Abstract Objective. Functional lung avoidance (FLA) radiotherapy treatment aims to spare lung regions identified as functional from imaging. Perfusion contributes to lung function and can be measured from the determination of pulmonary blood volume (PBV). An advantageous alternative to the current determination of PBV from positron emission tomography (PET) may be from dual energy CT (DECT), due to shorter examination time and widespread availability. This study aims to determine the correlation between PBV determined from DECT and PET in the context of FLA radiotherapy. Approach. DECT and PET acquisitions at baseline of patients enrolled in the HI-FIVE clinical trial (ID: NCT03569072) were reviewed. Determination of PBV from PET imaging ( PBV PET ), from DECT imaging generated from a commercial software (Syngo.via, Siemens Healthineers, Forchheim, Germany) with its lowest ( PBV syngo R = 1 ) and highest ( PBV syngo R = 10 ) smoothing level parameter value (R), and from a two-material decomposition (TMD) method ( PBV TMD L ) with variable median filter kernel size (L) were compared. Deformable image registration between DECT images and the CT component of the PET/CT was applied to PBV maps before resampling to the PET resolution. The Spearman correlation coefficient (r _s) between PBV determinations was calculated voxel-wise in lung subvolumes. Main results. Of this cohort of 19 patients, 17 had a DECT acquisition at baseline. PBV maps determined from the commercial software and the TMD method were very strongly correlated [r _s( PBV syngo R = 1 , PBV TMD L = 1 ) = 0.94 ± 0.01 and r _s( PBV syngo R = 10 , PBV TMD L = 9 ) = 0.94 ± 0.02]. PBV PET was strongly correlated with PBV TMD L [r _s( PBV PET , PBV TMD L = 28 ) = 0.67 ± 0.11]. Perfusion patterns differed along the posterior-anterior direction [r _s( PBV PET , PBV TMD L = 28 ) = 0.77 ± 0.13/0.57 ± 0.16 in the anterior/posterior region]. Significance. A strong correlation between DECT and PET determination of PBV was observed. Streak and smoothing effects in DECT and gravitational artefacts and misregistration in PET reduced the correlation posteriorly.