Prostate cancer is the most frequently diagnosed type of cancer among males. Brachytherapy has become a common treatment for this disease. In this form of radiotherapy, sealed radiation sources are placed through a implant catheters inside the prostate. This treatment affects the tumor cells locally and reduces damage to healthy tissues. However, accessing all relevant areas is met by difficulties. Innovating brachytherapy instruments can overcome these challenges. These new instruments need to be tested in a lab environment before patient testing is allowed. Phantom models provide a good validation model for testing new medical instruments. The goal of this study is to design and develop a cancerous prostate phantom model which can be used to test newly developed instruments for brachytherapy. A material study was conducted to find the best mechanically tissue-mimicking materials. The effects of different concentrations, varying volumes, coolant additive and dimethyl-sulfoxide additive on the Young's modulus of poly-vinyl alcohol was tested. Unconfined compression tests were performed after each freeze-thaw cycle for a total of 7 cycles. These results showed that poly-vinyl alcohol can form material which, based on its Young's modulus, can mimic prostate tissue and adipose tissue. An increase in poly-vinyl alcohol concentration, volume, coolant additive, dimethyl-sulfoxide additive and the number of freeze-thaw cycles were each found to increase the Young's modulus of the material. Three cancerous prostate phantom models were made of poly-vinyl material with each a different stiffness value achieved by varying the poly-vinyl alcohol concentrations. A low Young's modulus for model 1, medium for model 2 and high for model 3. The poly-vinyl alcohol was solved in a mixture of distilled water:DMSO (10:90 ratio) to produce a transparent material. Each model included a prostate, urethra and surrounding adipose tissue. The models were surrounded by a transparent casing which included an opening and template for needle insertion at the front. A pubic bone was added to model 2 and 3 to simulate prostate blockage by this tissue. Model 1 and 2 did not achieve the transparency that was required. The transparency was found to be sufficient in model 3. A needle insertion experiment was conducted to validate the prostate phantom models. An 18Gauge brachytherapy needle was inserted 13, 10 and 20 times in model 1, 2 and 3 respectively with a velocity of 5 mm/s. Mean peak forces, describing the force upon puncture of the prostate material, for model 1 (0.57 N) and 2 (3.54 N) were lower than multiple peak forces found in literature. The median peak force of model 3 (6.17 N) came close to the peak force found during in patients with prostate cancer (6.28 N) in the study of Podder et al. (2006). Higher Young's moduli values produced higher peak forces. The insertion force in the adipose tissue of the models was lower than the results found in literature. This study has shown that poly-vinyl alcohol can function as an easily controlled tissue mimicking material. The material study created an overview of the effects of concentration, volume, coolant, DMSO and freeze-thaw cycles on the Young's modulus of poly-vinyl alcohol. An easy to manufacture cancerous prostate phantom model was made of poly-vinyl alcohol and DMSO. Model 3 developed in this project can function as a test model for newly developed instruments meant for brachytherapy.