Comparison of supine or prone crawl photon or proton breast and regional lymph node radiation therapy including the internal mammary chain

Abstract

We report on a dosimetrical study comparing supine (S) and prone-crawl (P) position for radiotherapy of whole breast (WB) and loco-regional lymph node regions, including the internal mammary chain (LN_IM). Six left sided breast cancer patients were CT-simulated in S and P positions and four patients only in P position. Treatment plans were made using non-coplanar volumetric modulated arc photon therapy (VMAT) or pencil beam scanning intensity modulated proton therapy (IMPT). Dose prescription was 15*2.67 Gy(GyRBE). The average mean heart doses for S or P VMAT were 5.6 or 4.3 Gy, respectively (p = 0.16) and 1.02 or 1.08 GyRBE, respectively for IMPT (p = 0.8; p < 0.001 for IMPT versus VMAT). The average mean lung doses for S or P VMAT were 5.91 or 2.90 Gy, respectively (p = 0.002) and 1.56 or 1.09 GyRBE, respectively for IMPT (p = 0.016). In high-risk patients, average (range) thirty-year mortality rates from radiotherapy-related cardiac injury and lung cancer were estimated at 6.8(5.4-9.4)% or 3.8(2.8-5.1)% for S or P VMAT (p < 0.001), respectively, and 1.6(1.1-2.0)% or 1.2(0.8-1.6)% for S or P IMPT (p = 0.25), respectively. Radiation-related mortality risk could outweigh the ~8% disease-specific survival benefit of WB + LN_IM radiotherapy for S VMAT but not P VMAT. IMPT carries the lowest radiation-related mortality risks.

Figure 1

Transverse and sagittal dose distributions of prone photon (left) and proton (right) plans. The main difference is larger dose spread outside the target volumes in VMAT than in IMPT plans. Larger dose spread is seen in de dorsal shoulder region (indicated by the red number (1) and inside the thorax (indicated by the yellow number (2).

Figure 2

Maximum (D02) and minimum (D98) dose in breast and lymph node targets. Dose range: 34–46 Gy/GyRBE. P1–P6: patients (n = 6) for whom CT-simulation was performed in supine and prone crawl positions. Paired t-tests.

Figure 3

Skin dose. Dose range: 0–40 Gy/GyRBE for patients 1–6. Color lines drawn on the CT-scan image represent the different layers OAR-skin-0–1mm to OAR-skin-4–5mm. Data points are mean dose to these structures.

Figure 4

Risk-benefit classification of high-risk patients for cardiac events and lung cancer. Data points represent mean heart and mean lung doses of individual patients treated supine or prone crawl using photons or protons. Line of regret 8%: data points on the line represent 8% absolute 30-year survival loss from combined radiotherapy-related cardiac and lung cancer mortality.