In this work, we propose an image synthesis mechanism based on diffusion, which models the reversal of the sequential addition of noise to an image. We further develop conditioning mechanisms for this approach, such that image synthesis can be conditioned on information relevant to the clinical tasks-of-interest. We demonstrate the conditional synthesis capabilities of such models via an example application of multi-sequence prostate MR image synthesis, conditioned on text, to control lesion presence and sequence, and on images, to generate paired MR sequences e.g., generating diffusion-weighted MR from T2-weighted MR, which are two challenging tasks in pathological image synthesis. We validate our method using 2D image slices from real suspected prostate cancer patients. The realism of the synthetic images was validated through a blind evaluation by an expert radiologist, specialising in urological MR with 4 years of experience. The radiologist was able to distinguish between real and fake images with an accuracy of 59.4%, only slightly above the random chance of 50%. For the first time, we also evaluate the realism of the generated pathology by blind expert identification of the presence of suspected lesions. We find that the clinician performs similarly for both real and synthesised images, with a 2.9 percentage point difference in lesion identification accuracy between real and synthesised images, demonstrating the potentials for radiological training. Additionally, we demonstrated that a machine learning model trained for lesion identification exhibited improved performance (76.2% vs 70.4%, a statistically significant increase) when augmented with synthesised data compared to training solely on real images, highlighting the utility of synthesised images in enhancing model performance.