HSV-tk + Valacyclovir Therapy in Combination With Brachytherapy for Recurrent Prostate Cancer

This study is testing whether adding a gene therapy (called HSV-tk, which makes tumor cells sensitive to a virus-killing drug called valacyclovir) to standard radiation seed treatment (brachytherapy) improves outcomes for people with a recurrent brain tumor called glioblastoma. **You may be eligible if...** - You have been diagnosed with glioblastoma (an aggressive brain tumor) that has come back after prior treatment - You are a candidate for brachytherapy (small radioactive seeds placed inside the tumor) - You are at least 18 years old and in reasonable overall health - Your tumor meets certain size and location requirements **You may NOT be eligible if...** - You cannot take the antiviral drug valacyclovir (for example, due to kidney problems or allergy) - You have had certain prior brain radiation treatments that would make this combination unsafe - You are pregnant or breastfeeding - You have serious infections or significantly reduced immune function - Your overall health is too poor to tolerate the procedure Talk to your doctor to see if this trial is right for you.