Evaluating the improvements of placing the treatment isocentre at the boost centre of mass (CoM) in a hybrid treatment for breast cancer radiotherapy.

Twenty-two patients were planned in two isocentre locations with two forward intensity-modulated radiation therapy (fIMRT) tangentials to the breast and a volumetric-modulated arc therapy (VMAT) to the boost. A simultaneous integrated boost technique was used. Breast Boost (BB) Vector was investigated as a criterion for selecting an appropriate isocentre placement. Various metrics for boost, breast and hybrid plans were analysed using analysis of variance statistics.

Comparing hybrid plans at the boost CoM vs. hybrid plans at the breast CoM, no significant differences were found. Analysis of relative variations of planning target volume (PTV) boost coverage vs. BB Vector indicated an upgrade in boost CoM isocentre strategy. Dose to organs at risk was comparable: V5Gy (26·24 vs. 25·69%, p = 0·8), V20Gy (14·66 vs. 14·58%, p = 0·959) and the mean dose (7·37 Gy vs. 7·26 Gy, p = 0·879) to ipsilateral lung; V5Gy (15·60 vs. 15·22%, p = 0·903), and the mean dose (4·91 Gy vs. 4·86 Gy, p = 0·950) to heart and dose to free breast of boost (46·71 Gy vs. 46·62 Gy, p = 0·408).

The hybrid fIMRT–VMAT technique centred at the boost CoM resulted equivalent to plans centred at the breast CoM, while benefiting from an enhancement in PTV boost coverage for patients with BB Vector superior to 5.