Quantitative performance evaluation of 124I PET/MRI lesion dosimetry in differentiated thyroid cancer

Detection and quantification of small and low-uptake lesions for differentiated thyroid carcinoma using non-time-of-flight iodine-124 PET/MRI

BACKGROUND

124-iodine (124I) is used for positron emission tomography (PET) diagnostics and therapy planning in patients with differentiated thyroid cancer (DTC). Small lesion sizes (<10 mm) and low 124I uptake are challenging conditions for the detection of DTC lymph node lesions.

PURPOSE

The aim of this study was to systematically investigate the lesion detectability and quantification performance under clinically challenging imaging conditions using non-time-of-flight (TOF) PET/magnetic resonance imaging (MRI) in the clinical context of radionuclide therapy planning of DTC patients.

METHODS

PET/MR measurements were performed on the Siemens Biograph mMR using a small lesion NEMA-like phantom (six glass spheres, diameters 3.7-9.7 mm). 60 min list-mode data were acquired for nine activity concentrations (AC) ranging from 25 kBq/mL to 0.25 kBq/mL using a sphere-to-background ratio of 20:1. PET list-mode data were divided into five timeframes (60, 30, 16, 8, and 4 min) and reconstructed using either ordered-subsets expectation maximization (OSEM) or OSEM+ point spread function (PSF) algorithm. For all reconstructions, the smallest detectable sphere size was investigated in a human observer study. Partial volume effect (PVE) corrected PET images (contour and oversize-based approach) were analyzed considering a ± 30% deviation range between imaged and true AC as acceptable. Clinical data of eight DTC patients with small lymph node lesions were evaluated to assess agreement between the PVE correction approaches.

RESULTS

Longer PET acquisition times, higher ACs, and PSF reconstructions resulted in improved PET image quality and overall improved lesion detectability. The smallest 3.7 mm sphere was only visible under the best imaging conditions. Using a typical clinical 124I whole-body PET/MRI protocol with an acquisition time of 8 min using OSEM reconstructions, all lesions of ≥ 6.5 mm in diameter could be detected and the quantification provided reliable results approximately above 5.0 kBq/mL. An accurate quantification of ACs in the 4.8 mm sphere was not feasible in this study. In the clinical evaluation of 10 lesions, a good agreement between oversize- and contour-based PVE corrections was observed (<15% deviation).

CONCLUSIONS

The results showed that a reliable quantification of 124I uptake with PET/MRI is feasible and, therefore, could be used to perform radioiodine pre-therapy lesion dosimetry and individualized therapy planning in DTC patients.

Impact of tissue-independent positron range correction on [68Ga]Ga-DOTATOC and [68Ga]Ga-PSMA PET image reconstructions: a patient data study

PURPOSE

The positron range effect can impair PET image quality of Gallium-68 (68Ga). A positron range correction (PRC) can be applied to reduce this effect. In this study, the effect of a tissue-independent PRC for 68Ga was investigated on patient data.

METHODS

PET/CT data (40 patients: [68Ga]Ga-DOTATOC or [68Ga]Ga-PSMA) were reconstructed using Q.Clear reconstruction algorithm. Two reconstructions were performed per patient, Q.Clear with and without PRC. SUVmax and contrast-to-noise ratio (CNR) values per lesion were compared between PRC and non-PRC images. Five experienced nuclear medicine physicians reviewed the images and chose the preferred reconstruction based on the image quality, lesion detectability, and diagnostic confidence.

RESULTS

A total of 155 lesions were identified. The PRC resulted in statistically significant increase of the SUVmax and CNR for soft tissue lesions (6.4%, p < 0.001; 8.6%, p < 0.001), bone lesions (14.6%, p < 0.001; 12.5%, p < 0.001), and lung lesions (3.6%, p = 0.010; 6.3%, p = 0.001). This effect was most prominent in small lesions (SUVmax: 12.0%, p < 0.001, and CNR: 13.0%, p < 0.001). Similar or better image quality, lesion detectability, and diagnostic confidence was achieved in PRC images compared to the non-PRC images as those assessed by the expert readers.

CONCLUSIONS

A tissue-independent PRC increased the SUVmax and CNR in soft tissue, bone, and lung lesions with a larger effect for the small lesions. Visual assessment demonstrated similar or better image quality, lesion detectability, and diagnostic confidence in PRC images compared to the non-PRC images.

