Theranostics SPECT/CT Dosimetry

What it does

After each cycle of [177Lu]Lu-PSMA or [177Lu]Lu-DOTATATE, many centres image the patient with quantitative SPECT/CT, at one or several time points, to see where the activity went and what dose the kidneys, salivary glands, bone marrow and tumours absorbed. Upload the post-therapy SPECT/CT series of a cycle; the run segments organs at risk and lesions on the CT and SPECT, fits time-activity curves and computes absorbed dose.

The result reports absorbed dose in gray per organ and per lesion, a voxel dose map, warnings where an organ at risk approaches its dose limit, and a cycle-to-cycle comparison when earlier cycles are supplied. It follows the EANM 2022 dosimetry recommendations and the MIRD formalism.

Intended use

Decision support for the theranostics team between cycles: a consistent, documented dose estimate for kidneys and salivary glands to weigh against the next administration, and a tumour-dose trend across cycles. Dose-limit warnings compare each organ dose with its reference limit. Camera calibration and partial-volume effects drive every dosimetry result, so the inputs and assumptions are stated with every number.

Who it is for

Nuclear-medicine physicians who run radioligand therapy programmes, medical physicists responsible for patient-specific dosimetry, and researchers comparing dose–response across cycles and centres.

Accepted input

• Quantitative SPECT/CT: DICOM NM tomographic series reconstructed for quantification, with the paired CT, for each imaging time point of one cycle.
• Time points: one or more acquisitions after the same administration, typically over the following days.
• Earlier cycles (optional, in prior_files): enable the cycle-to-cycle comparison and cumulative organ dose.

Requirements

Requirement	Why
Camera calibration factor (counts per second per MBq)	Converts SPECT counts into activity
Administered activity and administration time	Normalisation and the time axis of the time-activity curve
Acquisition date and time of every time point	Curve fitting and time integration
Attenuation and scatter-corrected reconstruction with CT	Quantitative organ and lesion activity
Organs at risk in the field of view (kidneys; salivary glands for PSMA ligands)	Absorbed dose is reported only for organs that were imaged

With several time points, the run fits the patient's own time-activity curve. With a single time point, time-integrated activity depends on population kinetic assumptions, and the result says so.

Optional context

The radiopharmaceutical, cycle number and cumulative activity given so far can be passed in clinical_context; they are used for the cycle comparison and the guidance.

The result

Section	Content
Organ and lesion segmentation	Kidneys, salivary and lacrimal glands, liver, spleen, bone marrow and lesions on the SPECT/CT
Dosimetry	Time-activity curves, time-integrated activity, absorbed dose in Gy per organ and per lesion, voxel dose map, dose-limit warnings and cycle comparison
Draft report	Opt-in (options.report): an English narrative that interprets the result for the reader (findings, impression, limitations); every number is checked against the findings

Each absorbed dose carries its source: the segmented volume, the time points used, the fit and the calibration factor, so a physicist can reproduce or audit the estimate.

Standards

• DICOM: Segmentation and RTSTRUCT for organs and lesions; Parametric Map with Real World Value Mapping for the absorbed-dose map; TID 10021 Radiopharmaceutical Radiation Dose for the administered activity; Encapsulated PDF.
• Dosimetry: EANM 2022 dosimetry recommendations and the MIRD formalism for organ and voxel absorbed dose.
• Therapy guidance: EANM/SNMMI 177Lu-PSMA guideline (2023).
• Units: UCUM units throughout (MBq, h, Gy).

Cycle comparison

When earlier cycles of the same patient are included, organs and lesions keep the same identifiers across cycles. The result reports dose per cycle and cumulative organ dose, and the change in tumour dose from cycle to cycle.

Related models

• nm/psma-pet: pre-therapy PSMA PET/CT with PSMA expression and eligibility findings.
• nm/sstr-pet: SSTR PET/CT with the Krenning score before PRRT.