[68 Ga]Ga-DOTATATE PET/CT is now recognised as the most sensitive functional imaging modality for the diagnosis of well-differentiated neuroendocrine tumours (NET) and can inform treatment with peptide receptor radionuclide therapy with [177Lu]Lu-DOTATATE. However, somatostatin receptor (SSTR) expression is not unique to NET, and therefore, [68 Ga]Ga-DOTATATE PET/CT may have oncological application in other tumours. Molecular profiling of gastrointestinal stromal tumours that lack activating somatic mutations in
To investigate (i) the diagnostic role of [68 Ga]Ga-DOTATATE PET/CT; and, (ii) to investigate the potential of this imaging modality to guide treatment with [177Lu]Lu-DOTATATE in patients with wtGIST.
[68 Ga]Ga-DOTATATE PET/CT was performed on 11 patients with confirmed or metastatic wtGIST and one patient with a history of wtGIST and a mediastinal mass suspicious for metastatic wtGIST, who was subsequently diagnosed with a metachronous mediastinal paraganglioma. Tumour expression of somatostatin receptor subtype 2 (SSTR2) using immunohistochemistry was performed on 54 tumour samples including samples from 8/12 (66.6%) patients who took part in the imaging study and 46 tumour samples from individuals not included in the imaging study.
[68 Ga]Ga-DOTATATE PET/CT imaging was negative, demonstrating that liver metastases had lower uptake than background liver for nine cases (9/12 cases, 75%) and heterogeneous uptake of somatostatin tracer was noted for two cases (16.6%) of wtGIST. However, [68 Ga]Ga-DOTATATE PET/CT demonstrated intense tracer uptake in a synchronous paraganglioma in one case and a metachronous paraganglioma in another case with wtGIST.
Our data suggest that SSTR2 is not a diagnostic or therapeutic target in wtGIST. [68 Ga]Ga-DOTATATE PET/CT may have specific diagnostic utility in differentiating wtGIST from other primary tumours such as paraganglioma in patients with sporadic and hereditary forms of wtGIST.
Luigi Aloj and Olivier Giger contributed equally to this study and share first authorship
Gastrointestinal stromal tumours (GIST) are the most common mesenchymal tumours of the gastrointestinal tract, and the majority of adult onset GIST, so-called ‘tyrosine kinase mutant’ GIST (TK-mutant GIST), are driven by activating somatic mutations in the
While clinical behaviour of wtGIST is generally indolent, there are no effective treatments for patients with more aggressive progressing disease. Management of TK-mutant GIST has been greatly improved by the introduction of the tyrosine kinase inhibitor imatinib which provides an effective standard of care treatment in this setting [
Previous studies have reported high-level expression of the somatostatin receptor 2 (SSTR2) subtype in GIST tumours with unknown mutational status [
Based on this information, it is possible that SSTR PET/CT may be relevant in the clinical management of patients with wtGIST. This study aims to investigate the diagnostic role of [68 Ga]Ga-DOTATATE PET/CT in patients with wtGIST and to assess the potential for theranostic application of the 177Lu/68 Ga-DOTATATE pair in this setting.
