Investigations on MRI changes after 5-aminolevulinic acid mediated interstitial photodynamic therapy for malignant gliomas

The treatment of glioblastoma still faces challenges due to poor prognosis, necessitating alternative therapies such as interstitial photodynamic therapy (iPDT). Using oral 5-ALA administration and stereotactic light application, iPDT aims to cause a selective, phototoxic effect to the tumor cells. To gain a deeper insight into the intracranial tissue reaction and morphological changes associated with iPDT, this study examined retrospectively MRI sequences (T1, T2, DWI, ADC) in 16 patients at three time points: preoperative (within 7 days before iPDT), postoperative (within 1 day after iPDT), and the first post treatment follow-up (after 3 or 6 months). Significant variations in hypointense, necrotic looking areas, contrast enhancement, edema, diffusion restriction, and hemorrhagic blood volume were found. Postoperative MRI on the day after iPDT showed increased edema volume and diffusion restriction. In T2 images after iPDT, a treatment-related circumscribed structure was identified, which we called "iPDT remnant". It showed substantial overlap with the original tumor. At the first follow-up, the iPDT remnant generally reduced in size, with no further diffusion distortion detected. A small overlap between the initial contrast-enhanced tumor volume and the contrast enhancement on the day after iPDT correlated with more prolonged progression-free survival in this cohort, suggesting it as a potential indicator for iPDT treatment response. These results provide insights into the morphological MRI changes resulting from the iPDT and highlight the importance of MRI, conducted at specific times after the iPDT, for assessing and monitoring the iPDT response. In addition, differences from other forms of therapy and conventional interpretations in neuroradiological evaluation need to be considered.