A total of 429 g-ray-induced thymic lymphomas were obtained from F 1 and backcross mice between BALB/c and MSM strains, about a half of which carried a p53-de®cient allele. A genome-wide allelic loss analysis has revealed two loci exhibiting frequent allelic losses but no allelic preference, one is localized within a 2.9 cM region between D12Mit53 and D12Mit279 loci on chromosome 12, and the other is near the D16Mit122/D16Mit162 loci on chromosome 16. The frequency of allelic loss in the D12Mit279 region is 62% and does not dier in tumors between the presence and absence of the p53-de®cient allele. In contrast, the loss frequency of D16Mit122 is raised by the existence of p53-de®cient allele: 62% for p63(7/+) and 13% for p53(+/+), suggesting cooperative function of the two losses. The D12Mit279 and D16Mit122 regions probably harbor dierent types of tumor suppressor gene that play key roles in lymphoma development.

We assess diffusion-weighted MR images in the differential diagnosis of intracranial brain tumors and tumor-like conditions. Heavily diffusion-weighted (b = 1100 or 1200 s/mm2) axial images were obtained with single-shot echo-planar technique in 93 patients with pathologically confirmed various intracranial tumors and tumor-like conditions with diffusion gradient perpendicular to the images. We compared signal intensity of the lesions with those of gray and white matter, and cerebrospinal fluid (CSF). In 29 cases (31.1%) the lesions were isointense to gray and/or white matter. However, 5 cases (5.4%) showed extremely increased signal intensity: two epidermoid cysts; two chordomas; and one brain abscess. The entire portion of a tumor was markedly hyperintense in 10 cases (10.8%): four malignant lymphomas; four medulloblastomas; one germinoma; and one pineoblastoma. A CSF-like hypointense signal was seen in many cystic tumors, and cystic or necrotic portions of tumors. A neurosarcoid granulation was the only solid lesion showing characteristically a hypointense signal like CSF. The combination of markedly hyperintense and hypointense signals was seen generally in hemorrhagic tumors. Diffusion-weighted echo-planar MR imaging is useful in the differential diagnosis of brain tumors and tumor-like conditions, and suggests specific histological diagnosis in some cases.

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Germ-cell tumors of the central nervous system generally develop in the midline, but the tumors can also occur in the basal ganglia and/or thalamus. However, MR images have rarely been documented in the early stage of the tumor in these regions. We retrospectively reviewed MR images obtained on admission and approximately 3 years earlier in two patients with germinoma in the basal ganglia, and compared them with CT. In addition to hyperdensity on CT, both hyperintensity on T1-weighted images and a small hyperintense lesion on T2-weighted images were commonly seen in the basal ganglia. These findings may be early MRI signs of germinoma in this region, and the earliest and most characteristic diagnostic feature on MRI was atrophy of the basal ganglia, which was recognizable before development of hemiparesis.