## 1 Introduction  
Controlled localised delivery systems have been replacing systemic administration of cancer therapies for some time [1]. These sustained-release injectable formulations are usually designed as microparticles, implants or gels [1]. They extend the presence of therapy at the tumour site by gradually releasing treatment as they degrade, improving treatment efficacy and avoiding excessive treatment loss to neighbouring tissue or the lymphatic system. Sustained-delivery systems have improved the effectiveness of a range of cancer treatments, including chemotherapy [2; 3], oncolytic virotherapy [4; 5; 6; 7] and immunotherapy [8; 9; 10]. Oncolytic virotherapy and immunotherapy are two promising cancer therapies that have gained significant attention over recent years [11]. Oncolytic virotherapy focuses on genetically engineering viruses to infect, replicate within, and lyse tumour cells. Many oncolytic viruses can accommodate gene insertions and thus be \"armed\" with therapeutic transgenes to produce immunostimulatory signals that stimulate an antitumor immune response, making them useful immunotherapy agents [12; 13]. Immunotherapy treatments typically involve administration of additional cytokines, molecules or immature immune cells. Interleukin 12 (IL-12) and granulocyte-monocyte colony-stimulating factor (GM-CSF) are cytokines used in immunotherapy clinical trials [14; 15]. IL-12 is known to have potent antitumor effects through promotion of the immunity of helper T cells and activation of killer T cells, whereas GM-CSF is known to enhance the processing and presentation of antigen on antigen-presenting cells (APCs) [13]. Immature immune cells, such as dendritic cells (DCs), are also tested in immunotherapy trials with the goal of overcoming the induction of energy or tolerance by the cancer [16; 17]. Intratumoural administration of DCs increases the probability of tumour antigen recognition and subsequent activation of helper T cells and killer T cells, leading to heightened tumour cell killing [18]. An increasing number of studies has shown that combined virotherapy and immunotherapy (viro-immunotherapy) leads to a prolonged antitumor response, where both viruses and immune cells work simultaneously to eradicate cancer cells [19; 20; 10; 13; 21]. Conventionally, oncolytic virotherapy and immunotherapy are administered intratumourally or intravenously. This can result in rapid clearance of the virus and functional impairment of immune cells in the tumour microenvironment [10]. To overcome this, there have been many investigations into the potential of treatment loaded onto injectable gels or polymer matrix systems [5; 6; 7; 10]. [PERSON] et al. [10] showed that an oncolytic adenovirus expressing IL-12 and GM-CSF loaded onto a hydrogel with immature DCs had a higher retention in tumour tissue. Additionally, the use of the gel resulted in greater antitumor activity due to the sustained release of therapy. Immune checkpoint blockade therapy and agonists were also shown to be considerably more effective when loaded onto an degradable gel matrices [8; 9]. A major challenge for sustained-release mechanisms is determining a release profile that achieves optimal therapeutic benefit. Mathematical models describing the biological response to disease can be used as an important tool in therapy planning and optimisation [22; 23; 24; 25]. Deterministic systems of ordinary differential equations (ODEs) are used to describe the key biophysical interactions and are then analysed to determine optimal therapeutic protocols [23]. In oncology, these techniques have been used to successfully optimise chemotherapy and immunotherapy treatment [26; 27; 28; 29; 30]. [PERSON] et al. [31] optimised the scheduling of one cycle of chemotherapy drug VP-16 and found a new optimal drug regime which improved the clinical efficacy. For combined immunotherapy and chemotherapy, [PERSON] et al. [32] developed an ODE system that predicted which treatment protocols would result in tumour elimination.