Background: Field-in-field (FIF) technique for treatment of breast cancer has become a widely performed method over the recent years. However, there was no study in the application of FIF technique in patients with breast cancer undergoing mastectomy and lumpectomy. This study is an attempt to compare dosimetric outcomes after applying the FIF technique in these patients. Materials and Methods: Twenty-four patients with right and left breast cancer participated in this study. The FIF planning technique was carried out for patients undergoing mastectomy and lumpectomy using the TiGRT treatment planning system (TPS). For the comparison purpose, we used two main indices, i.e. dose homogeneity index (HI) and conformity index (CI), the number of subfields, as well as mean, maximum, and minimum doses, doses received by 2% (D₂) and 98% (D₉₈) of the target volume, volumes received greater than 107% (V>107%) and less than 95% (V<95%) of the prescribed dose, doses to organs at risk (OARs), and total monitor units (MUs). Results: The results indicated that CI and HI are better in patients with right and left breast lumpectomy surgery (p<0.038 and p<0.047) relative to mastectomy patients (p<0.037 and p<0.029), respectively. Other parameters mentioned in Materials and Methods did not show any significant difference between the two groups of patients (p>0.05). Conclusion: The use of alternative subfields resulted in better dose distribution in target volume with the increase in breast volume. Moreover, to disappear the hot spot areas in isodose curves, it is essential to elevate the number of subfields.