Purpose: The role of fusion imaging methods SPECT-CT and PET-CT is currently being investigated in radiotherapy (RT) treatment planning. SPECT and PET provide extra information for gross tumor volume (GTV) delineation. The purpose of this study was to evaluate the role of SPECT-CT in RT. Methods: SPECT-CT studies were performed in 55 patients with 99mTc-MIBI/Tetrofosmin (TF), with 99mTc-Nanoco, with 99mTc-MDP and with 185 MBq 131I scan. All studies were performed with SPECT-CT camera, using low dose CT (130KeV, 30 mA, 3-5 mm step). Corresponding slices on SPECT-CT were compared to the diagnostic CT projections from the simulator. Diagnostic CT scans were performed using 3-5 mm slices both to aid volume definition and to create high quality digitally reconstructed radiographs (DRRs) to aid verification. The volumetric CT data were sent to the treatment planning system. Results: Application of SPECT-CT with 99mTc-MIBI/TF allowed to diagnose residual malignant mass or a recurrent tumor after combined (surgery and chemotherapy/ hormonotherapy/ therapy in studied patients with breast cancer, thymomas and lung cancer. SPECT-CT99mTc-MIBI/ TF contributed to the definition of clinical target volume (CTV). SPECT-CT imaging of sentinel lymph nodes (SLNs) in medially located breast cancer was important for correct topography of intercostal space in cases with parasternal drainage. Irradiation of this subgroup of patients in conjunction with the chest wall and the supraclavicular region should be considered electively for each patient. The combined application of baseline whole body bone scintigraphy, followed by SPECT-CT fusion gave the possibility for correct radiotherapy planning of the GTV in tumor-induced bone disease. SPECT-CT is useful for diagnosis of recently discovered non-responsive to 131I treatment patients with differentiated thyroid cancer. SPECT-CT data are important to avoid prescribed 131I therapy and to define CTV for external beam RT (EBRT). Conclusion: SPECT-CT images could be applicable in RT planning to precisely delinate tumor volume.