Speaker
Description
Background
Proton therapy for breast cancer is usually given in free breathing (FB). With the use of deep inspiration breath-hold (DIBH) technique, the location of the heart is shifted more caudally, away from the internal mammary nodes (IMN) and, thus, the dose to the heart could potentially be reduced. The aim of this study was to compare proton therapy in FB with proton therapy in DIBH for patients with breast cancer in respect to minimizing heart exposure.
Material and Methods
In this study, 16 patients with left-sided breast cancer receiving loco-regional proton therapy (including the IMN) were included. The FB and DIBH plans were created for each patient using spot-scanning proton therapy with 2-3 en face fields, single field optimization and robust optimization. All plans were created with a prescription dose of 40 Gy relative biological effectiveness (RBE) in 15 fractions. To reduce treatment delivery time for the DIBH plans, minimum monitor unit per spot and the distance between spots was increased. The magnitude of the caudal shift of the heart in DIBH was measured as the longitudinal difference in heart position relative to the IMN CTV on the CT scans in FB and DIBH.
Results
All plans complied with target coverage constraints. The median MHD was statistically significant reduced from 1.1 to 0.6 Gy RBE by applying DIBH. The median caudal shift of the heart in DIBH was 2.7 cm (range, -0.7 cm to 5.2 cm) which may explain the reduced MHD in the DIBH plans. No statistical significant difference was seen for mean dose and V17Gy RBE to the ipsilateral lung. The median of the treatment delivery time for the DIBH plans was reduced by 27% compared to the FB plans without compromising the plan quality. The estimated number of breath-holds (assuming that patients perform breath-holds of 20 seconds) ranged from 11 to 15 breath-holds with the DIBH plans.
Conclusion
Even though the median absolute dose reduction to the heart was limited, it could still be relevant to consider DIBH for a subset of these patients with the largest reduction in mean dose. However, a large caudal shift of the heart in DIBH does not necessarily imply a large reduction of dose to the heart compared with FB, and, thus, comparative proton treatment planning in FB and DIBH will be needed to identify patients with the largest dosimetric benefit.
