Preclinical studies for effects on tumour tissue in a mouse model

The effectiveness of pulsed particle beams in cancer therapy will be studied in treatment of cancer-infected nude mice, using a large field pulsed proton beam at the Munich tandem accelerator (25 MeV for treatment to tissue depths up to 6 mm). For comparison, matched controls will be treated with a comparable dose of a 75 kV X-ray beam generated from an Orthovolt Radiotherapy Unit (RT 100, MRI/TUM). For preparation, 1 mm³ tumour will be subcutaneously implanted on the mice’s right hind leg or back and closely watched for tumour growth by ultrasound measurements with a high-frequency linear probe. Tumours 30–40 mm³ in volume (corresponding to diameters of 3.5–4.5 mm, ellipsoid formula) will be randomly allocated to the experimental arm (tandem proton beam) or the control arm (RT 100) for radiotherapy. After therapy, tumour volume is continuously monitored for regression or progression to a predefined maximum, when the animals are sacrificed and the tumour tissues asserved for further histological analysis.

The time schedule would encompass the preparation of all practical aspects and the proposal for the etical committee on animal studies in the first year. For the choice of an appropriate tumour model and irradiation set-up the initially limited penetration depth of the pulsed beam and the necessary access for tumour monitoring has to be taken into account. Close interaction with project D.1.1 will be mandatory for dosimetry of the ultrashort ion pulses and development of means of spatial beam modulation to cover the whole tumour volume. Similarly, collaboration with project D.1.2 will be essential, since their first results on possible changes in RBE will be indispensable to defining dose levels for the first animal experiments on single-shot radiotherapy. These are scheduled for the 2^nd and 3^rd years, again with the first in-vivo results on RBE from growth delay experiments needed to define a meaningful dose range for the TCD50 (50 % tumour control dose) experiments. Both are necessary to plan the fractionated experiments, being scheduled for the 4^th and 5^th years.

For short-term objective (3 years) we could expect an answer to the questions whether intense ultra-short ion pulses (ns pulses) will lead to enhanced RBE in tumour tissue in-vivo and whether beam handling will allow a clinically meaningful dose distribution, i. e. covering all tumour voxels (if not, tumour regrowth from underexposed parts). For medium-term objective (5 years) first answers can be expected how the RBE for this new beam modality is modified by fractionation, with explicit evaluation of possibly different developments concerning repair, repopulation, redistribution and reoxygenation. Depending on the results of the first three years, explanted and stored tumours from growth delay experiments will be examined histologically for evaluation of structural tissue effects.

Subsequently, with promising first results and availability of the LDA beam, experiments should be repeated with the LDA beam and extended to several tumour entities to define its RBE on a broader level. Similarly, with the prospect of implementation of this new treatment modality into the clinic, investigations of the effects on normal tissue (skin, myelon, kidney, lung, bowel, etc.) are mandatory (6^th – 10^th years).