Session Item

Treatment planning and delivery of bilateral breast cancer (BBC) in radiotherapy is a challenging task. Nowadays many advanced techniques are used to improve plan quality, reduce side effects to critical organs and better clinical outcome. The purpose of this retrospective study was to evaluate influence of combination of different photon beams with single and dual isocenter (DIC) bilateral breast volumetric modulated arc therapy (VMAT). 

A retrospective study was conducted for five bilateral carcinoma breast cancer patients (Post chemotherapy and Modified Radical Mastectomy) treated with Elekta Synergy® linear accelerator. Clinical target volume (CTV) was defined as the volume covering bilateral chest wall along with Supraclavicular Fossa (SCF), level III and Internal mammary lymph nodes delineated by clinically experienced radiation oncologists. Clinically approved plans (reference plan) were generated for VMAT technique in Monaco™ Version 5.11 treatment planning system using 6MV (6X) photon beam with single isocenter (SIC) (SIC-6X). For comparison, three more plans were generated such as SIC with combination of 6MV & 4MV (SIC-6X4X), DIC with 6MV (DIC-6X) and combination of 6MV&4MV (DIC-6X4X). Conformity index (CI), Homogeneity index (HI), Integral dose (ID), Monitor units(MU), 95% of prescribed dose (D95%) received by left CTV (CTV_left ) and right CTV (CTV_Right), max dose to CTV (CTV_max), mean dose, max dose and dose volume received by heart, lung, spine, and esophagus were evaluated using dose volume histogram. In addition, gamma pass rate of 3%, 3mm and 3%, 2mm were evaluated for both 2D and 3D. 

A total of 20 VMAT plans were generated for dosimetric comparison. The results of CI, HI, ID, and CTV_max did not show any significant difference between SIC-6X, SIC-6X4X, DIC6X and DIC-6X4X (p>0.05). The D95% of CTV_left and CTV_Right were slightly higher in DIC-6X4X as compared to SIC-6X, SIC-6X4X, and DIC6X. However, no significant difference were observed (p>0.05). A reduction of 2 Gy mean heart dose was observed in SIC-6X4X when compared to SIC-6X, DIC6X and DIC-6X4X. Similarly, a slight decrease of total lung dose (V20) was seen in DIC-6X4X than SIC-6X, SIC-6X4X and DIC6X. An increased MU was found in DIC6X, DIC-6X4X and SIC-6X4X as compared to SIC-6X (p<0.05). The result of 3%, 3mm and 3%, 2mm were observed as ≥98% and ≥95% in 2D and≥98% and≥95% in all the VMAT plans. 

Overall analysis, the use of SIC and DIC with 6X did not show any significant difference in the VMAT plan quality. However, the combination of 6X4X was a suitable option to reduce dose to critical structures around CTV on both SIC and DIC without compromising quality of the BBC VMAT plan.