Ultrasonic irradiation of multicellular tumor spheroids

Abstract

In vitro multicell spheroids from a mouse mammary sarcoma (EMT6) were exposed to therapeutic levels of ultrasond (1 MHz, 1–5 W/cm 2, 1–5 min cw) in a polystyrene tube which contains standing waves and promotes cavitationally related damage. Spheroid sizes were varied from 1‐day old, 20‐micron‐diameter cell aggregations (∼11 cells/spheroid) to 25‐day, 800‐micron‐diameter multicell spheroids (∼80,000 cells/spheroid). Damage was assessed by histology, scanning electron microscopy (SEM) and growth studies on individual spheroids and by survival of individual component cells. Dye exclusion (0.05% eosin) and histology indicated that peripheral damage occurred; SEM revealed the presence of surface abnormalities (tears and holes in cell membranes) in exposed spheroids. Additionally, necrotic loci within spheroids were noted after a 3 W/cm2 1 min exposure. Growth studies and surviving fraction indicated that damage is inversely proportional to spheroid size. With initial spheroid diameters ⩾ 200–250 microns, a decrease in spheroid diameter occurred 24 h post‐sonication (3 W/ cm2, 5 min cw); the threshold was between 3–5 min of exposure. With initial diameters ⩽200 microns, reduction was produced by a 1‐min exposure. Ultrasonically induced surviving fractions of individual cells also increased with increase in spheroid size.