P1.18-15 Dosimetric and Toxicity Benefits of Adaptive IMRT in Patients with Stage III Non-Small Cell Lung Cancer

Abstract

Multiple studies observed anatomical changes or tumor shrinkage during concurrent chemoradiotherapy in patients with non-small cell lung cancer (NSCLC). Mid-treatment CT based adaptive radiotherapy targeting to the shrunken tumor can reduce the dose to adjacent normal tissue or potentially deliver a higher dose to the tumor. We aimed to quantitatively analyze the benefit of intensity-modulated radiotherapy (IMRT) adapting to CT changes at the 20th fraction in stage III NSCLC patients. We retrospectively evaluated consecutive patients with unresectable stage III NSCLC treated with adaptive IMRT from November 2017 to August 2018. The eligibility criteria included a mid-treatment CT simulation for replanning at the 20th fraction and a follow-up of at least 6 months. The prescribed dose was 64-66 Gy in 30 fractions unless exceeding the dose limit. Normal tissues were delineated according to RTOG1106 atlas on organs at risk under the supervision of a senior physician. Dose-volume histograms were calculated for the initial plans, composite adaptive plans, and lung isotoxic boost plans. Radiation pneumonitis (RP) and esophagitis (RE) were graded per CTCAE v4.03. Univariate logistic regression was applied to analyze the correlation between dosimetric factors and adverse events. 53 patients were eligible in this study. The average GTV shrinkage was -40.9% at the 20th fraction. Comparing the dosimetric factors of the composite adaptive plans to the initial ones, the GTV coverage was found marginally higher (P=0.002). The doses to normal tissues were significantly lower (all Ps<0.001) in heart mean dose by 109.5 cGy, esophagus V60 by 1.53%, cord maximum dose by -272.7 cGy, lung V20 and mean lung dose (MLD) by 1.11% and 79.2 cGy, respectively. The tumor targets could potentially get an average lung isotoxic boost of 481 cGy. Eight patients (15.1%) had grade 2 RP while no grade 3 or higher RP occurred. Twenty-three patients (43.4%) developed grade ≥ 2 RE. MLD was significantly associated with grade 2 RP with an odds ratio of 1.39 per 100 cGy increase (95% CI, 1.01 to 1.91; P=0.042). Esophagus V60 was significantly associated with grade ≥ 2 RE with an odds ratio of 1.15 per 1% increase (95% CI, 1.04 to 1.28; P=0.009). (Table 1)Tabled 1FactorsInitial PlansAdaptive PlansMean difference95%CIP ValueTargetsPGTV (%)92.9693.810.850.331.370.002PTV(%)94.1394.540.410.350.800.033LungV5(%)46.7745.72-1.05-0.70-1.41<0.001V20(%)25.1524.04-1.11-0.80-1.42<0.001V30(%)18.6217.60-1.02-0.77-1.27<0.001MLD (cGy)1411.41332.2-79.2-60.1-98.4<0.001HeartV30(%)17.4015.11-2.29-0.94-3.620.001V40(%)10.879.06-1.81-0.98-2.64<0.001V55(%)4.062.79-1.27-0.76-1.77<0.001Mean Dose(cGy)1504.51395.0-109.5-67.88-151.22<0.001PericardiumV30(%)32.1730.43-1.74-0.77-2.700.001V40(%)25.7024.00-1.7-0.76-2.640.001V55(%)13.8711.87-2-1.34-2.66<0.001Mean Dose(cGy)2192.92091.2-101.7-60.00-143.3<0.001EsophagusV40(%)39.4336.49-2.94-1.62-4.27<0.001V50(%)27.8924.08-3.81-2.36-5.27<0.001V60(%)7.576.04-1.53-0.96-2.09<0.001Max Dose(cGy)6498.36336.7-161.6-101.99-221.3<0.001CordMax Dose(cGy)4113.03840.3-272.7-209.51-335.93<0.001 Open table in a new tab By adapting to the changes on CT scans at the 20th fraction, the adaptive IMRT approach provides significant dosimetric benefits and has the potential to lower the risk of symptomatic pneumonitis and esophagitis in stage III NSCLC.