Zoran Rumboldt
I was a brand new neuroradiology attending in my first week at an academic institution, eager to teach the trainees and be useful and helpful to the clinical colleagues, when an MRI of a child with a 4th ventricle mass appeared on the board. It was a large cyst with a small enhancing nodule, and naturally, I called it a pilocytic astrocytoma (PA). But, much to my surprise and disappointment, it turned out to be a medulloblastoma. I went back and looked at all the available preoperative images and noted how the nodule was darker than the intact cerebellum on ADC maps. Over the following months I started paying attention to apparent diffusion coefficient maps in pediatric cerebellar tumors and noted that all medulloblastomas were dark and all PAs were bright. At the time there were already a few studies in the literature suggesting that medulloblastomas may have decreased diffusion, presumably due to their small, densely packed cells.1,2 Direct comparison of different histologic types of pediatric cerebellar tumors had not yet been performed.
So we designed a retrospective study with a hypothesis that medulloblastomas would have lower and PAs higher diffusion than the normal-appearing brain. In addition to measurements, a blinded junior radiology resident independently qualitatively assessed the tumors on ADC maps. An abstract with our results was then rejected at two national meetings. Although not too enthusiastic, we were about to submit our paper to AJNR, when a study showing utility of ADC for differentiation of brain tumors came out,3 reinforcing our confidence. The paper did get accepted and our hypothesis was correct. This article is also a good example of a research study from clinical practice and for clinical practice, without any dedicated additional funding.
In that article we concluded that “ADC values and ratios could prove reliable for distinction of intracranial tumors, if used in a selective manner to answer specific questions, combined with patient’s age, tumor location, and other imaging findings. Isolated analysis of diffusion properties does not provide universally reliable identification of different brain tumor types and grade; however, this may not be clinically relevant, because diagnosis is never based on a single sequence but rather on careful analysis of entire brain MR imaging study.”4 There was some overlap in ADC values of ependymomas with other tumors; however, their very heterogeneous appearance on other sequences is usually highly suggestive of the diagnosis. PAs, hemangioblastomas, and schwannomas are posterior