Prone Breast and Lymph Node Irradiation in 5 or 15 Fractions: A Randomized 2 × 2 Design Comparing Dosimetry, Acute Toxicity, and Set-Up Errors

Purpose Prone whole breast irradiation results in lower dose to organs at risk compared with supine position, especially lung dose. However, the adoption of prone position for whole breast irradiation + lymph node irradiation remains limited and data on lymph node irradiation in 5 fractions are lacking. Although the study was ended prematurely for the primary endpoint (breast retraction at 2 years), we decided to report acute toxicity for prone and supine positions and 5 and 15 fractions. Additionally, dosimetry and set-up accuracy between prone and supine positions were evaluated. Methods and Materials A randomized open-label factorial 2 × 2 design was used for an acute toxicity comparison between prone and supine positions and 5 and 15 fractions. The primary endpoint of the trial was breast retraction 2 years after treatment. In total, 57 patients were evaluated. Dosimetry and set-up errors were compared between prone and supine positions. All patients were positioned on either our in -house developed prone crawl breast couch or a Posirest-2 (Civco). Results No difference in acute toxicity between prone and supine positions was found, but 5 fractions did result in a lower risk of desquamation (15% vs 41%; P = .04). Prone positioning resulted in lower mean ipsilateral lung dose (2.89 vs 4.89 Gy; P < .001), mean thyroid dose (3.42 vs 6.61 Gy; P = .004), and mean contralateral breast dose (0.41 vs 0.54 Gy; P = .007). No significant difference in mean heart dose (0.90 vs 1.07 Gy; P = .22) was found. Set-up accuracy was similar between both positions. Conclusions Unfortunately, the primary endpoint of the trial was not met due to premature closure of the trial. Acceleration in 5 fractions resulted in a lower risk of desquamation. Prone positioning did not influence acute toxicity or set-up accuracy, but did result in lower ipsilateral mean lung dose, thyroid dose, and contralateral breast dose.