[en] Background and purpose: Involved site radiotherapy clinical target volume (CTV) for lymphoma requires an expansion to account for the absence of radiotherapy treatment-position pre-chemotherapy imaging, which is not widely implemented. This prospective imaging study aims to quantify CTV expansion required for neck radiotherapy. Materials and methods: 10 patients from a prospective single centre imaging study underwent a pre-chemotherapy FDG-PET-CT in both the diagnostic and radiotherapy treatment position, and subsequently received neck radiotherapy post-chemotherapy. CTV_INRT and CTV_diagPET were delineated on the planning CT, following co-registration of the radiotherapy position PET-CT and side-by-side assessment of diagnostic PET-CT respectively. Results: Intra-observer variability was limited, with delineation of CTV_INRT highly reproducible and slightly lower for CTV_diagPET (mean DICE 0.88 and 0.8 respectively). Superiorly, CTV_diagPET varied by −10 to +15 mm from CTV_INRT. Inferiorly, CTV_diagPET varied by −18 to +6 mm from CTV_INRT. Comparing CTV_INRT and CTV_diagPET in the axial plane, the mean DICE was 0.74. Mean sensitivity index was 0.75 (range 0.59–0.91), showing that on average 75% of the CTV_INRT was encompassed by the CTV_diagPET. Conclusions: In the absence of treatment-position PET-CT, CTV expansion cranially and caudally by 10 mm and 18 mm respectively, along with generous contouring in the axial plane, was required to encompass pre-chemotherapy disease.