Role of adaptive radiotherapy during the application of three-dimensional conformal radiation therapy for small cell lung cancer

Tumor shrinkage is frequently observed during conventionally fractionated (chemo-) radiotherapy for small-cell lung cancer (SCLC). As the tumor reduces in size during treatment, the margin added around the gross tumor volume (GTV) effectively becomes larger, which can result in the excessive irradiation of normal lung tissue. The specific goals of this study are to evaluate GTV changes during the course of radiation therapy (RT) and examine its potential use in adaptive radiotherapy for tumor dose escalation or normal tissue sparing in patients with SCLC. A total of 10 SCLC patients eligible for chemoradiotherapy underwent computed tomography after 13 (at a nominal dose of 2340cGy) and 23 fractions (at a nominal dose of 4140cGy). The GTV was delineated on the repeat CT scans and two treatment plans were generated with or without adaptation to tumor shrinkage during RT for each patient. A dosimetric and volumetric analysis was performed. Average GTV reduction observed over 13 fractions was 58.5% (range, 13.2 to 92.3%; p<0.001), and 23 fractions was 70% (range, 36.9 to 84.5%; p<0.001). Compared to plan without adaptation, adaptive radiotherapy resulted in mean lung dose relative decreases of 8.7%, mean lung volume receiving ≥ 20Gy (V20) relative decreases of 5%, mean lung volume receiving ≥ 6 Gy (V5) relative decreases of 10%, mean medulla spinalis dose relative decreases of 21cGy, mean esophagus volume receiving ≥ 50Gy (V50) relative decreases of 19%, mean heart volume receiving ≥ 42 Gy (V42) relative decreases of 13%. The benefits of adaptive therapy are the greatest for tumor volumes ≥ 30 cm3 and are directly dependent on GTV reduction during treatment. Adaptation of the treatment plan twice during conventionally fractionated (chemo-) RT of SCLC achieved a significant benefit in terms of normal tissue sparing and significantly decreases tumor volume and dose to normal tissues, allowing for the possibility of-dose-escalation.