The dilemma of the target delineation with PET-CT in the radiotherapy planning for malignant tumors

Questions are currently still not clearly conclusively answered, when contouring target with PET/CT in the radiotherapy planning. Who needs to contour the PET volume, the radiation oncologist or the nuclear medicine physician? Which factors will contribute to the increase of the inter-observer variability between them? What should be taken as the optimal SUV threshold to demarcate malignant tumors from the benign tissue? When the PET volume is not in accordance with the CT finding in the local area, which part should be contoured as the target? When PET positive while CT negative or PET negative while CT positive were shown in the regional lymph node draining area or in the remote region, in which way the target could be decided and selected? Further study concerning the corresponding relationship between PET-CT and its registered pathological specimen are needed. The long-term clinical results of increased therapeutic ratio will finally verify the applicability of the guideline to contour the target with PET-CT in the radiotherapy planning.