Optimizing advanced stereotactic radiosurgery techniques for brain cancer treatment

脑癌是加拿大青少年和年轻人癌症死亡的第二大原因（加拿大癌症协会，2013年）。放射外科（RS）是一种常见的治疗脑部和颅底癌性肿瘤的技术，而RS治疗的准确性对成功根除癌症至关重要。由于辐射对健康组织和癌变组织都有损害，高质量的放射治疗计划必须避免向健康组织提供过量剂量，同时向肿瘤（通常位于脑干附近）提供大剂量。与其他类型的放射治疗不同，放射治疗通常由一种显著放射剂量的治疗组成，而不是许多小剂量的治疗（称为部分治疗）。

Brain cancer is the second-highest cause of cancer death in Canadian adolescents and young adults Canadian Cancer Society, 2013). Radiosurgery (RS) is a common treatment technique for cancerous tumours in the brain and base of the skull, and the accuracy of the RS treatment is critical to successful cancer eradication. Because radiation is damaging to healthy tissue as well as cancerous tissue, high-quality RS treatment plans must avoid delivering excessive dose to healthy tissue while simultaneously delivering a large dose to tumours, which are commonly positioned adjacent to the brainstem. Unlike other types of radiation therapy, RS usually consists of one treatment of significant radiation dose, rather than many treatments (called fractions) of small dose. The Elekta Gamma Knife Perfexion (PFX) is the state-of-the-art radiation delivery unit for RS, used to treat an estimated 50,000 patients each year (Elekta AB, 2013). In PFX, the patient lies on a couch, surrounded by eight banks of radiation sources (called sectors) all aimed at a single location (called an isocentre) inside a tumour. The couch then moves to irradiate a different location in the tumour, and then to another isocentre, etc., until the target is treated; this process is called “step-and-shoot”. At each isocentre, the sectors can emanate radiation beams of three different diameters (called collimator sizes) to create any radiation dose deposition at any isocentre. The selection of couch positions and collimator configurations can be formulated as a mathematical model to optimize treatment quality. Currently, clinicians manually select isocentres and collimators, which can lead to sub-optimal treatments and long planning times. Additionally, the step-and-shoot process can yield lengthy treatment times and limits the ability to treat large targets. Thus, there is significant clinical interest in transitioning to “continuous path” treatments, where radiation is delivered continuously while the device moves (like painting the tumour with radiation). There is also clinical interest in using PFX for multi-fraction RS (MF-RS), which has challenging dose homogeneity requirements, unlike single-fraction RS (SF-RS), where target dose can be up to twice the prescription dose. With MF-RS, organ motion and setup errors must be addressed. It is also believed that small amounts of organ motion are present even in SF-RS. To address these challenges in planning treatments, this research program will develop methods and tools within the fields of mathematical modelling and optimization to provide high-quality, personalized cancer care for patients receiving SF-RS and MF-RS with PFX. This research will allow more accurate elimination of tumours using PFX, and thus has the potential to save many lives and spare many patients from side effects arising from suboptimal treatments. Further, the use of continuous path treatments will allow larger targets to be treated, expanding the population who is eligible to be treated by PFX, giving clinicians more treatment options, and increasing utilization of PFX equipment. Additionally, continuous treatments will be much faster to delivery than step-and-shoot treatments, allowing for more throughput on PFX units. Thus, Canadians will benefit from both improved health outcomes in cancer treatments and improved utilization of costly healthcare equipment The research field will benefit from the development of techniques to model continuous motion within mathematical models, as well as new techniques to incorporate dimension-specific uncertainties within convex penalty-based optimization models. The methods to generate continuous paths that cover a closed volume have applications in robotics and other areas.