Precision Radiotherapy: 18F-FDG PET-based radiotherapy planning in Head and Neck cancers

Abstract

Precision medicine is gaining importance in this era of molecular imaging where the molecular features of a disease can be noninvasively assessed and treated with personalized medicine. This is especially suited for head and neck cancers (HNCa). Early stage HNCa are ideally managed with radiotherapy (RT) or surgery. Head and neck (HN) is a complex region and its tumors respond to RT differently due to dissimilar structures and moving organs such as tongue. Radiation oncologists are always in the process of trying and investigating newer RT techniques in order to achieve precise and targetted therapy to tumour/s. One such innovation is Intensity modulated RT (IMRT) using 3 Dimensional conformal RT (3DCRT). This 3DCRT resizes the radiation beams to match the shape of the tumor. Such focused dose escalation may improve local control in HNCa. Image guided RT in conjunction with IMRT is the most advanced form of RT planning being used these days. Simulation computerized tomography (CT) images are usually incorporated into RT planning module. But limitations of CT such as poor soft tissue contrast than magnetic resonance imaging and inability to clearly define solid / cystic / necrotic areas and viable tumour exist. Functional imaging such as Positron Emission Tomography (PET) has established its superiority over CT in delineating the actual site and extent of HN tumors. A combination of IMRT with BTV (Biological Tumour Volume) may be the most ideal technique to deliver a homogeneous radiation boost to tumour. This review shall discuss PET based RT planning, challenges, practical tips, and how to optimize therapy with the least side effects to the normal surrounding tissues.

Keywords: Biological tumor volume; FDG-PET/CT; PET based RT planning; gross tumor volume.

Figure 1

Different radiotherapy planning volumes (terminologies) used with pictorial color code depiction

Figure 2

Transaxial PET images in a patient with floor of mouth cancer, CT barely delineating the lesion. PET-based lesion contouring has been depicted in fused image

Figure 3

Image registration and steps for incorporating PET-CT images into RT planning system

Figure 4

(a) Ideal positioning of PET-based RT planning patient on a flat table covered with a thermoplastic mask and bite block; (b) transaxial PET image showing a large FDG avid lesion involving right tonsillar fossa and posterior aspect of right side of tongue crossing the midline; (c) PET guided GTV contouring of the primary lesion on regional image data sets

Figure 5

A standard PET-CT patient positioning and acquisition protocol is depicted (1) Following FDG inj and uptake, the patient is positioned for the CT scan a scout image is acquired (10 s). (2) CT imaging is acquired and reconstruction begins (60 s). (3) While CT reconstruction completes, the patient is automatically positioned for the PET imaging. PET attenuation correction factors are computed. Usually 7-8 bed positions are acquired for each patient during wholebody PET acquisition starting from thigh end of the table. For RT planning, an additional regional PET-CT (region of interest) acquisition with flat table is performed. Reconstruction is done for each bed position. 4) Whole body CT, PET and fused PET-CT images are reviewed (cross hair marker denotes the nodal mass in left cervical station) for nodal / metastases evaluation