title: Advances towards the use of radical radiotherapy in malignant pleural mesothelioma
Creators: name: Ashton, Miranda Jane
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Mesothelioma, radiotherapy, clinical trial, radiobiology, radiosensitisation.

description: Background: The complex shape of the pleural cavity and the close proximity of normal radiosensitive structures render the delivery of radical radiotherapy in malignant pleural mesothelioma (MPM) challenging. However, the advent of conformal, intensity modulated radiotherapy (IMRT), where dose is selectively delivered to the tumour whilst sparing normal tissues, can facilitate safe dose escalation. SYSTEMS-2 is the only randomised controlled trial of radiotherapy dose escalation to be attempted in MPM and is comparing the palliative efficacy of two hypofractionated radiotherapy regimes to sites of pain using conformal techniques. Although traditionally associated with unacceptable late normal tissue toxicity, the success of stereotactic radiotherapy (SABR) and the discovery that two common malignancies exhibit low α/β ratios, has enhanced the popularity of hypofractionated regimes. While the radiobiology of MPM is not well understood, its slow growth and apparent radioresistance suggests that it may exhibit a low α/β ratio and therefore that it may respond more favourably to dose hypofractionation. Aims of thesis: To investigate the possibility of further radiotherapy dose escalation in MPM, beyond that delivered in the SYSTEMS-2 study. Methods: I. Novel radiotherapy dose constraints were generated for use in the SYSTEMS-2 study and tested on five patients from the SYSTEMS study. II. Multi criteria optimisation (MCO) software was used to assess whether the original dose escalated radiotherapy plans for the Glasgow cohort of SYSTEMS-2 could be improved, without compromising target volume coverage. III. A clinically relevant 3D in vitro spheroid model was used to investigate the radiobiology of two independent MPM cell lines (H2052 and 211H). Spheroids were established and exposed to the same total dose of ionising radiation (IR) delivered in different doses per fraction. Data was used to investigate response to dose fractionation and to estimate the α/β ratio of this tumour. IV. The response of H2052 and 211H spheroids to two radiosensiting agents was investigated in combination with fractionated radiotherapy. Spheroids were incubated with increasing concentrations of either NU7441 (a DNA-PKcs inhibitor) or A1331852 (a BH3 mimetic) before being exposed to fractionated IR. The immunohistochemical (IHC) expression of DNA-PKcs and Bcl-xL was explored in diagnostic biopsies obtained from MPM patients to investigate clinical validity of the targets. V. IHC expression of nine proteins, selected for their potential to impact on radioresponse, was analysed in diagnostic tumour tissue collected from SYSTEMS and SYSTEMS-2 patients. Expression data was correlated with baseline clinical trial data in all patients, and with clinical trial outcome data from SYSTEMS patients. Results: I. Initial planning studies showed that none of the five SYSTEMS patients met all of the SYSTEMS-2 dose constraints, but the plans were deemed to be potentially clinically acceptable and the constraints were taken forward in the trial. The value of familiarity with a planning technique was evidenced by the fact that all constraints were achieved when the cases were re-planned by the same staff member in April 2019. II. MCO re-planning of dose escalated SYSTEMS-2 plans achieved clinically significant dose reductions to organs at risk (OAR) without compromising target volume coverage in 13/20 cases. Plans which did not meet OAR constraints or conform to the prescribed target volume coverage may still have been clinically acceptable. III. In vitro studies confirmed that growth of MPM spheroids can be delayed by IR. Spheroids demonstrated sensitivity to changes in dose per fraction, with the greatest volume reductions observed in hypofractionated radiotherapy regimes. This data implies that these MPM cell lines may exhibit a low α/β ratio, a suggestion which was further supported by in vitro multi-fraction IR studies. IV. Data suggest that NU7441 and A1331852 are potent radiosensitisers of MPM spheroids and that both are valid clinical targets in MPM. The supposition that a BH3 mimetic may offer tumour specific radiosensitisation, combined with the observation that A1331852 demonstrated greatest efficacy with hypofractionated IR, suggests that this agent may be clinically valuable in the radiosensitisation of MPM. V. No statistically significant correlations were found between baseline clinical characteristics and expression of the proteins of interest and no potential biomarkers of radiosensitisation were identified in the SYSTEMS cohort. Conclusions: Novel dose constraints are being used to facilitate the delivery of hypofractionated, dose escalated palliative radiotherapy in the SYSTEMS-2 study. Results from this trial may guide future dose escalation in this disease and data from MCO planning studies suggest that further dose escalation to the target volume may be feasible without breaching OAR limits. In vitro studies suggest that MPM is sensitive to IR, responds more effectively to dose hypofractionation and may have a low α/β ratio. This data may be helpful in determining dose and fractionation regimes in future MPM radiotherapy trials. Combination of BH3 mimetics with IR may provide MPM specific radiosensitisation, achieving greatest efficacy with dose hypofractionation. Ongoing IHC analysis of tumour samples from the SYSTEMS-2 study may identify a biomarker of radiotherapy response which would be helpful in guiding radiotherapy treatment decisions for future patients. In summary, this thesis has investigated ways in which radiotherapy could be delivered with radical intent in MPM. Practical aspects of radiotherapy planning and delivery have been considered and are presented in conjunction with laboratory data to demonstrate how technical advances can be combined with an appreciation of disease radiobiology to facilitate radical treatment.