F-Fluorothymidine-positron emission tomography with CT fusion ([F]FLT-PET/CT) offers a unique and non-invasive method for three-dimensional localization and quantification of functional bone marrow. [F]FLT-PET/CT has potential application in radiotherapy planning when risk of marrow toxicity is a significant clinical concern. In this patient with chemo-refractory, transformed lymphoma and treatment-induced cytopenias, [F]FLT-PET/CT was a novel and useful adjunct to (1) image the distribution of functional bone marrow reserve, (2) guide "marrow-sparing" radiotherapy planning, and (3) quantify the effects of radiotherapy-induced bone marrow suppression both in-field and out-of-field.