Session Item

Today, proton therapy for breast cancer is mostly performed in free breathing as breathing motion is assumed to have minor impact on the radiological path length when using an en face field arrangements. This study examines the effect of respiratory motion in patients with breast cancer receiving loco-regional proton therapy based on 4DCT scans. 

Twenty-five patients with breast cancer (20 left-sided, five right-sided), treated at Danish Centre for Particle Therapy in 2019 and 2020, were included. Patients were immobilised in supine position with arms above the head. A planning CT scan in free breathing and a 4DCT scan, both with 2 mm slice thickness, were acquired for all patients on a Siemens Somatom Definition Edge scanner. The 4DCT scan was sorted in ten phases according to amplitude. Breast or chest wall, internal mammary nodes (IMN), interpectoral nodes and level 1-4 lymph nodes (level 1 irradiation was required for 15 patients) were contoured as clinical target volumes (CTVs) on the planning CT following the ESTRO guidelines. Relative biological effectiveness (RBE) was fixed at 1.1. Patients were prescribed either 40 Gy RBE in 15 fractions (nine patients) or 50 Gy RBE in 25 fractions (16 patients). Clinical planning objectives: CTV breast/chest wall V95% (relative volume receiving 95% of the prescribed dose) and CTV lymph nodes V90% should be at least 98%. Spot-scanning proton therapy plans using two to three en face fields, single-field optimisation and range shifter were created in Eclipse v13.7 (Varian Medical Systems). CTVs were robustly optimised using 14 scenarios: 0 mm ±3.5% and ±5 mm ±3.5%. The plan was recalculated on all ten phases of the 4DCT scan with fixed monitor units. All ten phases in the 4DCT scan were deformable image registered to the planning CT in Velocity v4.0 (Varian Medical Systems), and the deformed doses from the phases were accumulated on the planning CT. A paired, two-tailed Wilcoxon signed-rank test was used to compare dose metrics from the nominal plan and the 4D evaluation. 

The median (range) amplitude of respiratory motion measured at sternum position in the axial plane on the 4DCT scan was 2 mm (1 to 4 mm). Dosimetric results from the nominal plan and the 4D evaluation can be seen Table 1 and an example of a dose-volume histogram for a patient can be seen in Figure 1. CTV coverage was below 98% for level 4 in one nominal plan and three 4D evaluations, for IMN only in one 4D evaluation and for breast/chest wall in one nominal plan. In the 4D evaluation, mean dose to the heart increased to 0.3 Gy RBE (from 1.9 to 2.2 Gy RBE) and decreased to 0.5 Gy RBE (from 1.6 to 1.1 Gy RBE), and mean dose to the ipsilateral lung increased to 1.0 Gy RBE (from 9.8 to 10.8 Gy RBE) and decreased to 0.8 Gy RBE (from 12.7 to 11.9 Gy RBE).

Respiratory motion has a modest impact on the dose distribution in proton therapy plans for breast cancer when using robust and single-field optimisation and an en face beam arrangement.