Richard P. Baum and Harshad R. Kulkarni
THERANOSTICS Center for Molecular Radiotherapy and Molecular Imaging, ENETS Center of Excellence, Zentralklinik Bad Berka, Germany
4.2
PET positron emission tomography pNETs pancreatic NETs
PRRT peptide receptor radionuclide therapy
SI‐NETs small intestinal (ileum/jejunum/duodenum) NETs SMS somatostatin
SSTR somatostatin receptor SUV standardized uptake value SUV T/S SUVmax tumor‐to‐spleen ratio TACE transarterial chemoembolization USG ultrasonography
WHO World Health Organization
introduction
Neuroendocrine tumors (NETs) are heterogeneous group of tumors originating from pluripotent stem cells or differentiated neuroendocrine cells. The hallmark of NETs is the expression of somatostatin receptors (SSTRs), which forms the ratio-nale for the use of radiolabeled somatostatin (SMS) analogues for imaging and therapy. Another unique feature is their endocrine metabolism, that is, decarboxylation of amine precursors, hence previously referred to as amine precursor uptake and decarboxylation (APUD)‐omas. NETs occur predominantly in the lungs and the gastroenteropancreatic system (GEP). Taking into account the complex and diverse histology of NETs and to allow optimal prognostic stratification, a new system of classification was devised in 2010 by the WHO (Table 4.2.1) [1].
The heterogeneous nature, the indolent course, and the possibility of multiple and variable anatomic site of primary make it difficult to evaluate patients with NETs. The clinical manifestations due to the secretion of a wide range of biogenic amines typify NETs. However, they do not provide adequate information to allow the clinician to decide upon a treatment regime, which therefore requires imaging.
tAble 4.2.1 WHo 2010 classification of net with corresponding values for mitoses per 10 high‐power fields (Hpf) and proliferation rate (as Ki‐67/Mib‐1 index in %), determining the grade (g) of net
Categories Mitoses (/10HPf) Ki‐67/MIB‐1 index (%)
Neuroendocrine tumor
g1 <2 ≤2
g2 2–20 3–20
Neuroendocrine carcinoma
g3 >20 >20
Mixed adenoneuroendocrine carcinoma (MANEC)
Hyperplastic and preneoplastic lesion
SMS ANALOGUES fOR PET/CT IMAGING 57
Computed tomography (CT) scans, magnetic resonance imaging (MRI), and endoscopic ultrasonography (USG) (EUS) are the morphological imaging modalities in the diagnostics of NETs. However, these do not give the functional status of the tumor, which is often essential for defining the prognosis, as molecular changes precede the morphologic changes. One of the biggest disadvantages of USG is its operator dependency. It also fails to differentiate liver metastases of NETs from other type of liver metastases. Hypervascularity, one of the most common features of NET liver metastases, can be very well documented using Doppler techniques and contrast‐
enhanced USG. However, these hypervascular solitary lesions may be misdiagnosed as hemangiomas. A combination of functional and morphological imaging often helps in initial diagnosis and staging, deciding upon the treatment regime, and mon-itoring therapy response [2–4].
The discovery of overexpression of SSTRs for peptide hormones in NETs, more than two decades ago, has revolutionized the role of nuclear medicine in the diag-nosis and therapy of NETs [5, 6]. This was indeed paralleled by the development of various radiopharmaceuticals targeting these tumor‐related receptors, as well as various metabolic pathways peculiar to the NET cells [7].
Over the past two decades, 111In‐DTPA‐octreotide scintigraphy has been the fore-most functional imaging in the diagnostics of NETs [3, 8].
More recently, molecular imaging using diverse positron emission tomography (PET) radiopharmaceuticals has gained popularity in the diagnostic workup of patients with NET [9–11]. In addition, the amalgamation of PET with CT (PET/CT) enables fast and high‐resolution functional imaging with more accurate anatomical localization.
