PTC CLINIC
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Proton radiotherapy in breast cancer in practice – indications for proton radiotherapy in patients with breast cancer

• patients with left breast cancer, after previous cardiotoxic treatment – chemotherapy, biological treatment (anthracyclines, trastuzumab)
• patients with associated cardiac disease such as valve insufficiency, coronary heart disease, arrhythmias, etc., as radiation dose limits for these patients should be lower
• patients who need irradiation of the internal mammary nodes
• patients after previous radiotherapy of contralateral breast, where it is necessary to take into account the possible irradiation of the medial part of the chest and to adapt the irradiation plan accordingly
• bilateral breast irradiation, which represents a large irradiated volume and the risk of loading critical tissues is very high
• patients with genetic syndrome predisposing to increased radiation toxicity

The target volume for breast cancer is either the breast after partial surgery (with targeted boosting of the tumor bed) or the chest wall after total mastectomy. Irradiation of the descending nodes is represented by the axillary nodes (stages I, II, III), the supra-key, sub-key nodes and the internal mammary nodes. When irradiated with a proton beam, a significantly lower dose load can be achieved, especially to the heart, coronary arteries, lungs and the contralateral healthy breast. This advantage is especially beneficial for left breast cancer, chest or lymph node involvement. The advantage of proton therapy is also clear when it is necessary to irradiate the nodes around the sternum (internal mammary nodes). Irradiation of the internal mammary nodes at the Proton Therapy Center (PTC) is indicated in patients according to the recommendations of the German professional society GebFra. When internal mammary nodes are part of the target volume in the proton plan for breast cancer, the mean dose to the heart is increased by about 0-0.10 Gy against the proton plan without irradiation of these nodes. In contrast, for a photon plan, a given indication can be a difficult compromise between optimal coverage and critical organ toxicity.

Dosimetric advantage is typical for almost all proton comparison plans over photon radiotherapy. However, not every patient has a clinical benefit from the reduced dose, especially in reducing the risk of cardiovascular morbidity. Proton radiotherapy should be considered especially in patients with left-sided breast cancer and expected long-term survival.

Figure 1: Example of a photon plan created using the 3D CRT technique and a proton plan created using the IMPT technique. It is obvious that the proton plan is more sparing than the photon plan, particularly in terms of the maximum dose to the heart and also the dose to the lungs. With photons, the contralateral breast is also partially irradiated, which brings a risk of secondary malignancies.

PTC cooperates with many radiation oncology departments in the Czech Republic and Slovakia. Patients who do not meet the recommended dose limits as recommended by the DEGRO group are referred for proton radiotherapy.

The German group Deutsche Gesellschaft Für Radioonkologie (DEGRO) has summarized the recommendations for setting constraints, i.e. the recommended dose limits for the heart and its substructures when irradiating breast cancer (Table 1). They recall that it is up to the attending physician to decide whether to compromise between target volume coverage and cardiac output. The physician should consider the risk of possible recurrence versus treatment toxicity. When evaluating the irradiation plan, the mean heart dose (Dmean) is monitored – the recommended limit is <2.5 Gy. Increasing the mean dose to the heart leads to a linear increase in the risk of cardiac ischemia (1 Gy = 7.4%). Equally important is the median dose to the left anterior descending (LAD) coronary artery. When irradiated, the risk of artery stenosis and consequently the risk of heart muscle ischemia increases. The recommended median left ventricular dose is <3 Gy. At our workplace, all cardiac parameters in all irradiated patients are monitored and observed (Table 2).

Table 1: Recommended dose limits for the heart and its substructures according to Deutsche Gesellschaft Für Radioonkologie (DEGRO)

Table 2: Average doses to critical organs with proton radiotherapy

In 2012, Fontanila et al. shared that target volume coverage in photon radiotherapy may be difficult when irradiating internal mammary nodes. The authors retrospectively analyzed the coverage of the chest wall, nodal areas (axillary I, II, III, superclavicular and internal mammary nodes) in executed radiation plans. All plans were contoured according to RTOG recommendations. The required dose covered 74% of the volume at the chest wall, 80% of the volume at the internal mammary nodes, the gutter of the axillary and superclavicular nodes was very good. Despite these trade-offs, the volume of the heart was irradiated with more than 10 Gy in left-sided breast tumors and 11% in right-sided tumors. The volume of lungs irradiated with a dose of 20 Gy (V20) of the ipsilateral lung represented 28% in left-sided breast cancer and up to 34% in right-sided breast cancer. The results show a relatively high radiation load. When using proton radiotherapy, the need for compromises and unnecessary radiation exposure of healthy tissues is eliminated.