Ultrasound, ions and combined modalities for increased local tumour cell death in radiation therapy

Abstract: In this thesis, new approaches to improve tumour therapy for patients with inoperable, radiation- and chemoresistant tumours were in focus. Ultrasound and ionising radiation, especially accelerated ions, are both non-invasive agents that have high precision and deep penetration in tissues, enabling increased dose to the tumour while sparing the normal tissues. The first aim was to investigate cell survival and various cell responses of non-thermal ultrasound, both as a single treatment or in combination with radiation or a chemotherapeutic agent, and the second aim was to investigate the effects of accelerated ions. Treatment of solid tumours using focused high-intensity ultrasound for thermal ablation has been initiated. In the present study lower intensities of ultrasound, not inducing thermal increase but inducing cavitation and production of free radicals were used for investigation of cellular effects. Cells were exposed to 1 MHz continuous mode ultrasound at varying intensities, either alone or in combination with X- or gamma-radiation, or the chemotherapeutic agent cisplatin. Intensity-dependent clonogenic cell survival and induction of apoptosis were seen in five different human solid tumour cell lines (lung carcinoma, cervical carcinoma, two gliomas, and melanoma). The variation in ultrasound sensitivity was very small compared to the large variation after photon radiation. Both in a rodent (lung fibroblast) and a human solid tumour (lung carcinoma) cell line synergistic effects after combined exposures to ultrasound and radiation was found. Also, ultrasound enhanced the effect of cisplatin, induced intensity-dependent increase in instant cytolysis, induction in DNA damage, cells with increased membrane permeability and aberrant morphology. In radiation therapy the dominating modality is external therapy with photons or electrons. Accelerated protons and carbon ions have improved the treatment of inoperable and radioresistant tumours. Other charged particles and radiation qualities may further optimise the treatment. Therefore we investigated the effects of accelerated boron ions at four different linear energy transfer (LET) in a human solid tumor cell line (melanoma). The clonogenic cell survival decreased with increasing LET. Boron irradiation induced higher levels of apoptosis compared to photons at all time points studied although no clear LET-dependence due to the complex time-dependent response pattern. Even intermediate LETs were effective in increasing cell killing, apoptosis, cell cycle delay and reducing DSB rejoining. Both ultrasound and boron ions were effective in tumour cell elimination. By combining different treatment modalities cell killing was further enhanced due to interacting sublethal damage. This finding, in combination with the minimal variation in sensitivity among human solid tumour cell lines after ion and ultrasound irradiation, suggests some new avenues for treating heterogeneous tumours with varying radiationand chemoresistance.