Every oncologist who prescribes Lu-177 PSMA-617 writes a number in megabecquerels. Most dosimetry reports, when they exist, express results in gray. These two quantities are related but not interchangeable, and the distinction between them is not a technical nicety: it is the conceptual foundation for understanding why patient-specific dosimetry changes clinical outcomes rather than just adding a calculation step. This article is for the oncologist who receives a dosimetry report, or who is evaluating whether to add dosimetry to a theranostics program, and wants to understand what the numbers mean.
Administered Activity: What You Prescribe
Administered activity, measured in becquerels (Bq) or more practically in gigabecquerels (GBq) or megabecquerels (MBq), is the quantity of radioactive material injected. One becquerel is one nuclear disintegration per second. When you prescribe 7.4 GBq of Lu-177 PSMA-617, you are specifying that 7.4 billion atoms of lutetium-177 will disintegrate per second at the moment of injection, releasing beta particles and gamma photons as they decay toward stable hafnium-177.
Administered activity is what you control. It is the number you enter in the treatment order. It is the quantity measured in the dose calibrator immediately before administration. It is also the number used in all registration trial protocols, the number on which population safety and efficacy data are reported, and the number that current prescribing conventions are built around.
What administered activity does not tell you is how much energy the decaying atoms deposit in any particular tissue in this particular patient. That depends on where the atoms go after injection, how long they stay, and how the emitted radiation interacts with the surrounding tissue geometry. Two patients receiving identical 7.4 GBq prescriptions can experience very different biological effects because the distribution and retention of the radiopharmaceutical differs between them.
Absorbed Dose: What the Tissue Experiences
Absorbed dose, measured in gray (Gy), quantifies energy deposition per unit mass: one gray is one joule of energy deposited per kilogram of tissue. This is the quantity that determines biological effect. Ionizing radiation damages cells by depositing energy that breaks chemical bonds, generates reactive oxygen species, and causes DNA strand breaks. The amount of damage is proportional to absorbed dose, not to administered activity.
For the kidney of a patient receiving Lu-177 therapy, absorbed dose is the integral over time of the dose rate from all radioactive disintegrations occurring within and near the kidney. Lutetium-177's beta particles have a range in tissue of roughly 0.5 to 2.5 mm, so disintegrations within the kidney primarily deposit their energy in the kidney. The cumulative absorbed dose to the kidney across a treatment course is the sum of these cycle-by-cycle contributions, and the 23 Gy constraint that most protocols use as a kidney tolerance limit refers to this cumulative absorbed dose, not to any cumulative administered activity.
The same logic applies to tumor absorbed dose. A tumor that receives 15 Gy per cycle across four cycles accumulates 60 Gy total. Whether that is sufficient for the desired tumor response depends on radiobiological modeling and the tumor's characteristics, but it is a physically defined quantity that can be compared across patients and institutions. Administered activity cannot be compared the same way: 7.4 GBq in a patient with high PSMA expression and slow clearance delivers more than 7.4 GBq in a patient with lower expression and faster clearance.
The Conversion Factor: Why It Varies Between Patients
The absorbed dose delivered per unit administered activity, sometimes written as Gy/GBq or mGy/MBq, is the conversion factor that links the quantity you prescribe to the quantity the tissue experiences. This factor is not a fixed number. It is a patient-specific and organ-specific quantity that depends on how much of the administered activity localizes in that organ and how long it stays there.
For kidney dosimetry in Lu-177 PSMA therapy, published data show kidney absorbed dose ranging from roughly 0.2 Gy/GBq to over 0.9 Gy/GBq across different patients at the same time-activity measurement protocol. At 7.4 GBq per cycle, this range corresponds to kidney doses from about 1.5 Gy to 6.7 Gy per cycle. Over four cycles, cumulative kidney dose ranges from 6 Gy to 26.8 Gy, with the upper end exceeding the 23 Gy tolerance constraint. The patient at the high end of this distribution, receiving standard dosing without dosimetric monitoring, may accumulate kidney doses approaching or exceeding tolerance without any early clinical signal until laboratory evidence of renal impairment appears.
The conversion factor is what patient-specific dosimetry measures. Serial SPECT imaging post-administration quantifies the time-activity curve in each organ of interest. The time-integrated activity coefficient (TIAC) for that organ, obtained by fitting the serial data to a pharmacokinetic model and integrating, multiplied by the appropriate S-value from the MIRD system, gives the absorbed dose per administered activity for that patient and that organ. Once the conversion factor is known from the first cycle, subsequent cycles can be prescribed with a specific dose target in gray rather than a fixed activity in GBq.
Reading a Dosimetry Report: Key Quantities and Their Clinical Meaning
A dosimetry report for a single cycle of Lu-177 therapy typically contains several key quantities. Understanding each one is necessary for translating the report into clinical action.
The kidney absorbed dose per cycle, in gray, is the primary safety metric. It should be compared against the remaining tolerance: the 23 Gy cumulative constraint minus the accumulated dose from all prior cycles. If the current-cycle kidney dose is 5 Gy and three cycles have been completed at similar doses (total 15 Gy), one additional cycle at 5 Gy would bring the cumulative total to 20 Gy, within tolerance. A fifth cycle would raise the total to 25 Gy, exceeding the constraint. The dosimetry report makes this arithmetic explicit rather than relying on the assumption that the fixed-activity protocol is safe for this specific patient.
Tumor lesion absorbed dose, where available from quantitative SPECT analysis, is the primary efficacy metric. This is more technically demanding than organ dosimetry because tumor lesion volumes must be segmented and partial-volume effects corrected for each lesion separately. Where reported, it provides a physical basis for assessing whether target lesions are receiving doses in the range expected to produce the desired radiobiological effect.
The TIAC for each organ, sometimes reported alongside the absorbed dose, represents the total radioactive disintegrations per unit administered activity integrated over the clearance of the radiopharmaceutical. Its primary use in the report context is to document the measurement underlying the dose calculation, supporting transparency and reproducibility. Differences in TIAC between cycles for the same patient reflect changes in organ uptake or clearance kinetics, which may signal relevant pharmacological or clinical changes in the patient's condition.
What Dosimetry Changes About Clinical Decisions
The practical change that dosimetry introduces is substituting measured patient-specific dose data for assumed population-average biokinetics in the cycle planning process. For the majority of patients whose biokinetics fall near the population mean, dosimetry confirms that standard dosing is safe and effective, which is a useful confirmation rather than a trivial one. For patients at the tails of the distribution, dosimetry identifies the deviation and supports adjusting the prescription accordingly.
The YSOTOPE platform is designed specifically to make dosimetric data available at the clinical decision point: before the next cycle is prescribed, not after it is administered. This means the conversion factors estimated from prior cycles are available when the oncologist and nuclear medicine physician are deciding on the subsequent cycle's activity. The decision to modify, maintain, or terminate based on dosimetric data remains with the clinical team. What changes is the quality of dose information informing that decision.