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Breast radiotherapy in the prone position is less toxic versus the supine position in women with large breast size, according to a phase III randomized trial.

Among 357 patients, those treated in the supine position had significantly higher rates of moist desquamation anywhere in the breast compared with those treated in the prone position (39.6% vs 26.9%; OR 1.78, 95% CI 1.24-2.56, P=0.002), reported Danny Vesprini, MD, MSc, of Sunnybrook Health Sciences Centre in Toronto, and colleagues in .

This association was confirmed in a multivariable analysis (OR 1.99, 95% CI 1.48-2.66, P<0.001), along with other independent factors, including use of boost (OR 2.71, 95% CI 1.95-3.77, P<0.001), extended fractionation (OR 2.85, 95% CI 1.41-5.79, P=0.004), and bra size (OR 2.56, 95% CI 1.50-4.37, P<0.001).

"Treatment in the prone position has several dosimetric advantages for these patients," Vesprini and team explained. "It allows for more homogeneous dose distribution owing to the smaller separation when compared with the supine position, which decreases deposition of higher doses in the inframammary fold and axilla."

Among all women, there were fewer toxic effects of the skin when patients were treated with hypofractionated radiotherapy compared with extended fractionation, they added.

"Prone radiotherapy appears to be an excellent option for patients with large breast size and right-sided breast cancer, and may benefit many women with left-sided breast cancer with large breast size if acceptable cardiac avoidance is feasible," observed Dean Shumway, MD, of the Mayo Clinic in Rochester, Minnesota, and Katelyn Atkins, MD, PhD, of Cedars-Sinai Medical Center in Los Angeles, in an . "In summary, prone positioning for whole-breast radiotherapy represents a valuable addition to the armamentarium of treatment techniques to reduce the adverse effects associated with whole-breast radiotherapy."

Vesprini and colleagues also found that the supine position was associated with more grade 3 desquamation compared with the prone position (15.4% vs 8.0%; OR 2.09, 95% CI 1.62-2.69, P<0.001).

In addition, when broken down by treatment with either extended fractionation or hypofractionation, extended fractionation was associated with more:

• Toxic effects (43.3% vs 23.2%; OR 2.56, 95% CI 1.50-4.37, P<0.001)
• Grade 3 desquamation (17.2% vs 6.3%; OR 3.14, 95% CI 1.62-6.11, P=0.001)
• Pain (9.4% vs 3.4%; OR 2.97, 95% CI 1.06-8.31, P=0.04)

"These differences were primarily driven by the rates of toxic effects in patients treated in the supine position," the authors noted.

Specifically, in patients treated in the supine position, extended fractionation was associated with increased desquamation compared with hypofractionation (51.1% vs 27.8%; OR 2.72, 95% CI 1.18-6.24, P=0.02), and grade 3 desquamation (23.9% vs 6.7%; OR 4.40, 95% CI 1.83-10.57, P<0.001).

Extended fractionation was also associated with increased toxicity in patients treated in the prone position, although the link was less pronounced. Desquamation occurred in 35.2% of patients treated with extended fractionation versus 18.4% of patients treated with hypofractionation (OR 2.41, 95% CI 1.65-3.52, P<0.001), while grade 3 desquamation occurred in 10.2% versus 5.7% of patients (OR 1.87, 95% CI 1.05-3.32, P=0.03).

This trial took place at five centers across Canada from April 2013 to March 2018. Eligible patients included those with large breast size and invasive carcinoma or ductal carcinoma in situ who were treated with breast-conserving surgery and referred for adjuvant radiotherapy to the breast alone. Of the 357 women (mean age 61 years) included, 182 were treated in the supine position and 175 were treated in the prone position.

From April 2013 until June 2016, 167 patients received 50 Gy in 25 fractions (extended fractionation) with or without boost (range 10-16 Gy). After the trial was amended in June 2016, the majority of patients (93.2%) received the hypofractionation regimen of 42.5 Gy in 16 fractions.

Vesprini and colleagues noted that there were no differences in quality of life as measured by global health status, breast symptoms, or pain scales between the supine and prone groups.

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