Proton pencil beam scanning reduces secondary cancer risk in breast cancer patients with internal mammary chain involvement compared to photon radiotherapy

Abstract

Purpose: Proton pencil beam scanning (PBS) represents an interesting option for the treatment of breast cancer (BC) patients with nodal involvement. Here we compare tangential 3D-CRT and VMAT to PBS proton therapy (PT) in terms of secondary cancer risk (SCR) for the lungs and for contralateral breast.

Methods: Five BC patients including supraclavicular (SVC) nodes in the target (Group 1) and five including SVC plus internal-mammary-nodes (IMNs, Group 2) were considered. The Group 1 patients were planned by PT versus tangential 3D-CRT in free-breathing (FB). The Group 2 patients were planned by PT versus VMAT considering both FB and deep-inspiration breath hold (DIBH) irradiation. The prescription dose to the target volume was 50 Gy (2 Gy/fraction). A constant RBE = 1.1 was assumed for PT. The SCR was evaluated with the excess absolute risk (EAR) formalism, considering also the age dependence. A cumulative EAR was finally computed.

Results: According to the linear, linear-exponential and linear-plateau dose response model, the cumulative EAR for Group 1 patients after PT was equal to 45 ± 10, 17 ± 3 and 15 ± 3, respectively. The corresponding relative increase for tangential 3D-CRT was equal to a factor 2.1 ± 0.5, 2.1 ± 0.4 and 2.3 ± 0.4. Group 2 patients showed a cumulative EAR after PT in FB equal to 65 ± 3, 21 ± 1 and 20 ± 1, according to the different models; the relative risk obtained with VMAT increased by a factor 3.5 ± 0.2, 5.2 ± 0.3 and 5.1 ± 0.3. Similar values emerge from DIBH plans. Contrary to photon radiotherapy, PT appears to be not sensitive to the age dependence due to the very low delivered dose.

Conclusions: PBS PT is associated to significant SCR reduction in BC patients compared to photon radiotherapy. The benefits are maximized for young patients with both SVC and IMNs involvement. When combined with the improved sparing of the heart, this might contribute to the establishment of effective patient-selection criteria for proton BC treatments.

Keywords: Breast cancer; Proton therapy; Secondary cancer risk; Tangential 3D-CRT; VMAT.

Fig. 1

Summary of the dose distributions obtained with the different irradiation techniques. A representative Group 1 patient receiving PT is shown in a, while the corresponding 3D-CRT plan is displayed in b. Concerning Group 2, a typical dose distribution with FB treatment is shown for PT (c) and VMAT (d). The corresponding DIBH plan is finally reported for PT (e) and VMAT (f). The limited dependence on patient setup (FB vs DIBH) can be appreciated

Fig. 2

Cumulative DVHs obtained averaging over the patients included in the study for Group 1 (left column) and Group 2 (right column). DVHs are shown for the ipsilateral lung (upper panel), contralateral lung (middle panel) and contralateral breast (lower panel). The different planning techniques are shown for each OAR. Please notice that different scales are adopted for the X-axis

Fig. 3

EAR difference as a function of the age of exposure (EAR_30–70–EAR_50–70) for the single patients included in the analysis. The EAR difference is shown for the contralateral breast for Group 1 patients treated with tangential 3D-CRT vs PT (top panel) and for Group 2 patients treated with VMAT versus PT in the case of FB (middle panel) and DIBH (bottom panel)