History
A 15-year-old male patient, complaining of a growing lump on the right of his forehead over the past 20 days, came to the hospital for a check-up. He was otherwise asymptomatic. His medical history was unremarkable. Physical examination revealed no abnormalities. A contrast-enhanced CT scan was performed with a dual source photon-counting CT (PCCT), NAEOTOM Alpha, using an ultra-high resolution (UHR) scan mode, for assessment.
Diagnosis
CT images showed an expansive frontotemporal mass, measuring 66 mm x 65 mm in size, protruding into the right orbit, the right frontal sinus and subcutaneously. It was also compressing the right lateral ventricle, shifting the cerebral midline and both the anterior cerebral arteries (ACA) leftwards, and the right middle cerebral artery (MCA) backwards, and causing bone erosion of the squamous part of the right temporal bone and posterolateral orbital wall. The mass, homogeneous and slightly hyperdense in the virtual non-contrast (VNC) images, showed uniformed contrast enhancement in the virtual monoenergetic images (VMI) displayed at 55 keV, as well as in the iodine maps, with multiple hyperdense foci inside. Hypodense areas around the mass were seen, suggesting edema. The intraorbital segment of the optic nerve was compressed, however, not invaded. CT findings suspected a meningioma. A MRI was suggested for further assessment, also suspecting a meningioma. A lymphoma was considered in the differential diagnosis, however, there were no typical signs in either CT or MRI.
Subsequently, the patient underwent surgical removal of the mass. Histologic and immunophenotypic findings revealed an anaplastic large cell lymphoma (ALCL), anaplasticlymphoma-kinase (ALK) positive. The patient recovered well and was discharged two weeks later with scheduled outpatient follow-ups.
Fig. 1: Coronal view of a 0.2 mm UHR image (Fig. 1a) shows bone erosion (arrows) of the squamous part of the right temporal bone and the posterolateral orbital wall. 1 mm slices with spectral information are reconstructed from the UHR data to reformat coronal views of a VMI displayed at 55 keV (Fig. 1b), a VNC (Fig. 1c), and an iodine map (Fig. 1d). These images show an expansive frontotemporal mass, homogeneous and slightly hyperdense in VNC, uniformly contrast enhanced in VMI and iodine images, with multiple hyperdense foci inside, protruding into the right orbit and shifting the cerebral midline to the left.
Fig. 2: Oblique MPR images show the tumor protruding into the right frontal sinus and the right orbit, compressing the intraorbital segment of the optic nerve. The fine separation (arrows) between the tumor edge and optic nerve is better seen in 0.2 mm UHR image (Fig. 2a) and in VMI displayed at 55 keV (Fig. 2b), in comparison to a VMI displayed at 70 keV (Fig. 2c) corresponding to an image acquired at 120 kV in standard scan.
Fig. 3: cVRT images, reconstructed using 0.2 mm UHR slices, show bone erosion of the squamous part of the right temporal bone and the posterolateral orbital wall (Fig. 3a, arrows), the extracranial (Fig. 3b) and intracranial (Fig. 3c) feeding arteries (arrowheads) to the tumor in detail. Both ACA are shifted leftwards and the right MCA is shifted backwards.
Comments
Primary ALCLs, occurring in the central nervous system (CNS), are extremely rare. They tend to occur in children and young adults, predominantly male patients, with a prognosis correlated with ALK positivity. A favorable course has been exhibited in younger patients with ALK-positive ALCL. As the imaging characteristics are atypical, diagnosis and differential diagnosis are difficult with traditional imaging alone, especially when the tumor is massive and invasive, such as in this case.
A unique combination of high spatial resolution and spectral information, provided by PCCT, is helpful in visualizing the detailed cerebral, vascular and bony structures using the image data acquired from one UHR scan. The VMI of the cerebrum are routinely set to be displayed at a low keV level (55 keV) allowing for enhanced contrast. VNC images can be displayed with one mouse click, without having to perform a native scan. In daily practice, these different image types 鈥 VMI, VNC and iodine 鈥 can be interactively toggled, using Interactive Spectral Imaging (ISI) technique available in image reading. In UHR mode, small sub-pixels are defined by electric fields, instead of physical separations, and are read out separately to increase the spatial resolution. This approach improves the geometrical dose efficiency of the detector to enable UHR imaging without radiation dose penalty. In addition, PCCT optimizes the contrast-to-noise ratio (CNR) with a combination of missing down-weighting of the lower energy X-ray photons, the absence of the electronic noise and the inherent spectral information. A practical example is that the UHR images, acquired and reconstructed at 0.2 mm slice width, can be used to create lifelike three-dimensional images with cinematic volume rendering technique (cVRT). In this very rare case, although diagnosis based on imaging differs from histologic and immunophenotypic results, the detailed visualization and demonstration help the neurosurgeons understand the complexity of the case prior to surgical planning.
Examination Protocol