EFOMP policy statement NO. 19: Dosimetry in nuclear medicine therapy - Molecular radiotherapy

Organ and tumor dosimetry including method simplification for [177Lu]Lu-PSMA-I&T for treatment of metastatic castration resistant prostate cancer

BACKGROUND

Internal dosimetry in individual patients is essential for safe and effective radioligand therapy. Multiple time point imaging for accurate dosimetry is time consuming and hence can be demanding for nuclear medicine departments as well as patients. The objectives of this study were (1) to assess absorbed doses to organs at risk and tumor lesions for [177Lu]Lu-PSMA-I&T using whole body SPECT imaging and (2) to investigate possible simplified dosimetry protocols.

METHODS

This study included 16 patients each treated with 4 cycles of [177Lu]Lu-PSMA-I&T. They underwent quantitative whole body SPECT/CT imaging (3 bed positions) at four time points (TP) comprising 2 h, 24 h, 48 h and 72-168 h post-injection (p.i.). Full 3D dosimetry (reference method) was performed for all patients and dose cycles for organs at risk (kidneys, parotid glands and submandibular glands) and up to ten tumor lesions per patient (resulting in 90 lesions overall). The simplified dosimetry methods (SM) included (1) generating time activity curves for subsequent cycles using a single TP of imaging applying the kinetics of dose cycle 1, and for organs at risk also (2) simple extrapolation from dose cycle 1 and (3) from both, dose cycle 1 and 2.

RESULTS

Normalized absorbed doses were 0.71 ± 0.32 mGy/MBq, 0.28 ± 0.12 mGy/MBq and 0.22 ± 0.08 mGy/MBq for kidneys, parotid glands and submandibular glands, respectively. Tumor doses decreased from 3.86 ± 3.38 mGy/MBq in dose cycle 1 to 2.01 ± 2.65 mGy/MBq in dose cycle 4. Compared to the full dosimetry approach the SM 1 using single TP imaging at 48 h p.i. resulted in the most accurate and precise results for the organs at risk in terms of absorbed doses per cycle and total cumulated dose. For tumor lesions better results were achieved using the fourth TP (≥ 72 h p.i.).

CONCLUSION

Simplification of safety dosimetry protocols is possible for [177Lu]Lu-PSMA-I&T therapy. If tumor dosimetry is of interest a later imaging TP (≥ 72 h p.i.) should be used/added to account for the slower kinetics of tumors compared to organs at risk.

Theranostics: Timing is Everything

On stage, and in real life, timing is critical for success. Theranostic cancer care epitomizes the central role of timing in the evolution of efficacious molecular targeted radioligand therapy and its incorporation into routine clinical practice of oncology. Nuclear medicine has returned to its therapeutic roots, having been founded as a medical specialty, over three-quarters of a century ago, with radioiodine therapy of thyroid cancer. The very recent oncologist acceptance of 68Ga/177Lu/225Ac-PSMA effectiveness in treating prostate cancer has re-established the role of the physician in nuclear medicine. This article addresses various important issues in respect of timing related to this resurgence. Training of the required new workforce in technical -omics expertise and physicianly virtues is an urgent priority. Precision in radioligand therapy requires definition of individual radiation absorbed dose (Gy) to tumor and to critical normal organs, preferably prospectively. It is time to abandon one-size-fits-all administration of fixed activities (GBq) in arbitrary cycle intervals and duration. The time has also come to design combination sequenced theranostic-immuno-chemotherapeutic approaches to metastatic cancer to address unmet needs, particularly in pancreatic carcinoma; exploiting the potential of new fibroblast activation protein inhibitor radioligands targeting the tumor microenvironment. Public perception of all things "nuclear," including nuclear medicine, has recently recovered from the general opprobrium and radiophobia of the last half-century. Nuclear is the new green. At last, there have arisen propitious circumstances for the future development of theranostics: The timing is right, now.

Symposium on molecular radiotherapy dosimetry: The first of a series?

The EFOMP Special Interest Group for Radionuclide Internal Dosimetry (SIG_FRID) organised its first scientific meeting, the Symposium on Molecular Radiotherapy Dosimetry, in Athens on November 9th-11th 2023. The Symposium was hosted by the Hellenic Association of Medical Physicists and the National and Kapodistrian University of Athens. This meeting gathered more than 180 scientists from 28 countries. Scientific, clinical and regulatory aspects were addressed by 8 invited experts. Two continuous professional development sessions were organised. A special round table gathering medical physics experts, physicians regulatory authority experts and patient representatives addressed the possibilities to increase clinical dosimetry dissemination. The event was supported by companies and a specific industry session allowed sponsors to present their products, innovations and future perspective in this field.

Implementation of dosimetry for molecular radiotherapy; results from a European survey

PURPOSE

The use of molecular radiotherapy (MRT) has been rapidly evolving over the last years. The aim of this study was to assess the current implementation of dosimetry for MRTs in Europe.

METHODS

A web-based questionnaire was open for treating centres between April and June 2022, and focused on 2020-2022. Questions addressed the application of 16 different MRTs, the availability and involvement of medical physicists, software used, quality assurance, as well as the target regions for dosimetry, whether treatment planning and/or verification were performed, and the dosimetric methods used.

RESULTS

A total of 173 responses suitable for analysis was received from centres performing MRT, geographically distributed over 27 European countries. Of these, 146 centres (84 %) indicated to perform some form of dosimetry, and 97 % of these centres had a medical physicist available and almost always involved in dosimetry. The most common MRTs were 131I-based treatments for thyroid diseases and thyroid cancer, and [223Ra]RaCl2 for bone metastases. The implementation of dosimetry varied widely between therapies, from almost all centres performing dosimetry-based planning for microsphere treatments to none for some of the less common treatments (like 32P sodium-phosphate for myeloproliferative disease and [89Sr]SrCl2 for bone metastases).

CONCLUSIONS

Over the last years, implementation of dosimetry, both for pre-therapeutic treatment planning and post-therapy absorbed dose verification, increased for several treatments, especially for microsphere treatments. For other treatments that have moved from research to clinical routine, the use of dosimetry decreased in recent years. However, there are still large differences both across and within countries.

The contest between internal and external-beam dosimetry: The Zeno's paradox of Achilles and the tortoise

Radionuclide therapy, also called molecular radiotherapy (MRT), has come of age, with several novel radiopharmaceuticals being approved for clinical use or under development in the last decade. External beam radiotherapy (EBRT) is a well-established treatment modality, with about half of all oncologic patients expected to receive at least one external radiation treatment over their disease course. The efficacy and the toxicity of both types of treatment rely on the interaction of radiation with biological tissues. Dosimetry played a fundamental role in the scientific and technological evolution of EBRT, and absorbed doses to the target and to the organs at risk are calculated on a routine basis. In contrast, in MRT the usefulness of internal dosimetry has long been questioned, and a structured path to include absorbed dose calculation is missing. However, following a similar route of development as EBRT, MRT treatments could probably be optimized in a significant proportion of patients, likely based on dosimetry and radiobiology. In the present paper we describe the differences and the similarities between internal and external-beam dosimetry in the context of radiation treatments, and we retrace the main stages of their development over the last decades.