Advances in Radioligand Theranostics in Oncology

Theranostics with radioligands (radiotheranostics) has played a pivotal role in oncology. Radiotheranostics explores the molecular targets expressed on tumor cells to target them for imaging and therapy. In this way, radiotheranostics entails non-invasive demonstration of the in vivo expression of a molecular target of interest through imaging followed by the administration of therapeutic radioligand targeting the tumor-expressed molecular target. Therefore, radiotheranostics ensures that only patients with a high likelihood of response are treated with a particular radiotheranostic agent, ensuring the delivery of personalized care to cancer patients. Within the last decades, a couple of radiotheranostics agents, including Lutetium-177 DOTATATE (177Lu-DOTATATE) and Lutetium-177 prostate-specific membrane antigen (177Lu-PSMA), were shown to prolong the survival of cancer patients compared to the current standard of care leading to the regulatory approval of these agents for routine use in oncology care. This recent string of successful approvals has broadened the interest in the development of different radiotheranostic agents and their investigation for clinical translation. In this work, we present an updated appraisal of the literature, reviewing the recent advances in the use of established radiotheranostic agents such as radioiodine for differentiated thyroid carcinoma and Iodine-131-labeled meta-iodobenzylguanidine therapy of tumors of the sympathoadrenal axis as well as the recently approved 177Lu-DOTATATE and 177Lu-PSMA for differentiated neuroendocrine tumors and advanced prostate cancer, respectively. We also discuss the radiotheranostic agents that have been comprehensively characterized in preclinical studies and have shown some clinical evidence supporting their safety and efficacy, especially those targeting fibroblast activation protein (FAP) and chemokine receptor 4 (CXCR4) and those still being investigated in preclinical studies such as those targeting poly (ADP-ribose) polymerase (PARP) and epidermal growth factor receptor 2.

Intra-Individual Comparison of 124I-PET/CT and 124I-PET/MR Hybrid Imaging of Patients with Resected Differentiated Thyroid Carcinoma: Aspects of Attenuation Correction

Simple Summary

This study evaluates the qualitative and quantitative differences between 124-iodine PET/CT and PET/MR in oncologic patients with differentiated thyroid carcinoma after thyroidectomy. The impact of improved MR-based attenuation correction (AC) using a bone atlas was analysed in PET/MR data. Despite different patient positioning and AC methods PET/CT and PET/MR provide overall comparable results in a clinical setting. The overall number of detected ¹²⁴I-active lesions and the measured average SUV_mean values for congruent lesions were higher for PET/MR when compared to PET/CT. The addition of bone to the MR-based AC in PET/MR slightly increased the SUV_mean values for all detected lesions.

Abstract

Background: This study evaluates the quantitative differences between 124-iodine (I) positron emission tomography/computed tomography (PET/CT) and PET/magnetic resonance imaging (PET/MR) in patients with resected differentiated thyroid carcinoma (DTC). Methods: N = 43 ¹²⁴I PET/CT and PET/MR exams were included. CT-based attenuation correction (AC) in PET/CT and MR-based AC in PET/MR with bone atlas were compared concerning bone AC in the head-neck region. AC-map artifacts (e.g., dentures) were noted. Standardized uptake values (SUV) were measured in lesions in each PET data reconstruction. Relative differences in SUV_mean were calculated between PET/CT and PET/MR with bone atlas. Results: Overall, n = 111 ¹²⁴I-avid lesions were detected in all PET/CT, while n = 132 lesions were detected in PET/MR. The median in SUV_mean for n = 98 congruent lesions measured in PET/CT was 12.3. In PET/MR, the median in SUV_mean was 16.6 with bone in MR-based AC. Conclusions: ¹²⁴I-PET/CT and ¹²⁴I-PET/MR hybrid imaging of patients with DTC after thyroidectomy provides overall comparable quantitative results in a clinical setting despite different patient positioning and AC methods. The overall number of detected ¹²⁴I-avid lesions was higher for PET/MR compared to PET/CT. The measured average SUV_mean values for congruent lesions were higher for PET/MR.

Comparing lesion detection efficacy and image quality across different PET system generations to optimize the iodine-124 PET protocol for recurrent thyroid cancer

Background

In recurrent differentiated thyroid cancer patients, detectability in ¹²⁴I PET is limited for lesions with low radioiodine uptake. We assess the improvements in lesion detectability and image quality between three generations of PET scanners with different detector technologies. The results are used to suggest an optimized protocol.

Methods

Datasets of 10 patients with low increasing thyroglobulin or thyroglobulin antibody levels after total thyroidectomy and radioiodine therapies were included. PET data were acquired and reconstructed on a Biograph mCT PET/CT (whole-body, 4-min acquisition time per bed position; OSEM, OSEM-TOF, OSEM-TOF+PSF), a non-TOF Biograph mMR PET/MR (neck region, 4 min and 20 min; OSEM), and a new generation Biograph Vision PET/CT (whole-body, 4 min; OSEM, OSEM-TOF, OSEM-TOF+PSF). The 20-min image on the mMR was used as reference to calculate the detection efficacy in the neck region. Image quality was rated on a 5-point scale.

Results

All detected lesions were in the neck region. Detection efficacy was 8/9 (Vision OSEM-TOF and OSEM-TOF+PSF), 4/9 (Vision OSEM), 3/9 (mMR OSEM and mCT OSEM-TOF+PSF), and 2/9 (mCT OSEM and OSEM-TOF). Median image quality was 4 (Vision OSEM-TOF and OSEM-TOF+PSF), 3 (Vision OSEM, mCT OSEM-TOF+PSF, and mMR OSEM 20-min), 2 (mCT OSEM-TOF), 1.5 (mCT OSEM), and 1 (mMR OSEM 4 min).