With the ever‐growing list of PET tracers currently being employed for the imaging of NETs, it is essential to first describe their respective molecular targets so as to understand and compare the results of this wide range of PET radiopharmaceu-ticals (Table 4.2.2). The potential molecular events/targets that can be currently tar-geted by PET‐based radiopharmaceuticals are:
a) SSTR expression
b) Serotonin production pathway c) Biogenic amine storage d) Catecholamine transport e) Glucose metabolism
f) Miscellaneous peptide receptor expression
sMs AnAlogues for pet/ct iMAging
SMS is a cyclic peptide hormone with primary action of inhibition of hormone secretion and modulation of neurotransmission and cell proliferation through specific membrane‐bound G‐protein‐coupled receptors. These SSTRs, which are also normally expressed in different organs such as the pituitary, thyroid, adrenals,
spleen (activated lymphocytes), kidneys, and gastrointestinal tract in different quan-tities, have generated immense clinical interest due to their expression on various tumor types. This offers the potential of labeling SMS and its analogues with differ-ent radionuclides for imaging and also for therapy. The advantages of small peptides are better pharmacokinetic characteristics and no (or very low) antigenicity as compared to antibodies, making them nearly ideal ligands for receptor‐based radio-nuclide imaging.
The basis of peptide receptor imaging using radiolabeled SMS is the overexpres-sion in NETs of SSTRs [5]. There are five different types of SSTR proteins, which have been cloned (SSTRs 1 through 5); SSTR2 consists of two subtypes, SSTR2A and SSTR2B. Though most of the tumors predominantly express SSTR2, it has been demonstrated that SSTR1, SSTR3, SSTR4, and SSTR5 are also expressed on many tAble 4.2.2 diagnostic pet radiopharmaceuticals for net
Radiopharmaceutical Receptor/metabolic target Indication and comments
18f‐fDG Glycolytic pathway All NETs. Observation of flip‐flop mechanism with SMS‐R PET
68Ga‐DOTATOC Somatostatin receptor (high affinity for SSTR2)
68Ga‐DOTATATE Somatostatin receptor (highest affinity for SSTR2)
11C‐5‐HTP Serotonin production pathway All serotonin‐producing NETs
11C‐DOPA Dopamine production pathway Pheochromocytoma,
paraganglioma, neuroblastoma;
short half‐life, cost of production, and difficulty in obtaining 11C major obstacle
18f‐DOPA Dopamine production pathway Pheochromocytoma,
paraganglioma, neuroblastoma, glomus tumor
18f‐fDA Catecholamine precursor Pheochromocytoma,
paraganglioma, neuroblastoma
64Cu‐TETA‐octretoide Somatostatin receptor All SSTR +ve NETs
18f‐fP‐Gluc‐TOCA Somatostatin receptor All SSTR +ve NETs
11C‐Ephidrine Catecholamine transporter Pheochromocytoma,
neuroblastoma, study of the sympathetic nervous system
11C‐Hydroxyephidrine Catecholamine transporter Pheochromocytoma,
neuroblastoma, study of the sympathetic nervous system
SSTR PET/CT USING 68Ga 59
tumors with varying percentage of expression [12]. The prevalence of expression of SSTRs by NET of midgut origin was found to be maximum for SSTR2 (95%), followed by SSTR1 (80%) and SSTR5 (75%).
111In‐DTPA‐D‐Phe1‐octreotide (111In‐pentetreotide; OctreoScan, Mallinckrodt, Inc., St. Louis, Missouri) was the first radiolabeled SMS analogue to be approved for scintigraphy of NETs and has been shown to be well suited for the scintigraphic localization of primary and metastatic NET [13, 14]. 99mTc (technetium) has also been labeled with SMS analogues to enable conventional nuclear medicine imaging [15–18]. 99mTc‐EDDA/HYNIC‐TOC has been demonstrated to be promising for the detection of SSTR‐positive tumors and metastases [19].
