What is the importance of dPET in the management of head and neck tumours? Read on to find it!
(The paper is attached below).
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Unmet clinical needs for PET/CT imaging like the improved detection of subcentimeter metastatic lesions
Improved characterization of indeterminate lesions on PET/CT
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There is agreement in the literature that FDG PET/ CT performed 8-16 weeks after completion of chemoradiation is useful to assess treatment response and to provide prognostic information because it correlates with local control, regional control, and survival
FDG PET/CT offers a distinct advantage over other conventional imaging modalities
FDG PET provides functional insights into tumor biology and tissue metabolism.
Pooled sensitivity of 89.3% vs 71.6% and a pooled specificity of 89.5% vs 78%,
The main advantage of PET/CT applies to clinically node positive HNC
Should be noted that PET/CT has a higher false negative rate for detecting nodal involvement in the setting of a clinically N0 neck.
Due to differences in tumor biology, the criteria used to evaluate treatment response may need to differ for HPV vs HPV¡ disease. This principle was suggested in the meta-analysis by Helsen et al where post- treatment evaluation of HPV tumors was associated with lower sensitivity (75% vs 89%; P =0.01) and specificity (87% vs 95%; P < 0.005
Incorporating PET imaging data into radiation therapy (RT) planning allows for accurately delineating metabolic gross tumor volumes (GTVPET). Studies have generally found that GTVPET are smaller than GTV guided by CT alone
The 3-year recurrence rate was higher in the HPV¡ complete response (CR) cases vs. non-CR cases (92% vs 63%, P < 0.01) but was not different in the HPV CR cases vs non-CR cases (98% vs 92%, P = 0.14).
Increased FDG PET uptake in recently irradiated tissues can persist for 12-16 weeks
Postradiation neck dissection is advisable for all non-CR HPV¡//non-CR N3 HPV cases, but it may be avoided for selected non-CR N2 HPV cases with a significant LN involution if they can undergo continued imaging follow-up.
Guidelines currently recommend waiting approximately 12 weeks before obtaining a restaging FDG PET/ CT
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Replacement of the conventional analog photomultiplier tube-based PET detectors with solid-state, digital photon counting (DPC) PET detectors
A prospective, randomized controlled trial evaluated the role of FDG PET/CT-guided active surveillance as compared with planned neck dissection in the treatment of patients with squamous cell HNC who have advanced nodal disease (stage N2 or N3) after completion of chemoradiation
Detection and visualization of a small radiotracer-avid node using the new dPET detector technology that is not clearly visualized with current cPET technology.
This study recruited 564 patients (282 in the planned-surgery group and 282 in the surveillance group) in which 84% of the patients had oropharyngeal cancer, and 75% had tumor specimens that stained positive for the p16 protein (surrogate for HPV disease
Another example of disruptive innovation is better delineation of true physiologic radiotracer distribution in small tissue structures like the adrenal and pituitary glands may be erroneously interpreted as discrete malignant/meta- static lesions (ie, pseudolesions)
Median follow-up of 3 years, PET-CT−guided surveillance resulted in fewer neck dissections than did planned dissection surgery (54 vs 221). The overall survival rate at 2 years was 84.9% in the surveillance group and 81.5% in the planned-surgery group.
There was no significant difference between the groups with respect to p16 expression. Active surveillance guided by FDG PET/CT resulted in considerably fewer operations and it was more cost-effective.
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Several factors can influence the detectability of a lesion. The FDG-avidity of the underlying tumor biology and the size of the lesion are 2 biological factors that influence lesion detectability. High-grade tumors are usually more FDG-avid than low-grade tumors. Smaller FDG- avid lesions are more susceptible to partial volume effects than larger lesions, which make smaller lesions harder to distinguish from background radiotracer activity. Some technical factors that can affect lesion detectability, include the amount of radiotracer dose administered, the time between radiotracer administration and PET imaging, image acquisition approaches, and image reconstruction methodologies.
These approaches require optimized reconstruction methodologies.
The new dPET technology has the capability to reconstruct images with smaller voxel lengths of 1-2 mm (ie, larger matrix sizes greater than 200-400) which enable high definition and ultra-high definition PET imaging
Another existing challenge for radiologists and nuclear medicine physicians who interpret FDG cPET/CT is the indeterminate lesion or lymph node detected on the anatomic CT images
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Subsequent validation of the 5-point Hopkins criteria demonstrated excellent inter-reader agreement and prediction of progression free survival in HNC patients
The capability of dPET/CT imaging with higher definition reconstructions can also aid in the evaluation of indeterminate lesions by more precisely localizing FDG activity within smaller voxel volumes, reducing partial volume effects and more precisely quantifying radiotracer activity within a lesion or lymph node on CT imaging
The most widely utilized quantitative FDG PET parameters include maximum standard uptake value (SUVmax), metabolic tumor volume (MTV), and tumor lesion glycolysis (TLG). MTV is defined as the sum of the volume of voxels with an SUV surpassing a threshold value in a tumor; and TLG is the MTV multiplied by the mean SUV.
MTV/TLG had a higher predictive value than SUVmax. MTV and TLG may be more representative of overall tumor heterogeneity and therefore serve as a more robust means of correlating FDG PET findings with clinical outcomes.
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The current challenge of surmising the value of interim PET from the existing literature is the variability in the PET metrics used across studies (including median SUVmax, mean SUVmax, SUVmax reduction ratio, complete PET response, MTV, and TLG), as well as the inconsistency in the time point during radiation therapy in which the interim PET study is performed.
The early dynamic dPET time series of the target lesion allows for the detailed assessment of both lesion perfusion and radiotracer uptake kinetics
In the future, the dDPPI approach may be helpful in characterizing tumor perfusion (ie, hyper- perfused vs hypoperfused), tumor differentiation (ie, well- differentiated vs poorly differentiated), and distinguishing between residual malignant disease from postradiation ther- apy inflammatory change.
Significantly lower FDG doses without affecting overall image quality or quantification when compared with cPET/CT.
dPET image acquisition times by more than 50% without affecting dPET image quality and quantification but these new approaches again require opti- mized reconstruction methodologies
Higher definition dPET reconstructions, improved lesion characterization with dDPPI, improved quantitative accuracy, faster patient PET imaging, reduced PET radiotracer dose levels, and more effective interim PET assessment strategies that can drive biologically adaptive radiation.
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FDG PET CT suffers from one major problem-the background noise in the tumour uptake. As such, there is no “threshold” value for the tumour delineation. For example, when anyone relies on the PET scan to form the biological tumour volume, it is difficult to define a cut off value for the tumour volumes. Traditionally, they had been advocating a SUVmax of 40% but it doesn’t seem to work that ways because it is an arbitrary value.
It is further complicated by the fact that each machine has a different characteristic of uptake which also makes subsequent comparisons challenging. Therefore, I chose this paper because digital PET CT would help to refine and standardise several criteria.
I have several reservations about the idea of “de-escalation” of therapy in oropharyngeal tumours because time to progression or systemic disease decreases. Head and Neck, usually follows a systemic progression (primary/nodes and then perhaps systemic). It is nodal region that determines systemic component.
Of course, in my clinical practise, I hardly see early stage tumours. Even if we get the HPV characteristics, de-escalation is still Phase II. I’d be happy to recommend it only if it had a strong radiobiological rationale.
RadOncNotes 