Patients were recruited from the National Paediatric and Adult wild-type GIST (PAWS GIST UK), and clinical genetics clinic at Cambridge University Hospital NHS Foundation Trust. Details of clinical phenotype, family history and germline molecular testing results were collated from patient records. All participants provided written informed consent. The study was approved by the East of England South Cambridge Research Ethics Committee (REC ID 14/EE/1059
Patients were administered 250 (± 10%) MBq of [68 Ga]Ga-DOTATATE. Scanning was started after urinary bladder emptying and 60 min after radiopharmaceutical injection on a General Electric Discovery 690 scanner (GE Healthcare, Milwaukee, WI, USA), using 4 min per bed position and low dose CT for attenuation correction and localisation. The ordered subsets expectation maximisation (OSEM) algorithm (2 interations, 24 subsets) was used to reconstruct the emission images with time-of-flight modelling. Emission data were corrected for decay, dead time and random coincidences and normalised for injected dose and patient body weight. Studies were viewed on a Xeleris 4 (GE) workstation for clinical review, and region of interest analysis was performed for quantitative assessment. Maximum standardised uptake values (SUVmax) were recorded for tumour lesions as well as for normal liver. Lesion to liver uptake ratios was calculated for each case. Lesions with an uptake ratio < 1 were considered to have low/no receptor expression, a ratio of 1–2 was considered equivocal, and a ratio > 2 was defined as representing high receptor expression [
SSTR2 immunohistochemistry (IHC) was performed on 2 µm sections of paraffin embedded tumour tissue using a rabbit monoclonal anti-SSTR2 antibody (Abcam, catalogue number ab134152) at a dilution of 1:600. Neuroendocrine cells in gastric mucosa, normal adrenal tissue and a well-differentiated pancreatic neuroendocrine tumour were used as a positive control for SSTR2. Adjacent normal tissue was used as an internal positive control. All IHC results were reviewed by an experienced pathologist (OG) and reported using the following scoring system: 0 = no membranous staining; 1 = < 25% of cells show membranous staining; 2 = > 25% of cells show membranous staining; 3 = > 75% of cells show membranous staining. Data outlining GIST-specific immunohistochemistry markers for the 11 GIST cases included in the imaging study are provided in Additional file Clinical and molecular features of [68 Ga]Ga-DOTATATE PET/CT imaging cohort Case Sex Age Primary tumour site Metastases SDH status Germline mutation Tumour epi-mutation Other tumours 001 F 21 Gastric Liver nodal peritoneal dSDH No No 002 M 37 Gastric Liver, peritoneal dSDH No Carotid PGL 003 F 31 Gastric Liver, peritoneal dSDH No No 004 M 39 Gastric Liver, peritoneal dSDH No No No 005 F 15 Gastric Liver, peritoneal dSDH Yes No 006 M 68 Gastric Liver dSDH No No 007 M 21 Gastric Liver dSDH No No 008 F 33 Gastric Liver dSDH No NA No 009 M 73 Gastric Liver nodal dSDH Variant of uncertain significance in No No 010 M 60 Small bowel Liver cSDH No No No 011 M 63 Small bowel Liver cSDH No No No 012 F 15 Gastric Mediastinal mass dSDH No Mediastinal mass NA = not available
DNA was extracted from peripheral blood samples according to standard protocols. Next generation sequencing of a clinical gene panel that included
This was performed on DNA extracted from paraffin embedded tumour tissue and adjacent normal tissue using a pyrosequencing method previously described [
Standard of care imaging was reviewed alongside the [68 Ga]Ga-DOTATATE PET/CT for each patient. Eight patients had CT imaging (for two this was the registration CT of the [68 Ga]Ga-DOTATATE PET/CT), three patients had MRI, and one patient had CT and MRI at the time of the [68 Ga]Ga-DOTATATE PET/CT study. Six patients also had [18F]FDG PET/CT studies for comparison.
Statistical analysis was performed using MedCalc (version 18.2.1). A mean and standard deviation was calculated for all continuous variables. An unpaired student t test was employed to investigate differences between groups.
Eleven patients with a diagnosis of metastatic wtGIST and one patient with a mediastinal mass and suspected metastatic wtGIST (case 012) were recruited for [68 Ga]Ga-DOTATATE PET/CT imaging (Table
Tumour tissue for SSTR2 IHC was performed on 54 tumour samples and this included tumour samples from 8/12 (66.6%) patients who had [68 Ga]Ga-DOTATATE PET/CT imaging and 47 tumour samples from individuals who did not have [68 Ga]Ga-DOTATATE PET/CT imaging.
