Preclinical image-guided ionising radiation delivery systems are being developed, mimicking increasingly accurate patient radiotherapy. Numerous associated technical problems relate to dose delivery to small tissue volumes that ‘move’ due to breathing. We have developed sensors of this motion, coupled to very fast radiation beam gating. The accuracy of the radiation delivery critically depends on the quality of target imaging; conventional methods of cone beam CT are being complemented by optical fluorescence, ultrasound and other imaging techniques.
Fluorescence optical image guidance has also been developed for use during human surgery (Figure 1). Here we exploit the near infrared region of the optical spectrum (650-950 nm) to allow real-time imaging at tissue depths of <20 mm. This has been applied to lymph node imaging. An exciting new development is molecular optical imaging of prostate tumours to ensure that the surgeon is able to perform correct excision of extra-prostatic tumour tissue.
Time-resolved fluorescence microscopy techniques have been developed and enhanced over many years. Once excited by a light pulse, a fluorophore carries on emitting fluorescence light for a few nanoseconds. The kinetics of this process inform on resonance energy transfer between suitably chosen fluorophores tagged to proteins of interest (Főrster Resonance Energy Transfer, FRET). This is strongly dependent on the inter-fluorophore distance and the method can be used as a molecular ruler, capable of measuring distances of 1-10 nm, way below any competing methods. We have developed medium throughput, high content automated microscopy platforms to allow protein interaction screening of patient biopsies (Figure 2). The aim here is to determine which drugs are likely to be effective at treating the cancer.
We have also developed a unique system, based around an in-house 6 MeV electron linear accelerator coupled to a robot and fluorescence microscopy, to study radiation-induced cell DNA repair kinetics using high resolution time-lapse imaging. (Figure 3). We can resolve double strand breaks formed within seconds and are able to follow the temporal evolution of the repair of individual breaks.
Figure 1: Right: Near infrared fluorescence image guidance system developed by the group used during surgery at the Churchill Hospital. Left: Sentinel node removal during endometrial cancer laparoscopic surgery using our guidance system.
Figure 2: Typical images of fluorescence intensity (left) and pseudo-coloured image (right) of interacting fractions between specific proteins in a patient breast tumour tissue slice (collaboration T Ng and T Coolen, King’s College, London.
Figure 3: the 6 MeV accelerator.