FEASIBILITY OF IMAGING PHOTODYNAMIC INJURY TO TUMORS BY HIGH-RESOLUTION POSITRON-EMISSION-TOMOGRAPHY

One early effect of the treatment of tumours by the new modality photo dynamic therapy (PDT) is a reduction in tumour glucose levels. We have employed the widely used positron-emitting glucose analogue flurorine -18 fluoro-2-deoxy-D-glucose ([F-18]-FDG), to determine whether, in pr inciple, PDT-induced injury might be delineated non-invasively and qua ntitatively by positron emission tomography (PET). The scanner was of the high-density avalanche-chamber (HIDAC) type with a resolution of 2 .6 mm. Subcutaneous T50/80 mouse mammary rumours, sensitised by haemat oporphyrin ester, were illuminated by graded doses of interstitial 630 nm light. Thirty hours later, any remaining viable tumour was detecte d (a) by region-of-interest analysis of the PET images and (b) by gamm a counting the excised tumour. PET measurements of % uptake of [F-18]- FDG into tumour correlated closely with ex vivo gamma counting (slope= 0.976, r(2)=0.995), validating the in situ method. Uptake into untreat ed, control tumours was 3.8%+/-1.1% of the injected activity. Uptake o f [F-18]-FDG into treated rumours decreased by 0.7% for every 100 mm(3 ) reduction in remaining viable histological volume. Outcome was furth er compared with that measured by (a) T2-weighted proton imaging on a 4.7-T magnetic resonance imaging (MRT) system and (b) histological ana lysis of subsequently sectioned tumours. PET using [F-18]-FDG describe d the absolute volume of surviving tumour histological mass to the sam e degree as high-resolution MRI. The conclusion of these initial studi es is that PET with [F-18]-FDG, although non-specific, quantitatively described at early times the extent of tumour destruction by PDT.