DOTA‐d‐Phe1‐Tyr3‐octreotide (DOTATOC) and DOTA‐d‐Phe1‐Tyr3‐Thr8‐ octreotide (DOTATATE) have a very high affinity for the SSTR2, low or negligible affinity for SSTR3 and SSTR5, and no significant affinity for SSTR1 and SSTR4 [20, 21]. The development of this next generation of SMS analogues opened the prospect for convenient radiolabeling with 68Ga for PET imaging [22]. DOTA‐1‐NaI3‐ octreotide (DOTANOC) was developed by amino acid exchange at position 3 of octreotide as a pansomatostatin analogue, covering a broader spectrum of SSTRs.
This compound has not only a higher affinity to SSTR3 and SSTR5 but also binds more avidly to SSTR2 [23].
fourth‐generation analogues to be have been studied preclinically are (DOTA‐1‐
NaI3, Thr8)‐octreotide (DOTA‐NOC‐ATE) and (DOTA‐BzThi3, Thr8)‐octreotide (DOTA‐BOC‐ATE) [24]. They have been shown to have very high affinity for SSTR2, SSTR3, and SSTR5 and intermediate high affinity to SSTR4. SSTR antago-nists [NH(2)‐CO‐c(DCys‐Phe‐Tyr‐DAgl(8)(Me,2‐naphthoyl)‐Lys‐Thr‐Phe‐Cys)‐
OH (SST(3)‐ODN‐8) and (SST(2)‐ANT)] have also been labeled with 111In, and their superiority over SSTR agonists (in murine models) for in vivo targeting of SSTR2‐
and SSTR3‐rich tumors has resulted in the shift in paradigm, and they are now being contemplated for use in tumor diagnosis [25]. Recently, the SSTR antagonist 111In‐
DOTA‐BASS has been demonstrated to have a favorable human biodistribution in five patients with metastatic thyroid carcinoma or neuroendocrine neoplasms [26].
sstr pet/ct using 68ga
68Ga is a diagnostic trivalent radiometal and is feasible for labeling with SMS ana-logues (DOTATOC, DOTATATE, or DOTANOC) with the help of chelator DOTA [27]. 68Ga is prepared from a TiO2‐based 68Ge/68Ga generator system, which has a half‐life of 288 days [28]. A GMP‐compliant, fully automated click‐and‐start cassette‐
based synthesis system with easy handling is now available (EZAG, Berlin, Germany) for the daily routine production of 68Ga‐labeled radiopharmaceuticals. Postprocessing of 68Ge/68Ga radionuclide generators using cation exchange resin provides chemi-cally and radiochemichemi-cally pure 68Ga (97 ± 2%) within a few minutes, ready for on‐site labeling with high overall product yields.
The most consequential feature of PET/CT is its ability to quantify the disease at a molecular level. In addition, superior resolution gives it a distinct edge over SPECT/CT
using gamma‐emitting radionuclides like Tc‐99m. The recent tremendous increase in the number of diagnostic imaging studies with 68Ga has indeed demonstrated its potential to become the Tc‐99m for PET/CT. Apart from detection of primary and metastatic disease (staging), assessment of molecular response to therapy, and long‐term follow‐up, PET/CT using 68Ga‐labeled SMS analogues like DOTATOC, DOTATATE, and DOTANOC (SSTR PET/CT) also helps to select patients who are likely to benefit from peptide receptor radionuclide therapy (PRRT) using the same analogue labeled with a beta emitter like 177Lu or 90Y. The successful use of 68Ga and
177Lu/90Y, respectively, for diagnosis and radionuclide therapy using the same peptide targeting SSTRs, has demonstrated that THERANOSTICS of neuroendocrine neo-plasms is already a fact today and not a fiction.
iMAging protocol
The guidelines for 68Ga‐SSTR PET/CT have been outlined [29]. Sandostatin LAR injections must be stopped 3–4 weeks prior to the scan, and subcutaneous (s.c.) treatment with octreotide should be stopped at least 1 day before. However, there are some centers that do not recommend stopping Sandostatin injection before imaging.
