Low-intensity pulsed ultrasound enhances cell killing induced by X-irradiation

•X-ray, an ionizing form of radiation, is known to induce cell killing mainly by induction of apoptosis in cancer cells.•Ultrasound, a nonionizing form of radiation, has also been shown to induce killing of cancer cells, including apoptosis.•Ultrasound enhances X-ray effects against cancer cells. To determine the effect of pulsed ultrasound (US) on radiation-induced cell killing, U937 and Molt-4 cell lines were exposed to 1.0MHz US with 50% of duty factor at 0.3W/cm2 and pulsed at 1Hz immediately after exposure to X-rays at 0, 0.5, 2.5 and 5Gy. The cells were assayed 24h after the treatments. The result showed significant enhancement of cell killing in the combined treatments. However, the ratio of apoptotic cells induced either by X-rays or US alone did not significantly change. These findings suggest that pulsed US can enhance the anticancer effect of X-irradiation due to US streaming under non-inertial cavitational condition. This combined treatment can potentially enhance the therapeutic effect of radiation therapy.