Changing Role of PET/CT in Cancer Care With a Focus on Radiotherapy

Abstract

Positron emission tomography (PET) integrated with computed tomography (CT) has brought revolutionary changes in improving cancer care (CC) for patients. These include improved detection of previously unrecognizable disease, ability to identify oligometastatic status enabling more aggressive treatment strategies when the disease burden is lower, its use in better defining treatment targets in radiotherapy (RT), ability to monitor treatment responses early and thus improve the ability for early interventions of non-responding tumors, and as a prognosticating tool as well as outcome predicting tool. PET/CT has enabled the emergence of new concepts such as radiobiotherapy (RBT), radioimmunotherapy, theranostics, and pharmaco-radiotherapy. This is a rapidly evolving field, and this primer is to help summarize the current status and to give an impetus to developing new ideas, clinical trials, and CC outcome improvements.

Keywords: cancer care; positron emission tomography computed tomography; precision radionuclide therapy; radiotherapy (rt); theranostics.

Figure 1. CT-based versus PET/CT based normal tissue volumes comparison

PET/CT: positron emission tomography/computed tomography, PET: positron emission tomography, CT: computed tomography, NTV: nodal target volume definition This image is reproduced from Van De Bosch et al. [13] - available via Creative Commons CC-BY-NC-ND license

Figure 2. CT - based versus PET/CT based comparison of invasive ductal cancer of the right breast

CT of the whole body: Axial (A) and coronal (D) views show enlarged node in the right axilla (A, arrow; C) associated with an irregular mass within the right breast (A, circle). PET of the whole body: Axial (B) and coronal (E) views demonstrate high flurodeoxyglucose uptake for both the mass and the enlarged node in the axilla that is also confirmed by hybrid imaging PET/CT (C, F) CT: computed tomography, PET: positron emission tomography, PET/CT: positron emission tomography/computed tomography This image is reproduced from Marino et al. [56] - available via Creative Commons CC-BY-NC-ND license

Figure 3. 18F-DOPA scan of neuroendocrine embryonal tumor before and after therapy documenting treatment response

¹⁸F-DOPA: Fluorine-18-I-Dihyroxyphenylalanine Axial (A) and coronal (B) magnetic resonance imaging - positron emission tomography fusion images and axial positron emission tomography/ computed tomography (C) images showing brainstem uptake (arrow); axial positron emission tomography/computed tomography (D) after systemic chemotherapy and proton therapy shows lower brainstem uptake (arrowhead) This scan image is reproduced from Masselli et al. [62] - permission obtained from e-Century Publishing Corporation

Figure 4. Use of PET/CT in the staging of lymphomas including the ability to diagnose bone marrow involvement

Maximal intensity projection image (A) in a patient of diffuse large B-cell lymphoma shows extensive sites of involvement visualized as areas of increased fluorodeoxyglucose uptake. Trans-axial computed tomography images (BB, C show axillary and abdominal lymphadenopathy (thin arrows) ad a large subcutaneous nodule (arrowhead). Fused PET/CT images (axial D, E; coronal F; and sagittal G) show sites of bone marrow involvement (thick arrows) in the sternum, spine, and iliac crest, over and above the lesions picked up on CT This image is reproduced from D'souza et al. [64] - available via Creative Commons Attribution - Noncommercial- Share Alike 3.0 Unported license

Figure 5. Demonstration of PET-directed prostate cancer oligometastatic disease treated with radiation therapy

(A) Avid abdominal node prior to RT (B) Same abdominal node showing less avidity post-RT (C) Avid neck node prior to RT (D) Same neck node post-RT (E) Avid pelvic nodes prior to RT (F) Same pelvic nodes post-RT PET: positron emission tomography, RT: radiation therapy This image is permitted through the signed consent from respective University of Mississippi Medical Center patient

Figure 6. Potential abscopal effect realized by follow-up PET demonstrating global reduction in PET-avidity after immunotherapy-boosted, hypo-fractionated RT to only the central liver lesion and intramammary node

(A) Prior to RT and immunotherapy (B) Post-RT and immunotherapy PET: positron emission tomography, RT: radiation therapy This image is permitted through the signed consent from respective University of Mississippi Medical Center patient

Figure 7. Lung tumor realized by 4DCT showing superior and inferior extremes of motion

(A) Inferior extremes of motion (B) Superior extremes of motion (C) Demonstration of PET/CT's potential not only for tumor identification, but also for tumor real-time tracking. 4DCT: Four-dimensional computed tomography, PET/CT: positron emission tomography/ computed tomography This image is permitted through the signed consent from respective University of Mississippi Medical Center patient