Editorial
FDG-PET in radiotherapy treatment planning: Pandora's box?

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Introduction

Radiation oncology is an evolving field, recently accelerating. New advancements in computer and radiologic sciences bring new technology, potentially adaptable for patient care. FDG-PET (fluorine-18 deoxyglucose–positron emission tomography), although first used in the 1970s for brain imaging, has stormed into prominence in the past few years. Recent studies show better staging at initial diagnosis and after therapy with the use of fused PET-CT images compared to either diagnostic test alone in the management of non–small-cell carcinoma of the lung 1, 2. The article by Bradley et al. from Washington University in this issue of the Journal shows that staging and treated radiotherapy volumes can change in 31% and 58% of patients, respectively (3). Although not a new finding, this report adds to the existing literature and confirms previous studies that show alterations in radiotherapy volume with the use of PET data 4, 5, 6, 7, 8, 9, 10. With the advent of PET-CT fusion in radiotherapy treatment planning, several questions need to be addressed.

Section snippets

What is the optimal PET volume for radiation therapy?

Unlike on CTs, where tumors have well-defined anatomic margins, the edges of tumors on PETs appear “fuzzy” to the contouring physician. Some have arbitrarily defined the FDG-avid volume as the region encompassed by the 50% intensity level relative to the tumor maximum 7, 8, whereas the current report by Bradley et al. uses the 40% intensity level (3). Others have not defined the margins of the PET volume employed 5, 9. Hence, the FDG-avid volume may have enormous interobserver variability. One

Who needs to contour the PET volume—the radiation oncologist or the nuclear medicine physician?

When CT simulation first came into widespread use, a number of radiation oncologists did not feel comfortable defining target and normal structures. Almost all physicians who participate in conformal treatments during the current era feel comfortable with outlining most normal and target structures. Some of the reasons for this include better education (i.e., more residency programs incorporating CT treatment planning and offering electives in diagnostic radiology, more textbooks including

Will treatment of PET-defined tumor improve outcome?

In theory, better delineation of the target should mean better tumor coverage and hence better local control. However, with respect to the treatment of non–small-cell lung cancer, defining the tumor with PET-CT does not obviate the need for a mechanism to correct for organ motion. Better target delineation will not help if we are missing the target because it moves. Respiratory gating with the aid of PET in the treatment of non–small-cell lung cancer will help deliver dose to a better target.

What about tumors that are positive on CT but negative on PET and vice versa?

It would be nice if all masses detected on CT were positive on PET and vice versa. However, such is not always the case. In the treatment of lymphomas, for example, one may still have a small mediastinal mass after chemotherapy with a negative PET. Because there is no increased metabolic activity in the mass, it likely represents fibrosis. That said, the current data are too incomplete to correlate negative PET data with long-term outcomes with any degree of comfort.

In the meantime, PET-CT

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