Dosimetric feasibility of an “off-breast isocenter” technique for whole-breast cancer radiotherapy

Abstract

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 t test.

Results

Comparing plan_ref vs. plan_off-breast, no significant differences in PTV coverage (V95%) were found (96.5% vs. 96.2%; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.361 to PTVbreast; 97.0% vs. 97.0%; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.977 to PTVtumor_bed). With regard to OARs, no substantial differences were observed in any analyzed metric: V5Gy (30.3% vs. 31.4%; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.486), V20Gy (10.3% vs. 10.3%; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.903) and mean dose (7.1[[ce:hsp sp="0.25"/]]Gy vs. 7.1[[ce:hsp sp="0.25"/]]Gy; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.924) to the ipsilateral lung; V5Gy (11.2% vs. 10.0%; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.459), V30Gy (0.7% vs. 0.6%; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.251) and mean dose (2.3[[ce:hsp sp="0.25"/]]Gy vs. 2.2[[ce:hsp sp="0.25"/]]Gy; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]0.400) to the heart; and average dose to the contralateral breast (0.4[[ce:hsp sp="0.25"/]]Gy vs. 0.5[[ce:hsp sp="0.25"/]]Gy; p[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]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.