Excluding either gross tumor volume or planning target volume from the normal lung volume in lung cancer irradiation: Evaluation of the dosimetric impact

Abstract

PurposeWhen evaluating dosimetric parameters predictive of lung toxicity in lung cancer, the total lung volume can be defined to exclude the gross tumor volume (lung-GTV) or to exclude the planning target volume (lung-PTV). The purpose of the study was to evaluate the impact of these 2 types of delineation on the dosimetric parameters V20, V30, and mean lung dose (MLD). Methods and MaterialsWe analyzed 69 patients with lung cancer treated with 3-dimensional radiation therapy. Normal lung volume was defined using 2 modalities of delineation: lung-GTV and lung-PTV. The lung volume inside the PTV, but outside the GTV, corresponded to the margins within the lung parenchyma applied to the GTV and the clinical target volume (CTV) to obtain the PTV. This volume (expressed in percentage of total lung volume) increases with the following: (1) the margins (GTV to CTV and CTV to PTV) increase within the lung parenchyma; (2) the GTV increases; and (3) the total lung volume decreases. ResultsMean reduction of lung volume was 5.1% (range, 1.4-10.0). With the delineation lung-PTV rather than lung-GTV, the mean reduction was 3.1% (P < 10-7), 3.3% (P < 10-7), and 2.1 Gy (P < 10-7) for V20, V30, and MLD, respectively. These reductions correlated strongly with reduction of lung volume (r2 range, 0.89-0.96). For 25% of patients having greater reduction of lung volume (high margins, high tumor volume, small lung volume), reduction of V20 ranged from 4.5%-6.3%, reduction of V30 ranged from 4.6%-7.0%, and reduction of MLD ranged from 2.9 Gy-4 Gy. ConclusionsThe dosimetric parameters V20, V30, and MLD are reduced with the delineation using lung-PTV rather than lung-GTV. These reductions correlate with lung volume in the PTV and can be significant.