X-ray-induced　photodynamic　therapy　offers　substantial　promise　for　treating　deep-seated　tumors,　but　it　is　still　limited　by　highly　inefficient　energy　transfer　processes　and　the　stringent　requirements　for　scintillators　with　high　luminescence　quantum　yield　and　significant　singlet-triplet　intersystem　crossing　ratios.　Herein,　we　describe　X-ray-induced　electron-dynamic　therapy　(X-eDT),　which　obviates　the　need　for　intersystem　crossing　by　exposing　nonluminescent　hafnium-silica　nanoparticles　to　X-rays,　to　generate　high-energy　electrons　that　can　sensitize　lower-lying　triplet　states　of　various　photosensitizers.　Our　approach　strongly　induced　the　production　of　singlet　oxygen　(6.18-fold)　in　vitro　even　at　lower　X-ray　doses,　and　in　mice　it　strongly　inhibited　the　growth　of　xenografts　derived　from　liver,　breast,　or　colon　cancer　cell　lines　(CDX),　and　growth　of　patient-derived　xenografts　(PDX)　of　hepatocellular　carcinoma.　In　these　CDX　preclinical　systems,　X-eDT　was　not　only　effective　against　the　irradiated　xenograft　but　also　against　untreated　xenografts　in　the　same　animal,　and　these　abscopal　effects　involved　enhanced　tumor　infiltration　by　CD4+T　cells,　CD8+T　cells,　and　IFN-γ-polarized　M1　macrophages　within　the　tumor　microenvironment.　X-eDT　even　stimulated　the　production　of　memory　T　cells　that　inhibited　rechallenges　after　treatment.　These　findings　suggest　that　X-eDT　can　be　effective　against　primary　and　metastatic　tumors　as　well　as　tumor　recurrence,　which　makes　it　much　more　powerful　than　conventional　X-PDT.　©　2025　American　Chemical　Society.