Care is taken for proper hydration of the patient. Just prior to the acquisition, the patient should be requested to void. Use of oral contrast media is recommended. The maximum tumor activity is reached within 70 ± 20 min after injection. PET/CT acqui-sition should start at 45–90 min (depending upon the radiolabeled analogues) after intravenous injection of approximately 120 MBq of the radiolabeled peptides. In order to increase renal elimination and to reduce radiation exposure to the urinary bladder, furosemide may be given at the time of injection of 68Ga‐DOTANOC. In order to use the full potential of the modern PET/CT cameras equipped with multislice CT, a three‐phase CT should be performed.
diAgnosis, stAging, And restAging
In a recent study in normal human tissues, expression of SSTR2 at the level of mRNA was found to correlate with the SUVmax obtained from 68Ga‐DOTATOC PET/CT [30]. Another recent study provided for the first time the proof of concept of the utility of SSTR PET/CT for quantification of the SSTR density on tumor cells: a close correlation between maximum SUV and immunohistochemical scores used for the quantitative assessment of the density of subtypes of SSTR in NET tissue [31].
This underlines the crucial role of molecular imaging of the SSTR expression by PET/CT using 68Ga.
68Ga‐DOTATOC has been demonstrated to be superior to 111In‐octreotide SPECT (CT was taken as the reference for comparison) in detecting upper abdominal metas-tases more than 10 years ago [22]. In a recent study, 68Ga‐DOTATOC PET/CT was proven to be superior to 111In‐octreotide in the detection of NET metastases in the lung and skeletal system and similar for the detection of NET metastases in the liver
DIAGNOSIS, STAGING, AND RESTAGING 61
and brain [32]. On a patient basis, the accuracy of 68Ga‐DOTATOC PET (96%) was found to be significantly higher than that of CT (75%) and 111In‐DOTATOC SPECT (58%) [33]. In 32/88 patients, 68Ga‐DOTATOC PET was not only able to detect more lesions than SPECT and CT but also was true positive where SPECT results were false negative. It was observed that for the staging of patients, PET was better than CT or SPECT as it could pick up more lesions in the lymph node (LN), in the liver, and in the bone. In addition, in comparison to the 111In‐octreotide scan, 68Ga‐
DOTATOC PET has been established to be superior especially in detecting small tumors or tumors bearing only a low density of SSTRs [34]. In patients with equiv-ocal or negative OctreoScan, 68Ga‐DOTATATE PET/CT detected additional lesions and changed the management [35]. Of the 51 patients included in the study, 47 showed evidence of disease on cross‐sectional imaging or biochemically. 68Ga‐
DOTATATE PET was found to be positive in 41 of these 47 patients (87.2%), detect-ing 168 of the 226 lesions (74.3%) that were identified with cross‐sectional imagdetect-ing.
68Ga‐DOTATATE PET also identified significantly more lesions than 111In‐DTPA‐
octreotide scintigraphy (P < 0.001) and changed management in 36 patients (70.6%), who were subsequently deemed suitable for peptide receptor‐targeted therapy. 68Ga‐
DOTATOC was also shown to perform better than CT or SRS for the early detection of skeletal metastases of NETs [36]. In a larger subgroup of patients (n = 90) with pathologically confirmed NET, a comparison of 68Ga‐DOTANOC PET/CT with conventional imaging (CI) CT and EUS showed the superiority of PET/CT over CI [37]. Considering PET/CT and CI concordant cases (47/90 [52.2%]), PET findings affected the therapeutic management in 17 of 47 (36.2%) patients. Although PET did not result in modification of disease stage, 68Ga‐DOTANOC detected a higher lesion number in most patients. PET resulted in a modification of stage in 12 patients (28.6%) and affected the treatment plan in 32 patients (76.2%). 