Background: The present work aims to study the out-of-field photon calculation accuracy of a commercial treatment planning system (TPS), Oncentra, for three-dimensional conformal radiotherapy (3D-CRT) and intensity modulated radiation therapy (IMRT) treatment techniques from Elekta Synergy medical linear accelerator. Materials and Methods: Accuracy of individual out-of-field dose components was studied by defining a square field of 10 x 10 cm² in our TPS and it is verified by using ionization chamber and TLDs (Thermoluminescent dosimeter) at 5 cm depth. Using 3D-CRT and IMRT techniques Ca (carcinoma) prostate treatment plans were created using 6 MV photon beam and calculated in Oncentra Masterplan v4.3 planning systems for out-of-field and further compared the same with TLD measurements. Results: Individual components study shows the poor out-of-field calculations in TPS, with respect to that obtained from TLD measurements. Underestimation increases from 10 % to 80 % as a contribution from various components while moving far from field edge. Complex IMRT plans resemble this underestimation of TPS calculations in greater extend. Conclusion: This study quantifies the poor accuracy for out-of-field dose calculations in TPS. No significant difference in 3D-CRT and IMRT plans is found at near field edge. While, as distance from field edge increases, underestimation of TPS in IMRT plan is higher. 10 % – 60 % difference in out-of-field dose was found as distance move from 2 cm to 7.5 cm far from field edge between TPS estimations and the measurements.