SABCS 2022 | Phase II randomized trial of conventional vs hypofractionated post-mastectomy proton radiotherapy

Robert Mutter, MD, Mayo Clinic Comprehensive Cancer Center, Rochester, MN, provides an overview of a Phase II randomized trial of conventional versus hypofractionated post-mastectomy proton radiotherapy. The trial compared 25 fractions delivered to 50 Gy RBE versus 15 fraction delivered to 40 Gy RBE for patients with indications for postmastectomy radiotherapy. Prior reports on proton therapy for breast cancer have used small doses per day, delivered across 5-6 weeks. However, evidence has suggested that side effects may be reduced when condensing the dose and length of treatments. The trial investigated whether condensing treatment to 15 fractions would be safe based on evidence that breast cancer is more sensitive to higher dose fractions than surrounding normal tissues. The conventional fractionated group received 50 Gy in 25 fractions of 2 Gy, and hypofractionation group received 40.05 Gy in 15 fractions of 2.67 Gy (RBE 1.1). Cardiotoxicity was assessed with serial transthoracic conventional and 2-dimensional speckle tracking echocardiography (2D-STE). No significant changes on conventional or 2D-STE at end-of-treatment or 3-month follow-up compared to baseline were observed. With a median follow-up of 38.3 months, the 24-month complication rate was conventional 14.6% vs hypofractionation 17.1%. There was no significant difference in 3-year disease-free survival between the conventional and hypofractionated. Proton PMRT provided excellent locoregional control and normal tissue sparing. This interview took place at the San Antonio Breast Cancer Symposium (SABCS) 2022 in San Antonio, TX.

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