Dosimetric feasibility of an "off-breast isocenter" technique for whole-breast cancer radiotherapy.

Aim: To investigate the viability of placing the treatment isocenter at the patient midline for breast cancer radiotherapy in order to avoid the risk of collisions during image-guided setup and treatment delivery.

Background: The use of kilovoltage orthogonal setup images has spread in last years in breast radiotherapy. There is a potential risk of an imaging system-patient collision when the isocenter is laterally placed.

Materials And Methods: Twenty IMRT plans designed by placing the isocenter within the breast volume ("plan_ref"), were retrospectively replanned by shifting the isocenter at the patient's midline ("plan_off-breast"). An integrated simultaneous boost (SIB) technique was used. Multiple metrics for the planning target volumes (PTVs) and organs at risk (OARs) were compared for both approaches using a paired test.

Results: Comparing plan_ref vs. plan_off-breast, no significant differences in PTV coverage (V95%) were found (96.5% vs. 96.2%;  = 0.361 to PTVbreast; 97.0% vs. 97.0%;  = 0.977 to PTVtumor_bed). With regard to OARs, no substantial differences were observed in any analyzed metric: V5Gy (30.3% vs. 31.4%;  = 0.486), V20Gy (10.3% vs. 10.3%;  = 0.903) and mean dose (7.1 Gy vs. 7.1 Gy;  = 0.924) to the ipsilateral lung; V5Gy (11.2% vs. 10.0%;  = 0.459), V30Gy (0.7% vs. 0.6%;  = 0.251) and mean dose (2.3 Gy vs. 2.2 Gy;  = 0.400) to the heart; and average dose to the contralateral breast (0.4 Gy vs. 0.5 Gy;  = 0.107).

Conclusions: The off-breast isocenter solution resulted in dosimetrically comparable plans as the reference technique, avoiding the collision risk during the treatment session.