68Ga‐DOTANOC PET/CT thus affected either stage or therapy in 50 of 90 (55.5%) patients. The most frequent impact was the initiation or continuance of PPRT, followed by the initia-tion or continuance of SMS analogue medical treatment and referral to surgery. Of importance is that PET could avoid unnecessary surgery in 6 patients and excluded from treatment with SMS analogues two patients with NET lesions that did not express SSTRs. Due to the broader range of SSTR expression, 68Ga‐DOTANOC is an excellent tracer for imaging SSTR‐positive tumors, which in addition, due to the high target to nontarget ratios, allows the detection of very small lesions, especially of LN and bone metastases [38]. PET using 68Ga‐DOTATOC has been found to be superior to 18f‐fDG PET in the detection of NETs, imaging 57/63 lesions in 15 patients, as compared with only 43/63 on fDG PET [39]. In malignant neural crest tumors (pheochromocytoma, paraganglioma, and medullary thyroid cancer), a direct comparison with 123I‐MIBG study showed the superiority of the 68Ga‐
DOTATATE PET/CT in terms of sensitivity [40]. In a study in pulmonary endocrine tumors, 68Ga‐DOTATATE was shown to have a definite incremental value over
18f‐fDG for typical bronchial carcinoids than in atypical carcinoids and higher grades of tumors [41]. Also, due to the probability of development of concomitant NETs, SSTR PET/CT with 68Ga could be useful in the detection and follow‐up of pulmonary NETs [42]. Indeed, 68Ga‐SSTR PET/CT provides a whole‐body
one‐stop‐shop approach to the identification and localization of NETs and their metastases (figs. 4.2.1 and 4.2.2).
68Ga‐DOTATOC PET/CT has been also found to be considerably cheaper than
111In‐DTPA‐octreotide with respect to both material and personnel costs [43]. In clinical practice, apart from higher resolution and excellent quality of the images, the other advantages of PET imaging with the 68Ga‐labeled SMS analogues over 111In‐
DTPA0‐octreotide scintigraphy are easy availability of the 68Ga generator, relative short scanning time, and low radiation exposure to the patient [44]. Two recent studies have taken into account the comparison between the 68Ga‐labeled SMS analogues. A preliminary intraindividual study comparing 68Ga‐DOTANOC and 68Ga‐DOTATATE demonstrated that 68Ga‐DOTANOC localized more lesions in, especially, the liver and pancreas, due to its broader SSTR affinity profile [45]. Another study demonstrated comparable diagnostic accuracy of 68Ga‐DOTATATE and 68Ga‐DOTATOC for detec-tion of NET lesions [46].
(a)
(b)
(c)
(d)
figure 4.2.1 68Ga‐DOTATATE PET/CT (maximum intensity projection image on extreme left) shows multiple primary neuroendocrine tumors in pancreatic tail with an SUVmax of 15.4 (a); in both adrenals, massive enlargement and partial necrosis in the left adrenal with an SUVmax of 29.7 (b) and comparatively smaller and less dense SSTR expression in the right adrenal with an SUVmax of 10.3 (c); and right ovary with an SUVmax of 11.6 (d). There was also widespread metastasis in the liver, multiple abdominal/extra‐abdominal lymph nodes, and possibly the pituitary gland (SUVmax of 5.4) (a, b, c, d—images in transverse view; CT on the left and fused PET/CT on the right). (See insert for color representation of the figure.)
DETECTION Of UNKNOWN PRIMARY TUMOR 63
Gluc‐Lys 18f fP‐TOCA is an 18f‐based radiopharmaceutical that targets SSTRs.
In a preliminary comparative study, Gluc‐Lys 18f fP‐TOCA PET was found to be superior to 111In‐DTPA‐octreotide scan in the diagnosis of NETs. The results also suggested that the sensitivity and specificity of Gluc‐Lys 18f fP‐TOCA is comparable to the reported sensitivity and specificity of 68Ga‐DOTATOC PET findings in NETs [47]. 64Cu, with a half‐life of 12.7 hours, is another potential positron‐emitting radio-nuclide for PET imaging [48]. The possibility of performing dosimetry for PRRT based upon 64Cu is one other possible advantage. In a preliminary study, 64Cu‐TETA‐
octreotide PET was found to have high sensitivity and favorable dosimetry and pharmacokinetics [49].
detection of unKnoWn priMAry tuMor
In a bicentric study, the role of 68Ga‐DOTANOC PET/CT in the detection of unknown primary NETs has been demonstrated (fig. 4.2.3) [50].
