Figure 5.

Transverse dose distribution differences between DIBH and SB photon and proton plans. For photon VMAT DIBH and SB (CT scan planes 1–6: a,c) plans, a large dose spread is seen outside the target volumes in the dorsal shoulder region and inside the upper thorax¹³. For IMPT proton plans, a larger dose spread is clearly observed in DIBH where tissues of high density were replaced by tissues of low density (CT scan planes 1–6: b,d). Plane 1: lung top. Plane 2: near the cranial edge of the left internal mammary lymph node chain. Plane 3: near the cranial edge of the left breast. Plane 4: near the caudal edge of the internal mammary lymph node chain at the central part of the left breast. Plane 5: through the caudal quadrants of the left breast. Plane 6: near the caudal edge of the left breast. CT scan planes 3–6 show in the SB setting (d) that distal edges of proton pencils are located in the heart, blood vessels and pericardial fat. These ‘high’ density tissues protect the lung in SB. In the DIBH proton setting (b), high density tissue is replaced by low density lung tissue resulting in increased proton range. Proton dose is deposited far outside the deep edge of the target into the left lung tissue in the DIBH plan (b) compared to SB plan (d).