Hepatocellular carcinoma (HCC) often presents as a diffuse or multifocal tumor making it difficult to control by surgery or radiation. Radio-inducible herpes simplex virus thymidine kinase (HSV-TK) gene therapy has been shown to enhance local tumor control after radiation therapy (RT), while limiting the expression of the transgene in the irradiated tumor tissues. To prevent liver tumor recurrence and control systemic disease while limiting the potential bystander toxicity of HSV-TK therapy, we proposed to stimulate endogenous dendritic cell (DC) proliferation with systemic adenovirus Flt3 ligand (Adeno-Flt3L) gene therapy, followed by primary tumor radiation therapy combined with a radio-inducible HSV-TK gene therapy. We hypothesized that adenovirus-expressing Flt3L gene therapy will stimulate DC proliferation, allowing the upregulated DCs to locally harness tumor antigens released from HSV-TK/RT-treated HCC cells, thereby converting irradiated tumors to an autologous in situ tumor vaccine in mice with primary liver tumors. To test this hypothesis, an expression vector of HSV-TK was constructed under the control of a radio-inducible promoter early-growth response (Egr-TK) and a recombinant adenovirus-expressing human Flt3L was constructed. The Adeno-Flt3L [10⁹ plaque forming units (pfu)] was administered intravenously on days 1 and 8 after radiation therapy. The murine hepatoma cell line (BNL1ME) was stably transfected by Egr-TK or Egr-Null (encoding no therapeutic gene). Palpable tumors in BALB/c mice were treated with a localized dose of 25 Gy of radiation followed by ganciclovir (GCV, 100 mg/kg, 14 days). Four treatment cohorts were compared: Egr-Null/GCV RT Adeno-LacZ; Egr-Null/GCV RT Adeno-Flt3L; Egr-TK/GCV RT Adeno-LacZ; and Egr-TK/GCV RT Adeno-Flt3L. There was no primary tumor regression in the Egr-Null tumors after radiation therapy alone. In contrast, Egr-TK tumors had nearly complete tumor regression for 3 weeks after radiation therapy (P < 0.01), however, long-term follow-up demonstrated primary tumor recurrence and death secondary to pulmonary metastasis. Flt3L expression was confirmed by serum bioassay (mean = 88 ng/mL) in these animals and Western blotting of tissue culture medium in Adeno-Flt3L-infected BaF/huFlt3L cells. Radiation therapy with Adeno-Flt3L gene therapy effectively retarded primary tumor growth when compared to radiation therapy alone. The trimodality therapy (Egr-TK/GCV RT Adeno-Flt3L) was the most efficacious with 40% complete tumor regression (>100 days) and <20% pulmonary metastases, indicating the development of sustained antitumor immune response. These studies provide a rationale for triple modality therapies with radiation-inducible HSV-TK gene therapy and Adeno-Flt3L when used in combination with primary tumor radiation therapy for improved local and systemic control of HCC.