Overall, 59 patients (33 men and 26 women, aged 65 ± 9 years) with documented NETs and unknown primary were enrolled. PET/CT was performed after injection of approximately 100 MBq (46–260 MBq) of 68Ga‐DOTANOC. The SUVmax were calculated and compared with SUVmax in known pancreatic NETs (pNETs) and ileum/jejunum/duodenum NETs (SI‐NETs). The results of PET/CT were also corre-lated with CT alone. In 35 of 59 patients (59%), 68Ga‐DOTANOC PET/CT localized
(a)
(b)
(c)
(d)
figure 4.2.2 Identification of rare metastases of neuroendocrine tumors with high‐sensitivity on 68Ga‐SSTR PET/CT: not only were myocardial metastases, which were otherwise difficult to appreciate on CT (a), localized on 68Ga‐DOTA‐SSTR PET/CT (b) but also pericardial metas-tases (c, CT; d, fused PET/CT). (See insert for color representation of the figure.)
the site of the primary: ileum/jejunum (14), pancreas (16), rectum/colon (2), lungs (2), and paraganglioma (1). CT alone (on retrospective analyses) confirmed the find-ings in 12 of 59 patients (20%). The mean SUVmax of previously unknown (cancer of unknown provenience (CUP)) pNETs and SI‐NETs were 18.6 ± 9.8 (range: 7.8–
34.8) and 9.1 ± 6.0 (range: 4.2—27.8), respectively. SUVmax in patients with previ-ously known pNET and SI‐NET were 26.1 ± 14.5 (range: 8.7–42.4) and 11.3 ± 3.7 (range: 5.6–17.9). The SUVmax of the unknown pNETs and SI‐NETs were signifi-cantly lower (P < 0.05) as compared to the ones with known primary tumor sites;
19% of the patients had high‐grade NET and 81% low‐grade NET. Based on 68Ga‐
DOTANOC receptor PET/CT, 6 of 59 patients were operated, and the primary was removed (4 pancreatic, 1 ileal, and 1 rectal tumor) resulting in a management change in approximately 10% of the patients. In the remaining 29 patients, because of the far advanced stage of the disease (due to distant metastases), the primary tumors were not operated. Additional histopathological sampling was available from one patient with bronchial carcinoid (through bronchoscopy). In this study, 68Ga‐DOTANOC
(a)
(b)
(c)
(d)
figure 4.2.3 In this case of CUP syndrome, 68Ga‐DOTATOC PET/CT (MIP image on extreme left) revealed somatostatin receptor‐positive primary tumor in the jejunum with SUVmax of 13.8 (a), along with multiple metastases in the liver (b), bone (c), and lymph nodes (d), and a metastasis in the left adrenal. (a, b, c, d—images in transverse view; CT on the left and fused PET/CT on the right.) (See insert for color representation of the figure.)
THERAPY PLANNING 65
PET/CT was found to be highly superior to 111In‐OctreoScan (39% detection rate for CUP according to the literature). It therefore has a major role to play in the management of patients with CUP‐NET.
tHerApy plAnning
Curative treatment of NETs usually requires the possibility of complete surgical resection of the primary tumor and perhaps regional LN metastases. However, effec-tive palliaeffec-tive therapies are also available at all stages of the disease and can be applied even to advanced stage. Depending upon tumor stage, size, localization, and degree of differentiation, treatment protocols for NET are currently based upon the
Curative treatment of NETs usually requires the possibility of complete surgical resection of the primary tumor and perhaps regional LN metastases. However, effec-tive palliaeffec-tive therapies are also available at all stages of the disease and can be applied even to advanced stage. Depending upon tumor stage, size, localization, and degree of differentiation, treatment protocols for NET are currently based upon the