Use of NTCP metrics to identify unbalanced levels of dose trade‐off between left lung and heart in lung cancer radiotherapy

Introduction The aim of this study is to determine the optimal level of dose trade‐off between left lung and heart during plan optimization in lung cancer radiotherapy, and also to examine the impact of planning target volume (PTV) size in reducing the overall risk of toxicity of lung and heart. Methods This work includes 20 left‐sided lung cancer patients and for each of them two VMAT plans (dosimetrically and radiobiologically optimized) were developed. In all the plans, a dose of 70 Gy in 35 fractions was prescribed to PTV. Each plan was evaluated in terms of PTV coverage, mean lung dose, lung‐V 20 , mean heart dose, and heart‐V 5 . The normal tissue control probability (NTCP) values of radiation pneumonitis and heart valve dysfunction were calculated using the Relative Seriality model. Results The patients with PTV < 300 cm 3 had a left lung NTCP of 70.7 ± 14.6% and a heart NTCP of 11.5 ± 5.5%, whereas for the patients with PTV > 300 cm 3 , the corresponding NTCP values were 86.0 ± 9.5% for left lung and 13.7 ± 4.9% for heart, respectively. After the NTCP‐based optimization, for the patients with PTV < 300 cm 3 , the corresponding lung NTCP values had a decrease of 12.8% and the heart NTCP values had an increase of 2.7%, whereas for the patients with PTV > 300 cm 3 , the lung NTCP values had a decrease of 6.5% and the heart NTCP values had an increase of 3.9%. Conclusion Our results indicate that there is a systematic unbalance in dose trade‐off between left lung and heart, which varies based on the PTV size. NTCP metrics were found useful in identifying this issue.