Session Item

Proton Therapy (PT) is gradually gaining consensus as a therapeutic alternative for selected cases of breast cancer (BC) patients. This is motivated by the potential to better spare organs at risk (OARs) distal to the tumour, such as heart and lungs, thus reducing the risk of treatment-related toxicity. In parallel, as a consequence of the good prognosis for early-stage BC, an increasing concern is registered toward the risks of radiation-induced secondary cancers, especially for young patients. In this context, we investigate how pencil beam scanning PT compares to 3D-CRT and VMAT in terms of expected secondary cancer risk (SCR).  

Seven BC patients with nodal involvement were included in the study. For each patient, a treatment plan was computed for 3D-CRT, VMAT and PT. The prescription dose to the target volume was 50 Gy delivered in 2 Gy/fraction. A constant RBE equal to 1.1 was assumed for PT. A Monte Carlo dose calculation algorithm was employed for both photons and PT plans. A single 30° beam was adopted for PT plans, which included also the presence of a range-shifter. The excess absolute risk (EAR) formalism was adopted to quantify the SCR associated to each plan. The EAR was estimated for patients attaining the age of 70 years after exposure at the age of 30 years. Different dose-response models were implemented in the analysis, namely the linear, the linear-exponential and the plateau model. The EAR was estimated for ipsi- and contra-lateral lung and for contra-lateral breast. A cumulative EAR was finally computed.

In terms of cumulative excess cancer cases per 10000 patients, our data indicate that PT results in an expected incidence of 48±5, 19±1, 17±1 (mean±SE) according to the linear, linear-exponential and plateau model, respectively. The corresponding incidence increased by a factor 2.3±0.4, 2.2±0.3, 2.4±0.3 for 3D-CRT and by a factor 4.0±0.6, 5.8±0.7 and 5.7±0.7 for VMAT plans. Remarkably, the EAR after PT is largely due to the dose released in the ipsi-lateral lung, while getting virtually zero for contra-lateral OARs (Figure 1). As a consequence of the different distribution of low and intermediate doses (Table 1), this is not the case for photon radiation therapy. In particular, even though the absolute numbers are low, the SCR for the contralateral OARs is the highest for VMAT, being up to a factor 10² higher than after PT.

We show that pencil beam scanning PT is associated to a significant reduction of SCR for BC patients compared to 3D-CRT and particularly to VMAT. Combined with the improved sparing of the heart, this information might contribute to the establishment of effective patient-selection criteria that are needed to extend the use of protons for the treatment of BC.


Figure 1. EAR for the different OARs as a function of treatment technique. Data indicate mean ± standard deviation.


Table 1. Dosimetric information for the OARs included in the analysis.