Figure 3.

PET–MRI in the evaluation of treatment response in breast cancer. The top row displays k_ep = (K ^trans/v_e) maps overlain on a sagittal T₁-weighted MRI of a woman with an invasive ductal carcinoma at baseline before treatment (left panel), after one cycle of neoadjuvant chemotherapy (middle panel), and at the conclusion of chemotherapy (right panel). k_ep represents the rate at which the contrast agent moves from the extravascular extracellular space back into the vascular space and a high k_ep value is indicative of malignancy. Note the increase in the number of red voxels observed after one cycle of chemotherapy (specifically, k_ep increased 32%, from 0.34 min⁻¹ to 0.45 min⁻¹) which indicates an increase in tumour aggressiveness. Conversely, the SUL of ¹⁸F-fluorodeoxyglucose during PET decreased by 30%, from 1.81 at baseline (bottom left panel) to 1.39 (bottom right panel) after one cycle of chemotherapy, indicating a possible reduction in metabolic activity; this finding argues against increased tumour aggressiveness. These apparently disparate findings indicate the potential importance of assessments that incorporate data from multimodality imaging, as well as potential complications that can arise during interpretation of such diverse parameters. That is, although such approaches can enable characterization of complementary aspects of tumour biology, substantial work is needed to determine the appropriate methods of synthesizing such data that ultimately provide the maximum benefit for patients. For this particular patient there was no residual tumour at the time of surgery (the patient achieved a pathological complete response) as seen by the images in the far right column, which were acquired within 1 week of surgery. Abbreviations: K^trans, volume transfer constant of contrast agent between the blood and the extravascular extracellular space; SUL, standardized uptake value normalized to lean body mass; v_e, fractional volume of the extravascular extracellular space.