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valuable mainly in the transition of the chest to the neck (Fig. 4.3f). Other artifacts like pulsations or increase of T1-signal of the CSF are more frequent in 3 T than in 1.5 T. To discriminate prominent cord veins from leptomeningeal seeding axial slices are advisable and a slice thickness of at least 4 mm is usually of better signal to noise ratio compared to extremely thin slices.
4.1.4Follow-Up Examinations
Response of tumors is traditionally evaluated by size measurement. Therefore comparable imaging parameters have to be used to guarantee as much accuracy as possible. In tumors only measurable on T2/FLAIR images like DIPG or nonenhancing tumors, a second plane of one or both of these sequences should be provided to enable a volume calculation.
4.1.4.1Differential Diagnosis Between Recurrence or Treatment Related Changes
In pilocytic astrocytomas, an intensification of enhancement or a new contrast enhancement must not be mistaken for a malignant degeneration and progression. In these tumors, enhancement is strongly varying and not related to prognosis with [125] and without treatment [126]. We have evaluated 83 children with measurable LGGs irrespective of treatment or during a watch-and-wait strategy according to their contrast behavior and found that in general growing tumors show increasing enhancement and vice versa. However, the groups of tumors with reduced and intensified enhancement also contained about 1/3 of tumors, which showed a contrary behavior like growing tumors with reduced enhancement and shrinking tumors with increasing enhancement. In conclusion, contrast enhancement alone is not decisive and does not mean progression in the absence of tumor growth.
New lesions like T2-hyperintensity and contrast enhancement in the brain parenchyma in embryonal tumors like MB after treatment with radiotherapy are frequently misdiagnosed as recurrence. MB and probably also the other embryonal entities and also ependymomas show two kinds of recurrences. They either recur locally in the surgical bed or as leptomeningeal dissemination or both in combination. However, we never saw a recurrence in the brain outside the surgical bed. Therefore treatment related changes like temporary (Fig. 4.4a–d) effects (early delayed reaction [127, 128]) or second tumors [129] (Fig. 4.4e, f) are much more likely than a true recurrence of the primary tumor. To raise the suspicion of a radiation reaction it has to be clarified with the radiotherapist that a sufficiently high dose has been applied to the affected region. Multimodal imaging like MR-spectroscopy, MR-perfusion, or aminoacid-PET can help in the differentiation.
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Fig. 4.4 (a–f) 9 months after complete resection of a medulloblastoma in the fourth ventricle, a new T2-lesion (a) with enhancement (b) is seen in the pons. This area was within the treatment dose area of radiation. Without specific treatment the changes vanished within 3 months of follow-up (c, d). However, early delayed radiation reactions may also increase or resolve more slowly. In a child, 7 years after completed treatment for a MB new and enlarging lesions developed in the cerebellum on the right hand side, the pons (e), and surrounding the lateral ventricles (arrows) with involvement of the corpus callosum and meaning subependymal spread (f). The histology of this second tumor was a glioblastoma
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Fig. 4.4 (continued)