NI-7 Diffusion-weighted imaging for monitoring acute response and recurrence after photodynamic therapy in malignant gliomas

BACKGROUND: Photodynamic therapy (PDT) subsequent to surgical tumor removal is a novel light-activated localized treatment for malignant glioma. Although PDT provides effective local control, even PDT cannot completely suppress local recurrence of malignant glioma. We previously reported that the acute response of malignant glioma to PDT could be detected as linear hyperintense signals on diffusion-weighted imaging (DWI) and a decline in apparent diffusion coefficient (ADC) values that were asymptomatic and transient. However, their long-term clinical significance has not yet been examined. This study aimed to clarify the link between the hyperintense signal on DWI as an acute response and recurrence after PDT in malignant glioma. METHODS: Thirty consecutive patients (16 men, 14 women; median age 60.5 years) underwent PDT for malignant glioma at our institution between 2017 and 2020. We analyzed signal changes on DWI after PDT and the link between these findings and the recurrence pattern. RESULTS: In all patients, linear hyperintense signals of 5–7 mm on DWI were detected at the surface of the resected cavity from day 1 after PDT. These changes matched the PDT-irradiated area and disappeared in about 30 days without any neurological deterioration. Of the 30 patients, 19 (63%) exhibited recurrence: local recurrence in 10 (33%), distant recurrence in 1 (3%), and dissemination in 8 (27%). All local recurrences arose from areas that did not show a hyperintense signal on DWI obtained on day 1 after PDT. Patients with distant recurrence or dissemination tended to have uninterrupted hyperintense signal on DWI obtained on day 1 after PDT. CONCLUSION: The local recurrence in malignant glioma after PDT occurred in the areas without hyperintense signal on DWI as the acute response to PDT. This characteristic finding could aid in the monitoring of not only PDT-irradiated area but also local recurrence site after PDT.