To compare the dosimetry and reproducibility of set-up with monoisocentric technique (MIT) and dual isocentric technique (DIT) in adjuvant breast radiotherapy (RT).

Breast cancer patients treated with MIT or DIT were retrospectively studied. The organ-at-risk dose was compared between two groups. All patients underwent set-up verification with an electronic portal imaging device, and set-up time was recorded for each fraction. Treatment reproducibility was assessed in terms of systematic and random error.

Twenty patients were included (11 right and 9 left-sided tumours) and ten received whole breast RT, while the rest received chest wall RT. Overall, the mean heart dose was less with MIT (0.40 versus 0.79, p = <0.001) as well as in left-sided tumours (0.37 versus 0.98, p = 0.003). The maximum dose at the field junction was significantly higher with DIT (43 Gy, 107%, p = 0.003). The maximum total error was 1 cm in lateral for supraclavicular field and 8 mm in superior–inferior in tangents for both techniques. There was no difference in set-up errors between the two techniques.

MIT resulted in better dose homogeneity at the field junctions and reduced mean heart dose as compared to DIT. MIT is safe for implementation in clinical practice for breast cancer treatment.

This study is one of the few studies comparing MIT with DIT in terms of the dosimetry and the first one to compare set-up errors between the two techniques. The ease of set-up and better dosimetry with MIT was achieved.