Background: Purpose of the study is to evaluate field-in-field (FIF) and sliding-window intensity modulated radiation therapy (SW-IMRT) treatment plans in terms of homogeneity index (HI), conformity index (CI) and normal tissue doses for organs at risk (OAR) in patients with endometrial carcinoma along with deriving a new expression for CI. Materials and Methods: Four different FIF plans with different MLC (multi-leaf collimator) margins and SW-IMRT plan were compared in 20 endometrial cancer patients who underwent adjuvant pelvic radiotherapy with 50.4 Gy in 28 fractions. Normal tissue doses were assessed for OAR from dose volume histograms (DVHs). HI and CI values were calculated according to the reports of International Commission on Radiation Units and Measurements (ICRU)-83, Radiation Therapy Oncology Group (RTOG) 1993 and ICRU-62. A new CI was developed and named CIOPT standing for optimized conformity index. Results: SW-IMRT provided a significantly better CI in comparison to other FIF plans (p< 0.001). Conventional-FIF (C-FIF) was the most homogeneous plan compared to all other FIF plans and SW-IMRT (p≤0.001). The absolute volume of small intestine that received ≥45 Gy (V45) and doses received by a 30% volume of rectum (D30) were dramatically reduced in SW-IMRT (p < 0.001). Conclusion: MLC margins had a substantial influence on OAR doses, HI and CI. A close proximity of CI_OPT to 1 indicated that this formulation of CI was a useful plan evaluation tool, which was also compatible with the RTOG 1993 and ICRU-62 reports.