Disclosed herein are salts of (R)-2-fluoro-10a-methyl-7,8,9,10,10a,11-hexahydro-5,6,7a,11-tetraazacyclohepta[def]cyclopenta[a]fluoren-4(5H)-one, and crystalline forms (polymorphs) thereof. Also disclosed herein are methods of preparing those crystalline forms, and methods of using at least one of those crystalline forms in treating a cancer responsive to inhibition of PARP, especially a cancer associated with BRCA1/2 mutant activities or other HR deficiencies, in a subject.