Hyponatremia after chemotherapy is not an uncommon clinical syndrome.
[ncbi.nlm.nih.gov]
Cisplatin is a well-known drug which can raise various adverse events, including hyponatremia.
[jstage.jst.go.jp]
Cuadrado, et al., “Incidence of Hyponatremia and Its Causes in Neurological Patients,” Endocrinol Nutrition, Vol. 5, No. 57, 2010, pp.182-186. doi:10.1016/j.endonu.2010.02.009
[scirp.org]