Reply: ¹⁸F-FDG PET in Planning Radiation Treatment of Non–Small Cell Lung Cancer: Where Exactly Is the Tumor?

REPLY: Thank you for the opportunity to comment on the letter to the editor from Drs. Dahele and Ung. The group from Toronto–Sunnybrook Regional Cancer Center has a longstanding interest and expertise on the subject of PET-planned radiation therapy. We thank them for their continued contributions to the field.

Our objective in publishing our article (1) was to point out that the current methods used to contour ¹⁸F-FDG PET for purposes of defining tumor volumes for radiation therapy are problematic. The use of a single threshold for all lung cancers is not appropriate because of the many factors that influence it, including tumor volume and heterogeneity. Currently, PET does not have the spatial resolution or tumor-tracking abilities of 4-dimensional CT. The ongoing RTOG-0515 trial (2) uses CT to contour lung cancer volumes, applying ¹⁸F-FDG PET as a guide. The spatial and motion issues are not managed well by PET presently. With the development of gated PET, improvements in the technology are coming.

Drs. Dahele and Ung point out that PET contouring has some advantages, such as improved interobserver consistency and forgiveness when the operator cannot distinguish between normal and abnormal anatomy. These advantages are likely to persist as technology advances. An obvious extension would be the development of automated tumor-contouring tools for treatment planning. Collection of PET-based tumor contours and histopathology specimens, as is being performed in the PET START Trial (3), would be an excellent database on which to develop and test such tools.

We agree that the correlation of PET with tumor pathology is of vital importance. We look forward to the completion of the trial and the reporting of the results.