**9. Lymphoma**

**Figure 11.** *Row A*: Fusion PET/CT shows a Ga-68 DOTATATE-avid focus directly adjacent to the right anterior clinoid process of the sphenoid bone. *Rows B and C*: Multiplanar MRI sequences show characteristic features of an intracranial

Currently, the primary indication for using gallium-68 (Ga-68)-labelled somatostatin ana‐ logue radiotracers is for PET imaging of carcinoid and other NETs which usually express a high density of somatostatin receptors to which these peptides bind with high affinity [55]. Some non-NETs are also known to express somatostatin receptors, including meningiomas [56–58] which express all of the five somatostatin receptor (SSTR) subtypes but predominant‐

Traditional scintigraphic imaging of meningiomas utilised indium-111-labelled octreotide and single-photon emission computed tomography (SPECT). However, PET imaging with Ga-68 labelled somatostatin analogue radiotracers such as 1,4,7,10-tetraazacyclododecane-1,4,7,10-


with SPECT. It also provides much higher target-to-background ratio of radiotracer uptake and more detailed tumour characterisation and has largely replaced octreotide SPECT imaging. This detailed spatial characterisation of meningiomas, when coupled with anatom‐ ical imaging by CT and MRI (in particular), allows for more accurate radiotherapy planning



meningioma on non-enhanced (*arrows*) and gadolinium-enhanced (*arrowheads*) imaging.

**8.1. Somatostatin analogue radiotracers**

ly SSTR1 and SSTR2.

194 Neurooncology - Newer Developments

Phe1 -Tyr3

tetraacetic acid (DOTA)-Tyr3

in patients with large, non-resectable tumours.

Almost all patients with primary CNS lymphoma have brain parenchymal lesions which have a predilection for the periventricular and superficial regions. Contrast-enhanced MRI remains the technique of choice when CNS lymphoma is suspected, although it is usually not possi‐ ble to conclusively differentiate CNS lymphoma from other malignant brain lesions on MRI.

FDG remains the most widely explored radiotracer for PET imaging in CNS lymphoma, though its utility is limited compared with FDG PET in extracranial lymphoma, due to the poor tumour-to-background contrast in the brain. FDG uptake is typically intense in lympho‐ ma—metabolic imaging with PET may help to differentiate lymphoma in the brain from lowgrade gliomas and meningiomas [60–62] and may also be suitable for early evaluation of posttreatment response [60]. Infectious pathologies in the brain of immunocompromised subjects can be discerned from lymphoma by their usually hypometabolic nature on FDG PET [63], and high uptake ratio on thallium-201 (Tl-201) imaging [64]. Steroid treatment can cause falsenegative results by reducing FDG uptake in CNS lymphoma [62].

MET PET typically shows intense uptake in CNS lymphomas which often involves a larger area than the corresponding enhancing abnormality on CT and MRI and may more accurate‐ ly delineate the actual tumour margins [65]. It may also be more accurate for the detection of residual or recurrent lymphoma after treatment [65].
