This phase II trial tests whether niraparib works to shrink tumor in patients with
leiomyosarcoma. Niraparib is an inhibitor of PARP, an enzyme that helps repair
deoxyribonucleic acid (DNA) when it becomes damaged. Blocking PARP may help keep cancer cells
from repairing their damaged DNA, causing them to die. PARP inhibitors are a type of targeted
therapy.