Contribution of 18F-Fluoro-ethyl-tyrosine Positron Emission Tomography to Target Volume Delineation in Stereotactic Radiotherapy of Malignant Cranial Base Tumours: First Clinical Experience
Increased amino acid uptake has been demonstrated in intracerebral tumours and head and neck carcinomas of squamous cell origin. We investigated the potential impact of using 18F-fluoro-ethyl-tyrosine (18F-FET)-PET/CT in addition to conventional imaging for gross tumour volume (GTV) delineation in stereotactic radiotherapy of skull base tumours. The study population consisted of 14 consecutive patients with cranial base tumours (10 with squamous cell histology, 4 others). All patients underwent a FET-PET/CT examination in addition to contrast-enhanced CT and 11 patients underwent MRI. All tumours and histologic types showed increased FET uptake. The GTV was defined by all voxels showing hyperintensity in MRI or CT (GTVMRI/CT) or enhancement in PET (GTVPET), forming a GTVcomposite that was used for the initial treatment fields. An additional volume of infiltrative growth outside the GTVMRI/CT of about 1.0 ± 2 cm3 (5% of the conventional volume) was demonstrated by FET-PET only (GTVPETplus) with significant enlargement (>10% of GTVMRI/CT) in three patients. From existing data, we found correlation between cellular density and the standardized uptake value (SUV) of FET. We were able to substantially reduce the volume of escalated radiation dose (GTVboost) by 11 ± 2 cm3 (24%) of the conventional volume. 1. Introduction It is assumed that the larger part of geometrical uncertainties in fractionated stereotactic radiotherapy (FSRT) is due to delineation errors during the treatment planning procedure [1]. This is especially serious if the errors lead to marginal tumour misses, resulting in a dismal prognosis, or to enlargement of the volume treated, increasing the frequency of severe late effects. Structures of the skull base (SB) with high signal intensity and high contrast-enhancement in magnetic resonance imaging (MRI) make it difficult to differentiate tumour tissue from normal structures [2] and to exactly delineate the target volume. Therefore, although costly, functional imaging is increasingly used for target volume delineation in SB radiotherapy. The diagnostic value of 2-((18)F)-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) for imaging intracranial tumours is hampered by the low imaging contrast between tumourous tissue and that of the normal brain due to the high glucose utilization of both and this may also be true for SB tumours and the neighbouring brain tissue [3]. The newly introduced tracer O-(2-[18F] Fluoro-Ethyl)-L-Tyrosine (18F-FET) allows a more precise estimation of intracerebral tumour borders than MRI [4]. Pauleit et
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