Radiation Pneumonitis In Small Cell Lung Cancer: Impact Of Induction Chemotherapy On Estimated Risk.

e17500 Background: Radiation pneumonitis (RP) is correlated with the volume of irradiated normal lung. Small cell lung cancer (SCLC) is considered highly chemosensitive. There are patients with bulky tumors for whom giving induction chemotherapy may result in less radiation to surrounding normal lung as compared to immediate concurrent chemoradiation. This study measured the change in gross tumor volume (GTV) designated as parenchymal and/or mediastinal disease, percent of lung receiving ≥ 20 Gy (V20), mean lung dose (MLD), and estimated risk of RP in SCLC patients who received chemotherapy alone, using simulated radiation therapy (RT) plans on the pre- and postchemotherapy CT scans. Methods: Between 2003 and 2009, 23 patients with extensive SCLC were treated with chemotherapy alone and had CT scans pre- and post-2 cycles of platin-based chemotherapy. Simulated RT plans were created as if to deliver 45 Gy to thoracic disease. Volumetric changes in GTV and normal lung were measured. Normal Tissue Complication Probability (NTCP) calculations were used to predict the pre- and postchemotherapy risk of pneumonitis ≥ Gr 2 based on simulated plans. Results: Nineteen of 23 patients treated with induction chemotherapy had ≥ 30% reduction in GTV, using either RECIST criteria or volumetric measurements, and therefore had pre- and postchemo RT plans created. The other 4 patients had stable disease. Among responders, the mean decrease in GTV was 66% (range -2 to 93%, p<0.01), the mean absolute decrease in V20, MLD, and NTCP was 7% (range -6.6 to 34%, p<0.01), 3.1 Gy (range -0.64 to 12.1 Gy, p<0.01), and 5.2 % (range -1.3 to 28.5%, p<0.01), respectively. The absolute risk of pneumonitis ≥ Gr 2 was decreased by ≥ 5% in 8 (35%) patients. Conclusions: Patients with limited SCLC will likely have a reduction in the risk of treatment related pneumonitis ≥ Gr 2 if treatment is planned following 2 cycles of induction chemotherapy, especially in patients who present with bulky mediastinal disease. Pre-chemo NTCP <15% Pre-chemo NTCP ≥15% p value Number of patients 9 10 Pre-chemo mean mediastinal GTV (cc) 39.5 215.7 0.03 Pre- chemo mean parenchymal GTV (cc) 182.9 211.7 0.39 Pre-chemo mean GTV (cc) 222.4 427.4 0.12 Mean absolute difference in NTCP (%) 1.8 8.2 0.03 No significant financial relationships to disclose.