Thresholding in PET images of static and moving targets

Abstract

Continued therapeutic gain in the treatment of non-small-cell lung cancer (NSCLC) will depend upon our ability to escalate the dose to the primary tumour while minimizing normal tissue toxicity. Both these objectives are facilitated by the accurate definition of a target volume that is as small as possible. To this end, both tumour immobilizations via deep inspiratory breath-hold, along with positron emission tomography (PET), have emerged as two promising approaches. Though PET is an excellent means of defining the general location of a tumour focus, its ability to define exactly the geometric extent of such a focus strongly depends upon selection of an appropriate image threshold. However, in clinical practice, the image threshold is typically not chosen according to consistent, well-established criteria. This study explores the relationship between image threshold and the resultant PET-defined volume using a series of F-18 radiotracer-filled hollow spheres of known internal volumes, both static and under oscillatory motion. The effects of both image threshold and tumour motion on the resultant PET image are examined. Imaging data are further collected from a series of simulated gated PET acquisitions in order to test the feasibility of a patient-controlled gating mechanism during deep inspiratory breath-hold. This study illustrates quantitatively considerable variability in resultant PET-defined tumour volumes depending upon numerous factors, including image threshold, size of the lesion, the presence of tumour motion and the scanning protocol. In this regard, when using PET in treatment planning for NSCLC, the radiation oncologist must select the image threshold very carefully to avoid either under-dosing the tumour or overdosing normal tissues.

Assessment of various strategies for 18F-FET PET-guided delineation of target volumes in high-grade glioma patients

“…At present, various methods are used in practice to delineate PET-based target volumes [16][17][18][19][20][21][22][23][24][25]. The manual delineation of target volumes using different window level settings and look up tables is the most common and widely used technique in the clinic.…”

“…The manual delineation of target volumes using different window level settings and look up tables is the most common and widely used technique in the clinic. However, the method is highly operator-dependent and is subject to high variability between operators [20,26]. Semiautomated or fully automated delineation techniques might offer several advantages over manual techniques by reducing operator error/subjectivity, thereby improving reproducibility.…”

Static and moving phantom studies for radiation treatment planning in a positron emission tomography and computed tomography (PET/CT) system

“…There have been many clinical and phantom studies on appropriate threshold values for PET images [21][22][23][24][25][26][27][28][29][30][31][32] (Table 1). Erdi et al [21] reported that 36%-44% of the maximum FDG activity was appropriate for delineating target volumes larger than 4 ml in a static phantom study.…”

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

“…A priori, it is therefore unlikely that a universal segmentation threshold would match nongated 18 F-FDG PET and non-breathhold CT GTVs in tumors of differing shapes and sizes. The findings of Biehl et al highlight the difficulty in defining a universal threshold as suggested by phantom studies (5).…”