The rationale for monitoring both skin and invasive temperatures in locoregional recurrent breast cancer patients treated with postoperative re-irradiation and superficial microwave hyperthermia exceeding 1 cm target depth

Background Thermoradiotherapy at 40–43 °C is an effective treatment for locoregional recurrent (LRR) breast cancer patients. During hyperthermia temperature monitoring is often limited to skin temperatures, but their predictive value for achieving locoregional tumor control (LRC) remains unclear for tumor depths ≥1cm. This study compared predictive value of skin versus invasive intratarget temperatures.Methods We reported earlier the thermal dose-effect relationship in 112 patients with LRR breast cancer treated in 2010–2017 with postoperative re-irradiation (8 × 4 Gy/23 × 2 Gy) and 4–5 weekly hyperthermia sessions with 434 MHz microwave applicators. Bolus cooling enabled semi-superficial target depths (1–4 cm), thermocouple thermometry monitored both skin and intratarget temperatures. The present study analyzes relationships between LRC and average skin and intratarget median temperature T50 and best session with highest median thermal dose CEM43T50 (cumulative equivalent minutes at 43 °C). Pearson correlation coefficient analysis evaluated correlation between intratarget and skin temperature parameters.Results Average median skin and invasive temperatures were T50 = 40.6 °C (Interquartile Range IQR 40.3–40.8) and T50 = 40.7 °C (IQR 40.0–41.3), respectively, without association between skin and intratarget temperatures. Median CEM43T50 for skin and intratarget were 2.8 (IQR 2.0–4.3) and 7.2 min (IQR 3.4–15.9) in the best hyperthermia session, respectively. Twenty-four patients developed infield recurrences; median time-to-recurrence was 3.4 years (IQR 2.7–4.6 years). Univariate and multivariate analysis showed significant association of intratarget temperature parameters with LRC (p = 0.015; p = 0.021, respectively), but not for skin temperature parameters and LRC (p = 0.131; p = 0.215, respectively).Conclusion Temperature monitoring using both skin and invasive intratarget measurements is key for ensuring good quality superficial hyperthermia delivery for tumor depths ≥1cm.