Evaluation of clinical target volume expansion required for involved site neck radiotherapy for lymphoma to account for the absence of a pre-chemotherapy PET-CT in the radiotherapy treatment position.

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 +15mm from CTV _INRT . Inferiorly, CTV _diagPET varied by -18 to +6mm 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 10mm and 18mm respectively, along with generous contouring in the axial plane, was required to encompass pre-chemotherapy disease.
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