Imaging description

Radiologists rely on enhancement in evaluating treatment response in glioblastoma multiforme (GBM). According to the widely used Macdonald criteria, >25% increase in enhancing tissue indicates tumor progression. Surgery when feasible followed by concurrent temozolomide (TMZ) and radiotherapy (RT) became the standard of care for GBMs. Recently, there has been an increasing awareness of progressive and enhancing lesions on MR images shortly after TMZ+RT treatment that improve or stabilize on follow-up imaging in the absence of further treatment [1]. This is termed “pseudoprogression” as it mimics true tumor progression. Up to 30% of GBM patients exhibit this phenomenon, and present with increased enhancement, edema, and mass effect within 3–4 months of treatment.

Conventional MRI sequences are not helpful in making a distinction between pseudo and true progression (Fig. 26.1) [2]. The definitive diagnosis is based on demonstration of the lesion getting better on follow-up MRI or biopsy, although perfusion images may show decreased rCBV and increased apparent diffusion coefficient (ADC) values, in contrast to true tumor progression (Fig. 26.2) [3].