**3.3. Localisation for biopsy**

FDG PET can be useful in selection of a biopsy site where uptake is highest in the tumour, thereby ensuring sampling of the most malignant tissue [12–16].

#### **3.4. Radiotherapy planning**

MRI is the current technique of choice for radiotherapy planning. Due to its relatively low tumour-to-background contrast, FDG PET has limited utility for conventional treatment planning. More recently, the addition of PET imaging of some radiotracers—in particular the amino acid analogues—in radiotherapy planning has been shown to be promising in the identification of microscopic residual tumour post-surgery and differentiation of tumourfrom brain tissue, thereby improving local control and reducing radiation to healthy brain paren‐ chyma (see Section 4).

#### **3.5. Assessment of treatment response**

The differentiation of tumour recurrence from radiation necrosis following treatment is one of the most common and important clinical indications for MRI and PET. Both viable tu‐ mour and post-radiotherapy necrosis demonstrate contrast enhancement on MRI. Similarly, increased FDG uptake cannot reliably differentiate residual/recurrent tumour from radiation necrosis. Furthermore, false-negative MRI and FDG PET can result from decreased enhance‐ ment (due to antiangiogenic therapy) and poor tumour-to-background contrast, respectively.

Whilst the criteria forResponse Assessment in Neuro-Oncology have been updated to mitigate these potentially confounding factors in MRI [17–19], advancements in response assessment in PET have focused on other (non-metabolic) radiotracers.
