Prospective SPECT-CT Organ Dosimetry-Driven Radiation-Absorbed Dose Escalation Using the In-111 (111In)/Yttrium 90 (90Y) Ibritumomab Tiuxetan (Zevalin®) Theranostic Pair in Patients with Lymphoma at Myeloablative Dose Levels

Patient-specific biokinetics and hybrid 2D/3D approach integration in OEDIPE software: Application to radioiodine therapy

BACKGROUND

The progression of targeted radionuclide therapy requires the development of dosimetry software accounting for patient-specific biokinetics. New functionalities were thus developed in the OEDIPE software, to deal with multiple 3D images or multiple planar images and a SPECT image.

MATERIEL & METHOD

Methods were implemented to recover patient biokinetics in volumes of interest. If several 3D SPECT images are available, they are registered to a reference CT scan. When several planar images and a single SPECT are available, the planar images are registered to the SPECT and counts of the planar images converted to activity. To validate these developments, six SPECT/CT and planar images of a Jaszczak phantom containing I-131 were acquired at different dates. Cumulated activity was estimated in each sphere using the SPECT/CT images only or the planar series associated to one SPECT/CT. Biokinetics and doses in lesions and in the lungs of a patient treated with I-131 for differentiated thyroid cancer were then estimated using four planar images and a SPECT/CT scan. Whole-body retention data were used to compare the biokinetics obtained from the planar and SPECT data.

RESULTS

Activities and cumulated activities estimated using OEDIPE in the phantom spheres agreed well with the reference values for both approaches. Results obtained for the patient compared well with those derived from whole-body retention data.

CONCLUSION

The implemented features allow automatic evaluation of patient-specific biokinetics from different series of patient images, enabling patient-specific dosimetry without the need for external software to estimate the cumulated activities in different VOIs.

Use of imaging-based dosimetry for personalising radiopharmaceutical therapy of cancer

Theranostics - i.e., the combination of molecular imaging and radiopharmaceutical therapy of cancer targeting a common biological feature - is a rapidly expanding field owing the recent successes of novel radiopharmaceutical therapies, such as ¹⁷⁷Lu-based prostate-specific membrane antigen radioligand therapy of prostate cancer and peptide receptor radionuclide therapy of neuroendocrine tumours. Despite the ongoing technical developments in imaging-based dosimetry, the existence of tumour absorbed dose-efficacy and organ absorbed dose-toxicity relationships, as well as the high interpatient variability in absorbed doses per unit activity, radiopharmaceutical therapies are still mostly administered in a fixed-activity, one-size-fits-all fashion. This is at odds with the principles of radiation oncology, where the absorbed doses to tissues are prescribed and their delivery is carefully planned and controlled for each individual patient to maximise the clinical benefits. There is a growing body of clinical evidence that dosimetry-based radiopharmaceutical therapy allows to safely optimise tumour irradiation, which translates into improved clinical outcomes. In this narrative review, we will present the reported prospective clinical experience to date on the use of imaging-based dosimetry to personalise radiopharmaceutical therapies.

Radionuclides for Targeted Therapy: Physical Properties

A search in PubMed revealed that 72 radionuclides have been considered for molecular or functional targeted radionuclide therapy. As radionuclide therapies increase in number and variations, it is important to understand the role of the radionuclide and the various characteristics that can render it either useful or useless. This review focuses on the physical characteristics of radionuclides that are relevant for radionuclide therapy, such as linear energy transfer, relative biological effectiveness, range, half-life, imaging properties, and radiation protection considerations. All these properties vary considerably between radionuclides and can be optimised for specific targets. Properties that are advantageous for some applications can sometimes be drawbacks for others; for instance, radionuclides that enable easy imaging can introduce more radiation protection concerns than others. Similarly, a long radiation range is beneficial in targets with heterogeneous uptake, but it also increases the radiation dose to tissues surrounding the target, and, hence, a shorter range is likely more beneficial with homogeneous uptake. While one cannot select a collection of characteristics as each radionuclide comes with an unchangeable set, all the 72 radionuclides investigated for therapy—and many more that have not yet been investigated—provide numerous sets to choose between.

Dosimetric Approaches for Radioimmunotherapy of Non-Hodgkin Lymphoma in Myeloablative Setting

Radioimmunotherapy (RIT) is a safe and active treatment available for non-Hodgkin lymphomas (NHLs). In particular, two monoclonal antibodies raised against CD20, that is Zevalin (⁹⁰Y-ibritumomab-tiuxetan) and Bexxar (¹³¹I-tositumomab) received FDA approval for the treatment of relapsing/refractory indolent or transformed NHLs. RIT is likely the most effective and least toxic anticancer agent in NHLs. However, its use in the clinical setting is still debated and, in case of relapse after optimized rituximab-containing regimens, the efficacy of RIT at standard dosage is suboptimal. Thus, clinical trials were based on the hypothesis that the inclusion of RIT in myeloablative conditioning would allow to obtain improved efficacy and toxicity profiles when compared to myeloablative total-body irradiation and/or high-dose chemotherapy regimens. Standard-activity RIT has a safe toxicity profile, and the utility of pretherapeutic dosimetry in this setting can be disputed. In contrast, dose-escalation clinical protocols require the assessment of radiopharmaceutical biodistribution and dosimetry before the therapeutic injection, as dose constrains for critical organs may be exceeded when RIT is administered at high activities. The aim of the present study was to review and discuss the internal dosimetry protocols that were adopted for non-standard RIT administration in the myeloablative setting before hematopoietic stem cell transplantation in patients with NHLs.

Dosimetry in Clinical Radiopharmaceutical Therapy of Cancer: Practicality Versus Perfection in Current Practice

The use of radiopharmaceutical therapies (RPTs) in the treatment of cancers is growing rapidly, with more agents becoming available for clinical use in last few years and many new RPTs being in development. Dosimetry assessment is critical for personalized RPT, insofar as administered activity should be assessed and optimized in order to maximize tumor-absorbed dose while keeping normal organs within defined safe dosages. However, many current clinical RPTs do not require patient-specific dosimetry based on current Food and Drug Administration-labeled approvals, and overall, dosimetry for RPT in clinical practice and trials is highly varied and underutilized. Several factors impede rigorous use of dosimetry, as compared with the more convenient and less resource-intensive practice of empiric dosing. We review various approaches to applying dosimetry for the assessment of activity in RPT and key clinical trials, the extent of dosimetry use, the relative pros and cons of dosimetry-based versus fixed activity, and practical limiting factors pertaining to current clinical practice.