American journal of clinical oncology 2015-08
Dosimetric definitions of total lung volumes in calculating parameters predictive for radiation-induced pneumonitis.   
The volume of normal lung receiving 20 Gy (V20) and the mean lung dose (MLD) represent dosimetric parameters used for identifying risk of radiation pneumonitis. However, the total lung volume for dosimetric analysis has been defined differently. Herein we investigate to quantify the dosimetric differences when analysis is based on either definition (ie, excluding planning target volume [PTV] vs. gross tumor volume [GTV] from the total bilateral lung volume).
Sixty-one patients with lung cancer who had undergone definitive radiation therapy were retrospectively reviewed. Dosimetric parameters were calculated when excluding GTV or PTV from the total bilateral lung volume.
Median GTV to PTV margin was 1.3 cm (range, 0.4 to 3.8 cm). Median heterogeneity-corrected RT dose was 74 Gy with the median GTV of 110 mL (range, 13.79 to 665.8 mL) and the median PTV of 346 mL (range, 39.8 to 1258 mL).The MLD, V5, V10, V20, and V30 were all slightly higher and significant when excluding GTV from the total bilateral lung volume compared with similar dosimetric parameters when excluding PTV (P<0.001). Average MLD was 14.8 and 16.7 Gy when excluding PTV and GTV, respectively. Mean V5, V10, V20, and V30 were 49.8%, 38%, 25%, and 18.8% when excluding PTV versus 51.3%, 40%, 28%, and 21.5% when excluding GTV. There were 4 patients with clinical pneumonitis and all had the V20>23% when excluding the PTV versus the V20>27% when excluding the GTV from total bilateral lung volume.
A small but significant difference exists between the 2 approaches used to calculate dosimetric variables for lung dose. This difference should be taken into account when comparing dosimetric information between different institutions and when optimizing treatment plans.

Related Questions

Do you define it as whole lungs (inc. tumor) or whole lungs minus GTV, CTV,  ITV, or PTV?