Conclusion

At a clinical standard acquisition time of 4 min per bed position, the new generation Biograph Vision using a TOF-based image reconstruction demonstrated the highest detectability and image quality and should, if available, be preferably used for imaging of low-uptake lesions. A prolonged acquisition time for the mostly affected neck region can be useful.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-021-00361-y.

Impact of prompt gamma coincidence correction on absorbed dose estimation in differentiated thyroid cancer using 124I PET/CT imaging

INTRODUCTION

Iodine-124 positron emission tomography/computed tomography (I PET/CT) is increasingly being used in the absorbed dose estimation in the radioiodine treatment of differentiated thyroid cancer (DTC). However, the produced prompt gamma coincidences (PGCs) associated with the I decay result in a bias in the absorbed dose estimation. The impact of a sinogram-based PGC correction approach on the absorbed dose estimation in I PET/CT DTC imaging is investigated.

METHODS

I phantom and patient measurements were performed on a Siemens Biograph mCT PET/CT system. All images were reconstructed with (PGCon) and without PGC correction (PGCoff). The phantom contained seven spheres (diameters: 6.6-37 mm). The spheres and background compartment were filled with a I solution, resulting in a low (9.4 : 1) and a high sphere-to-background activity concentration ratio (750 : 1). Sphere recovery coefficient (RC) values were determined. In addition, the impact of PGC correction on measured lesion uptake and calculated lesion-absorbed dose was assessed for 66 lesions identified in 24 DTC patients.

RESULTS

PGC correction systematically increased sphere RC values up to 71% for the smallest spheres. For the patient data, PGC correction significantly increased both the measured I uptake (P<0.005) and the calculated lesion-absorbed dose (P=0.008) by ∼3%. The percentage difference in the calculated lesion-absorbed dose ranged from -19% to 50%, showing that PGC correction had a variable and large impact for a few lesions.

CONCLUSION

PGC correction resulted in significantly higher sphere RC values, I lesion uptake values and estimated lesion-absorbed doses.

Factors Associated with Dose Determination of Radioactive Iodine Therapy for Differentiated Thyroid Cancer

Radioactive iodine (RAI) therapy for differentiated thyroid cancer has been successfully used for more than 70 years. However, there is still plenty of controversy surrounding the use and doses of radioiodine. There is insufficient evidence to answer the questions. Recent American Thyroid Association (ATA) guidelines seem to favor low-dose RAI, based on recent clinical trials and meta-analyses. However, long-term follow-up data remains limited, and there are additional factors we should consider that might affect the efficacy of RAI therapy. Therefore, until sufficient data are available, it is necessary to remain cautious about determining RAI doses by considering multiple patient-specific variables.

Quantitative performance of 124I PET/MR of neck lesions in thyroid cancer patients using 124I PET/CT as reference

BACKGROUND

In patients with differentiated thyroid cancer (DTC), serial 124I PET/CT imaging is, for instance, used to assess the absorbed (radiation) dose to lesions. Frequently, the lesions are located in the neck and they are close to or surrounded by different tissue types. In contrast to PET/CT, MR-based attenuation correction in PET/MR may be therefore challenging in the neck region. The aim of this retrospective study was to assess the quantitative performance of 124I PET/MRI of neck lesions by comparing the MR-based and CT-based 124I activity concentrations (ACs). Sixteen DTC patients underwent PET/CT scans at 24 and 120 h after administration of about 25 MBq 124I. Approximately 1 h before or after PET/CT examination, each patient additionally received a 24-h PET/MR scan and sometimes a 120-h PET/MR scan. PET images were reconstructed using the respective attenuation correction approach. Appropriate reconstruction parameters and corrections were used to harmonize the reconstructed PET images to provide, for instance, similar spatial resolution. For each lesion, two types of ACs were ascertained: the maximum AC (max-AC) and an average AC (avg-AC). The avg-AC is the average activity concentration obtained within a spherical volume of interest with a diameter of 7 mm, equaling the PET scanner resolution. For each type of AC, the percentage AC difference between MR-based and CT-based ACs was determined and Lin's concordance correlation analysis was applied. Quantitative performance was considered acceptable if the standard deviation was ± 25% (precision), and the mean value was within ± 10% (accuracy).

RESULTS

The avg-ACs (max-ACs within parentheses) of 74 lesions ranged from 0.20 (0.33) to 657 (733) kBq/mL. Excluding two lesions with ACs of approximately 1 kBq/mL, the mean (median) ± standard deviation (range) was - 4% (- 5%) ± 14% (- 28 to 29%) for the avg-AC and - 9% (- 11%) ± 14% (- 33 to 33%) for the max-AC. Lin's concordance correlation coefficients were ≥ 0.97, indicating substantial AC agreement.

CONCLUSIONS

Quantification of lesions in the neck region using 124I PET/MR showed acceptable quantitation performance to 124I PET/CT for AC above 1 kBq/mL. The PET/MRI-based 124I ACs in the neck region can be therefore reliably used in pre-therapy dosimetry planning.