MBq) of [6-3Hjthymidine (3H-Thd, Amersham Japan Ltd., Ja pan) and killed 1 hr later. The tumor was quickly dissected and prepared for frozen sectioning as reported previously (18). In brief, the trimmed tumor sample was embedded in OCT. com pound (Miles Inc.) and frozen in isopentane cooled with liquid nitrogen. The frozen sample block was sectioned on a cryostat at — 25ÂC. The 5@ tm-thicksection was mounted on a clean glass slide, air-dried and directly contacted with ARG film (MARG 3H-type, Konica, Japan) for 2 hr in order to produce the [18fl FDG image. Three days (about 39 half-lives of †8F) after the initial autoradiography when the †ÂFradioactivity had decayed, the same section was contacted with another film for 2 mo to produce the 3H-Thd image.