SSTR2 immunohistochemistry was performed on tumour samples from eight of the patients who underwent [68 Ga]Ga-DOTATATE PET/CT (patient 002, 003, 004, 005, 006, 007, 009 and 010). Single isolated tumour cells showed immunoreactivity in patient 009 (score 1); faint membranous staining (score 1) was seen in patients 005 and 007. Tumours from patients 002, 004, 003, 006 and 010 were negative for SSTR2 and were assigned a score of 0 (Table [68 Ga]Ga-DOTATATE PET/CT scan and IHC findings Case SUVmax of most avid lesion Location Clinical report Lesion to liver ratio SSTR2 IH score 001 3 Liver Negative < 1 NA 002 13 Liver Equivocal (GIST lesion) 2 0 60 Carotid PGL Positive (PGL) > 2.5 (9.6) NA 003 10 Liver Equivocal 1.5 0 004 2 Liver Negative < 1 0 005 3 Liver Negative < 1 1 006 12 Liver Negative < 1 0 007 4 Liver Negative < 1 1 008 4 Liver Negative < 1 NA 009 10 Liver, nodal Negative < 1 1 010 2.5 Liver Negative < 1 0 011 2.8 Liver Negative < 1 NA 012 45 Mediastinal mass Positive > 2.5 [ NA 40 Mediastinal node Positive > 2.5 [ NA = not available Images Panel A shows an axial fused [18F]FDG PET/CT image of the liver with a maximum intensity projection (MIP) image demonstrating highly FDG-avid liver and nodal metastases in case 001. Panel B shows the corresponding (case 001) trans-axial fused image of the [68 Ga]Ga-DOTATATE PET/CT and MIP image demonstrating low tracer uptake in the liver metastases (SUVmax 3) compared to background liver. Panel C, fused transaxial [68 Ga]Ga-DOTATATE PET/CT image and corresponding MIP image showing non-avid liver metastases in case 010 (SUVmax 2.5). Panel D shows negative SSTR2 expression of a liver metastasis on IHC for case 010
The 46 tumour samples from individuals who did not have [68 Ga]Ga-DOTATATE PET/CT imaging included; 14 dSDH wtGIST, 9 cSDH wtGIST and 23 TK-mutant GIST. The mean SSTR2 expression score was 0.73 for the dSDH wtGIST (8/14 had a score of 0, 3/14 had a score of 1, one case had a score of 2 and two cases had a score of 3) versus 0.22 in the cSDH wtGIST (7/9 had a score of 0 and 2/9 had a score of 1) and 0.22 in the TK-mutant GIST (19/23 had a score of 0, 3/23 had a score of 1 and one case had a score of 2). The median score was 0 for each molecular subgroup of GIST indicating low or absent expression of SSTR2 in the tumour tissue in all GIST tumours reviewed. No significant difference in the mean SSTR2 expression score was demonstrated between molecular sub-groups of GIST (
High-quality [68 Ga]Ga-DOTATATE PET/CT imaging was obtained for all twelve patients included in this study. Imaging findings are summarised in Table
In four cases, lesions showed [68 Ga]Ga-DOTATATE uptake which was equal to or above that of background liver. In one case (case 012), the mediastinal mass suspicious for a metastatic recurrence of a previous wtGIST was subsequently confirmed as a mediastinal paraganglioma explaining the high [68 Ga]Ga-DOTATATE uptake in this lesion. Similarly for case 002, where several liver lesions showed uptake slightly higher than liver, this patient had a synchronous carotid paraganglioma, and therefore, we cannot be certain that the liver metastases were all related to the wtGIST and not the co-existing paraganglioma as repeat biopsy was not performed and the patient is now deceased (Fig. Case 002. Panel A, transaxial fused [18F]FDG PET/CT image of the liver and MIP image demonstrating FDG-avid liver, peritoneal and nodal metastases as well as a synchronous left sided carotid paraganglioma (small arrow). Panel B, [68 Ga]Ga-DOTATATE PET/CT MIP image, demonstrating that few liver metastases show uptake higher than background liver (SUVmax 13, large arrow, vs 6.5 for background liver). It should be noted that not all liver lesions were biopsied so it remains unclear if all liver lesions were metastatic wtGIST deposits or if the carotid paraganglioma was also metastatic to the liver. The left sided carotid paraganglioma shows very intense uptake of the somatostatin analogue (SUVmax 60, small arrow). Panel C, negative SSTR2 expression on IHC in a biopsied dSDH wtGIST liver metastasis from case 002 Case 012. Panel A, [18F]FDG PET/CT MIP image and fused transaxial views showing a highly avid mediastinal mass and low-level uptake of an adjacent lymph node (C). This imaging was performed as part of surveillance for disease recurrence given a background of a resected wild-type GIST, and the mediastinal mass was a presumed recurrence of this earlier tumour. Panel B, 68 Ga-DOTATATE PET/CT, same views as above. Very intense uptake in the mediastinal mass (SUVmax 45) and adjacent lymph node (SUVmax 40). This patient had normal plasma metanephrine and 3-methoxytyramine values on biochemical testing. Biopsy confirmed a non-secreting mediastinal dSDH paraganglioma case 012
