Field match verification during combination proton, photon, and electron therapy for oligometastatic inflammatory...

Postmastectomy radiation therapy (PMRT) has been shown in randomized trials to improve overall survival for patients with locally advanced breast cancer. The standard PMRT clinical target volume (CTV) encompasses the chest wall and undissected regional lymphatics. Conformal isodose distributions covering the standard CTV with acceptable dose limits to normal tissue can typically be achieved with a combination of photon and electron fields. Field borders are marked on the patient's skin using a light field projection of each beam and are subsequently used to verify daily field matching clinically. Initial imaging of a patient with oligometastatic inflammatory breast cancer demonstrated direct extension of disease from the involved internal mammary lymph node chain into the anterior mediastinum as the only site of metastatic disease. The patient achieved a pathologic complete response to neoadjuvant chemotherapy and underwent mastectomy. The initial sites of gross disease, including the anterior mediastinal node was included in the CTV for PMRT, and treatment planning demonstrated a clear advantage to the inclusion of proton fields in this case. The absence of a light source on the proton delivery system that accurately projects proton field edges onto the patient's skin posed a significant challenge for daily verification of proton-to-photon and -electron field matching. Proton field-specific radiographic imaging devices were designed and used such that proton field edges could be delineated on the patient's skin and used for daily matching with photon and electron fields. Manufacture of the imaging devices was quick and inexpensive. Weekly verification of proton field alignment with the proton field delineation on the skin demonstrated agreement within 3-mm tolerance. The patient remains with no evidence of disease 18 months after completing radiation. Other patients with similar indications may benefit from multimodality radiation therapy.