)~70โ80%
- Objective response rate (empirical)~40โ55%
Y-90 Dosimetry โ Key Numbers
The most clinically important dosimetric reference values for patients and families.
- โฅ200 GyTumour absorbed dose target for ablative TARE (radiation segmentectomy)Studies show complete pathological response rates of 25โ35% and dramatically improved OS when tumour dose exceeds 200 Gy โ achievable only with personalised partition model dosimetry.
- <70 GyNormal liver dose limit to prevent radiation-induced liver diseaseNon-tumorous liver dose must be kept below this threshold โ the partition model allows simultaneous optimisation of tumour dose and normal liver protection.
- 3โ5รTypical tumour-to-normal liver uptake ratio in HCCHCC preferentially takes up ~3โ5ร more injected particles than normal liver per unit volume โ the biological basis for TARE's therapeutic selectivity.
Complete the TARE / Y-90 Resource Library
Explore every aspect of Y-90 radioembolization โ from the patient introduction and physics to platform selection and the pre-treatment workup.
Frequently Asked Questions About Y-90 Dosimetry
How do I know if my TARE centre is using personalised dosimetry?
Ask your interventional radiologist directly: 'Will you be using the partition model for my dose calculation, and what tumour dose are you targeting?' A centre performing personalised dosimetry will be able to tell you the expected absorbed dose in Gray (Gy) to your tumour and normal liver from their dosimetry calculation. A centre using only the BSA or empirical method will calculate activity in GBq based on your body size or tumour extent โ without a specific Gy target for the tumour. If your centre cannot provide a tumour dose estimate in Gy, CancerFax can identify specialist Y-90 centres in China and India that use partition model dosimetry routinely.
What is radiation-induced liver disease (REILD) and how is it prevented?
REILD is a complication of excessive radiation dose to the non-tumorous liver โ manifesting as jaundice, ascites, and liver dysfunction typically 4โ8 weeks after treatment. In its severe form it can progress to liver failure. REILD is prevented by keeping the mean absorbed dose to non-tumorous liver below the tolerance threshold (~70 Gy for non-cirrhotic, ~50 Gy for cirrhotic liver) โ a constraint that is enforced through partition model dosimetry. The risk is higher in cirrhotic patients and those with underlying liver disease, which is why HCC patients require particularly careful dosimetric planning.
Can the dose be adjusted between the mapping session and the actual treatment?
Yes โ the mapping MAA scan and dosimetry calculation are performed 1โ2 weeks before treatment specifically to allow this. If the initial dosimetry calculation shows that the prescribed tumour dose cannot be achieved without exceeding normal liver constraints, the approach can be modified: using selective lobar injection rather than whole-liver, adjusting the catheter position, or choosing a lower-than-ablative dose target. In some cases, if the dosimetry is unfavourable, the treating team may recommend TACE or an alternative approach instead. CancerFax provides guidance on what questions to ask about dosimetry results before agreeing to proceed with treatment.
How CancerFax Helps
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CancerFax does not guarantee treatment access, eligibility, or clinical outcome. Our role is to help patients access accurate information, structured review, and appropriate specialist pathways.
Ensure Your TARE Planning Includes Personalised Dosimetry
CancerFax connects patients with specialist Y-90 centres in China and India that perform partition model dosimetry โ maximising tumour dose while protecting normal liver function.
This content is for informational purposes only and does not constitute medical advice. Always consult a qualified interventional radiologist and medical physicist before making treatment decisions.