A negative or equivocal scan report for 11/12 patients with metastatic wtGIST correlated with the low or absent expression of SSTR2 ex vivo (Table
To our knowledge, this is the first study to investigate the clinical utility of [68 Ga]Ga-DOTATATE PET/CT in patients with wtGIST. In this study, SSTR2 expression was demonstrated to be low or absent for each of the 11 cases of metastatic wtGIST analysed using [68 Ga]Ga-DOTATATE PET/CT and in tumour samples from 8/12 patients using SSTR2 IHC. Notably, SSTR2 expression was also found to be low or absent in an additional 46 tumour samples, including 23 TK-mutant GIST and an additional 23 wtGIST samples. A previous study investigating the expression of SSTR1-5 in TK-mutant GIST demonstrated expression of SSTR1-5 using both quantitative real-time polymerase chain reaction (qPCR) and IHC. In this earlier study, the expression of SSTR2 was lower than other SSTR subtypes and no wtGISTs were included in the analysis [
Alternative molecular targets are being investigated for radionuclide-based theranostics in GIST such as the gastrin releasing peptide/bombesin receptor (GRPR). PET imaging with a 68 Ga-labelled GRPR agonist identified lesions in 7/17 patients with GIST [
One indication for [68 Ga]Ga-DOTATATE PET/CT is to confirm the diagnosis in patients with anatomic lesions that are suspicious for NET on conventional imaging [
The conventional radiological measurement of tumour response including uni- or bidimensional changes in tumour size (response evaluation criteria in solid tumours, RECIST) is routinely applied in clinical practice for both GIST and wtGIST using CT or MRI. The benefit of evaluating metabolic response using [18F]FDG PET/CT is well demonstrated in
Patients with dSDH wtGIST due to a germline variant in
In this study, [68 Ga]Ga-DOTATATE PET/CT imaging was negative for nine patients and equivocal for two patients with metastatic wtGIST. However, the uptake level in a synchronous carotid PPGL (SUVmax 60) in case 002 and a metachronous mediastinal PPGL in case 012 (SUVmax 45) was almost ten-fold higher than liver reference in these individuals and 9–tenfold higher than the average uptake seen in metastases of 11 cases of wtGIST (mean SUVmax 6). These results suggest that [68 Ga]Ga-DOTATATE PET/CT imaging may have specific utility for differentiating PPGL from wtGIST in clinical practice in patients with a genetic predisposition to develop both tumour types.
Our results address conflicting reports in the literature on the role of SSTR2 in wtGIST and indicate that this receptor system is not a viable diagnostic or therapeutic target. We show that [68 Ga]Ga-DOTATATE PET/CT may have an important diagnostic role in identifying and differentiating PPGL lesions from metastatic wtGIST in patients carrying genetic mutations predisposing to both conditions.
We thank the staff at the Tissue Bank and the staff in the PET/CT and Radiopharmacy Departments in Cambridge University Hospital Foundation Trust for their assistance with this study.
RC, LA, OG, BC, FG, IH, HC, IM, RTH, MM, EM, BA, MR, RJ, PD, DP and VRB were involved in data collection, LA, IM, IH, HC, OG, RC and RTH were involved in data analysis. RC, LA, BC and OG were involved in drafting of the manuscript. RC was responsible for study design and concept. All authors read and approved the final manuscript.
This study was funded by; GIST Support UK (RC, OG), NIHR Senior Investigator Award (ERM), European Research Council Advanced Researcher Award (ERM), The University of Cambridge has received salary support in respect of EM from the NHS in the East of England through the Clinical Academic Reserve. The views expressed are those of the authors and not necessarily those of the NHS or Department of Health. This work was supported by the Cancer Research UK Cambridge Centre at the University of Cambridge (Grant Code: A25177).
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All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. This article does not contain any studies with animals performed by any of the authors.
Consent for publication was obtained from all individual participants included in the study.
BGC is an employee of AstraZeneca; none of the other authors have a conflict of interest.
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