Towards Biological Target Volumes Definition for Radiotherapy Treatment Planning: Quo Vadis PET/CT?

Abstract

IntroductionRadiotherapy treatment planning (RTP) over the last few decades has been based on anatomical targets, namely gross tumor volume (GTV) and from it derived clinical target volume (CTV) as well as the planning target volume (PTV). Owing to its superb spatial reproducibility and the ability to provide the information on electron density (useful for heterogeneity corrections), computed tomography (CT) was, and it still represents, the backbone of modern/high technology radiotherapy treatment planning. The lack of sufficient soft tissue contrast in CT resulted in the incorporation of other imaging methods, such as the magnetic resonance imaging (MRI), into the target definition through the process of image co-registration (sometimes incorrectly termed as image fusion). Whereas the MRI has widely contributed to a better definition of the GTV, [1] which represents an anatomical volume, functional information has not been readily available in the framework of conformal 3D treatment planning and delivery in radiotherapy until recently. In addition, it has been widely accepted that a homogeneous dose delivery to the PTV is the standard of care one should pursue to achieve the best local tumor control.Integration of positron emission tomography (PET) and computed tomography (CT) scanners [2][3][4][5] introduced a new dimension in nuclear medicine by combining functional and anatomical imaging in one machine: the PET/CT scanner. Main advantages of PET/CT scanners are:9 Improved quality of reconstructed PET images with the use of a CT map for attenuation correction of emission PET scans 9 Decrease of about half of the PET acquisition time compared to the previous generation of PET scanners, which used an external radioactive source to acquire transmission scan for attenuation correction. In addition, new faster scintillation materials have also contributed to the shortening of the scanning time 9 The combined medical imaging modality is likely helping management of radiotherapy patients 9 Better understanding of tumor metabolic activity spatial localization by the ability to map the distribution of a specific radiopharmaceutical in co-registered PET/CT images, despite the relatively poor spatial resolution of PET image [6,7].With an increased access to PET/CT information and an apparent appreciation that different sections of the tumor functionally do not behave uniformly, radiation oncologists started to contemplate a change in the traditional concept of uniform dose to the PTV delivery. Instead, a notion of biological targeting and dose painting has come into the play. The first question that comes to our attention is how to define the biological target volumes (BTV) and what do they represent? Ten years after the introduction of the PET/CT scanners into the radiation oncology community, target delineation in radiotherapy treatment planning using FDG-PET/CT scans still has a lot of controversy. The aim of this article is to review the limitations of the current methods for the incorporation of FDG based PET/CT ...

Comparison of SUVmax and SUVpeak based segmentation to determine primary lung tumour volume on FDG PET-CT correlated with pathology data

“…Similarly to previous published studies [15,16], our results indicate that a single fixed threshold is not suitable to define the tumour volume. A higher percentage threshold is required to define the volume for smaller tumours [33]. The mean optimal percentage threshold in our study was found to be 50% +/-10% and 62% +/-15% for the SUVmax and SUVpeak respectively.…”

“…Our study has other limitations that have to be acknowledged. Variations in tumour histology, intratumour heterogeneity, shape and location as well as scanning time after injection, can affect the FDG uptake [32][33][34][35]. Patient characteristics such as variations in glucose levels, weight and age can also have an impact of the uptake of FDG by the tumour [35].…”

FDG-PET/CT and MR imaging for target volume delineation in rectal cancer radiotherapy treatment planning: a systematic review

“…Furthermore, as it has been suggested, appropriate multimodality imaging (integrated or non-integrated) may be used for complementary purposes rather than substituting existing methods 17,25,43 as absolute reliance on PET/CT may result in some geometrical misses especially in the evaluation of local tumour extent where MRI is indispensable. 13,28…”

“…Currently, attention has also focused on the use of 18 Fluorine-2fluoro-2-deoxy-D-glucose or fluoro-2-deoxy-d-glucose-positron emission tomography/CT (FDG-PET/CT) combining functional and anatomical imaging to provide more detailed information on tumour metabolic activity in RTP. 25 In contrast to morphological information provided by anatomical-base modalities, that is, MRI and CT, the advantages of PET/CT include: automatic creation of a delineation around the tumour thereby providing smaller but more accurate TVs. This allows for tumour dose escalation with minimal dose to the OAR, reduced interobserver variability, with the potential to alter treatment strategy.…”

The Path Toward PET-Guided Radiation Therapy for Glioblastoma in Laboratory Animals: A Mini Review

“…It was also shown that the volume of BTV is correlated with the overall survival in GB patients (23). With the increased availability of metabolic information and appreciation for tumor heterogeneity (see further), radiation oncologists started to consider an evolution from the traditional concept of a uniform dose distribution toward a non-uniform dose distribution (24).…”