Lung organ-at-risk volumes: A survey of practice and the need for a consistent definition in the 4DCT era.

Background: In the 4DCT era, the definition of lung organ-at-risk (OAR) volumes for dose-volume histogram (DVH) calculation is unclear, introducing potential for variability in practice. We aimed to identify definitions used clinically and evaluate the magnitude of DVH differences between these.

Methods: We surveyed Australian & New Zealand departments about lung radiotherapy protocols including lung OAR volume definition. We used these definitions to calculate lung DVHs on 10 patients prescribed lung IMRT (60-66 Gy/30-33 fractions). We calculated mean lung dose (MLD), V20 and V30 for 'Lungs - PTV', 'Lungs - CTV', 'Lungs - iGTV' (internal GTV in all respiratory phases) and 'Lungs - GTV_EX' (expiratory phase).

Results: The response rate was 39% (34/88). 14% and 29% of departments did not have a departmental protocol for OAR and tumour volume delineation, respectively. All permutations for lung OAR volumes were used with no clear preference. For conventional radiotherapy (n = 27), this included Lungs alone (n = 1), Lungs - PTV (n = 6), Lungs - CTV (n = 2), Lungs - iGTV (n = 6), Lungs - GTV in single phase (n = 5) and individual clinician preference (n = 7). The different lung OAR volumes resulted in MLD difference ranging from 0.9 to 4.15 Gy, V20 from 1.5% to 6.6% and V30 from 1.34% to 7.11%. The largest differences between subtraction of GTV_EX and iGTV were 0.32 Gy, 0.43% and 0.46% for MLD, V20 and V30, respectively.

Conclusions: A significant number of departments lacked lung cancer radiotherapy contouring protocols. Lung OAR volume definition was variable between and within departments. Potentially clinically significant differences in lung DVH parameters were seen according to the volume used.