BACKGROUND Synchronous bilateral breast cancer is rare. A case is presented where whole breast irradiation (WBI) was planned after breast conserving surgery in a patient with synchronous bilateral breast cancer. A comparison was made between the feasibility of helical tomotherapy and direct tomotherapy. CASE REPORT A 60-year-old woman was found to have bilateral breast nodules on routine mammographic screening, resulting in bilateral lumpectomy and sentinel lymph node biopsy. Histopathology showed a 6 mm diameter invasive ductal carcinoma in the right breast (Grade 1, hormone receptor positive, HER2 negative) and an 8mm diameter tubular carcinoma in the left breast (Grade 1, hormone receptor positive, HER2 negative). Lymph node biopsy and histology, chest X-ray, abdominal ultrasound scan, and bone scintigraphy were negative for metastases (both tumors were Stage 1). Adjuvant therapy with commenced with anastrozole, but no chemotherapy was given. Clinical target volumes (CTVs) were contoured on computed tomography (CT) images. For planning target volumes (PTVs), CTVs were expanded by 1 cm in all directions, except for the medial 5 mm. Since dose constraints to organs at risk (OARs) were beyond established limits, CTVs were expanded by 5 mm. For PTVs, OAR doses and homogeneity indices for helical tomotherapy and direct tomotherapy were compared. Helical tomotherapy provided better target volume coverage and OAR sparing than direct tomotherapy. CONCLUSIONS In a case of bilateral synchronous Stage 1 and Grade 1 breast cancer, helical tomotherapy appeared more suitable than direct tomotherapy.