ReviewPET scans in radiotherapy planning of lung cancer
Introduction
Radiotherapy is a key treatment modality in the curative treatment of patients with both non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC). Careful staging and patient selection is important to achieve the maximal chance of long-term survival with acceptable side effects. Similarly, accurate delineation of target volumes is crucial for preventing geographic misses. An incorrect definition of the gross tumor volume (GTV) (i.e. detectable tumor) or clinical target volume (CTV) (tumor plus a margin for microscopic extension) is a common source of error, which can lead to under-treatment and a reduced probability of tumor control. In recent years, it has been shown that FDG PET may be of significant value for radiotherapy planning in NSCLC, which is the focus of the present article.
Because the overwhelming majority of clinically relevant work has been done with 18F-2-deoxy-2-fluoro-d-glucose (FDG), the only widely available PET radiopharmaceutical, in the rest of this article, the term “PET” refers to FDG PET unless otherwise stated and FDG is the only tracer to be discussed.
In contrast with the diagnostic use of PET imaging, in which dichotomous outcomes are usually considered, (e.g. is cancer present at a particular location, yes or no?), and for which qualitative imaging is adequate, when PET-CT is used to plan radiotherapy, the volume shape and three-dimensional location of the tumor volumes are also of importance, making strict requirements for quantitative imaging even more crucial. When PET is used for radiotherapy planning, a range of uncertainties related to technical (e.g., relative calibration between dose calibrator and PET scanner), physical (e.g., image reconstruction parameters), biological (e.g., motion, tumor heterogeneity) and analytical (e.g., Region Of Interest, ROI, definition method) factors must be considered.
If a PET scan will be used for RT planning, the patient should be positioned and immobilised in the treatment planning position on the PET-(CT) scanner, according to protocols that are as strict as those used actually to deliver treatment. Due to the possible influence not only on diagnostic accuracy but also on tumor delineation, institutional standardisation is needed for image acquisition (including time interval between injection and scanning, adjustment of dose calibrator, reconstruction parameters, breathing protocol of the CT scan and many more known technical, physical and biological factors influencing the SUV), data display and analysis and for the transfer of PET-CT data to the planning system [2], [3], [4]. Here, strict clinical protocols must be used, as volume assessment and delineation (see below) assisted by PET is crucially dependent on many of the above-mentioned factors and significant errors can only be avoided by standardisation of procedures (Table 1) [1].
Section snippets
Target volume selection
Many clinicopathological studies have investigated the specificity and sensitivity of CT scans compared to FDG PET-(CT) scans. For the evaluation of indeterminate lung lesions, the sensitivity and specificity of PET(?) range from 79 to 96% and from 40 to 83% respectively (reviewed in Ref. [2]). FDG PET is the non-invasive method with the highest diagnostic accuracy for distinguishing benign from malignant pulmonary nodules. The main limitations of FDG PET in the evaluation of lung nodules are
Target volume selection
Several studies have investigated the specificity and sensitivity of CT scans vs. FDG PET-(CT) scans for the staging of small cell lung cancer, but its role has not been clearly established [33], [34], [35], [36], [37], [38]. In general, though, the accuracy for detection of mediastinal lymph nodes is higher with PET than with CT. Although, after CT-based radiotherapy planning, isolated nodal recurrences may be seen in over 10% of patients, in a prospective study selective nodal irradiation
Conclusions, perspectives
In patients with NSCLC, FDG PET scans allow more accurate staging, thus avoiding unnecessary aggressive and toxic therapies in patients who would be unable to benefit from them. PET based planning has the potential to reduce radiation treatment volumes because of the avoidance of mediastinal lymph nodes that are PET negative and hence reduces toxicity with the same radiation dose or enables radiation dose escalation with the same toxicity. Preliminary data are also encouraging for small cell
Conflict of interest statement
None declared.
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