Radiotherapy (RT) is an important modality in treatment against cancer. Developments in this modality are necessary for improvement of patient complications and survival. Aside from fractionation and precise irradiation, FLASH radiotherapy could be a third way to significantly increase the therapeutic window of radiotherapy.The FLASH effect is a biological, healthy tissue sparing effect that is found in tissue that receive a high dose (>10 Gy) within a very-short irradiation time (<100 ms). Although the mechanics behind the FLASH effect remain unknown, it is found to benefit a large variety of tissues and organs. Nevertheless, FLASH-RT still lacks clinical translation and implementation of FLASH irradiation is limited by current technology. However, the extremely high dose rates required for FLASH can readily be achieved using cyclotron accelerated proton transmission beams. Cancer patients with small lung tumours are a preferred starting point for clinical translation of FLASH because (i) transmission beams are of special benefit for lung tumours since they mitigate for range uncertainties and contain a sharp lateral penumbra, (ii) high doses are required for the FLASH effect to occur and hypofractionation is not uncommon for lung patients, and (iii) the very short irradiation times and high doses in combination with gating allow smaller treatment margins for moving tumours.In this study, we took a first step towards intensity modulated proton therapy (IMPT) with FLASH treatment plan optimisation by (i) investigating the most delivery-time effective way to irradiate a field, (ii) proposing an optimisation method to maximise FLASH within a treatment plan, and (iii)analysing and evaluating FLASH treatment plans. Because increased pencil-beam (spot) separation showed to benefit the pencil-beam scanning(PBS) delivery time of a field, spot overlap minimisation was proposed as optimisation method for FLASH. Treatment plans using 3, 5 and 7 transmission beams were generated with and without FLASH optimisation for 6 small lung tumour patients. Although spot overlap minimisation doesoccasionally improve delivery time for a limited number of beam angles, the overall treatment plan does not benefit from the optimisation. Therefore, spot overlap minimisation cannot be used for FLASH optimisation in clinical IMPT treatment planning. Recommendations for future research include optimisation on PBS delivery time and PBS pattern optimisation.