Table 5 Dose Constraints for Parallel Structures. Parallel structures require the use of a ‘critical volume’ (CV), also termed a ‘complementary volume’. For example, for lung, the CV1500cc is listed as 7 Gy for 1-fraction treatments, meaning that there must be 1500 cc of lung receiving 7 Gy or less. This is read from the left-hand side of a dose-volume histogram (DVH). For further information on calculating the CV, with DVH examples, see Application of Critical Volume-Dose Constraints for Stereotactic Body Radiation Therapy in NRG Radiation Therapy Trials at https://www.redjournal.org/article/S0360-3016(17)30241-9/abstract. The VX refers to the percent of lung (minus GTVs) receiving X Gy or more

From: A randomized phase III trial of stereotactic ablative radiotherapy for patients with up to 10 oligometastases and a synchronous primary tumor (SABR-SYNC): study protocol