This study showed that PET-CT was a useful tool for selecting potentially operable lung cancer patients at Rikshospitalet in the period 2007 – 2011. Pathological uptake of FDG in mediastinal lymph nodes led to targeted biopsies of N2 and N3 lymph nodes, thereby helping to ensure correct staging of the patients’ disease. This is consistent with previous studies (16). PET-CT also led to the detection of occult remote metastases in 9 % of our patients, who had this verified by means of biopsy or MRI of the lesion in question. Without PET-CT, these metastases would very likely not have been detected before they became symptomatic. Although some of these remote metastases can be detected by means of CT, whole body CT scans are not conducted as a matter of routine on asymptomatic patients (17). CT also has limitations when it comes to skeletal metastases, and a recently published meta-analysis (18) shows that PET-CT is superior to MRI or scintigraphy as an imaging method for detecting skeletal metastases. The number of patients with spreading to the contralateral lung (12 (25 %)) may appear high, but it proves to be consistent with a previous study by MacManus et al. (19). Surprisingly, they found metastases in 32 of 167 patients, where the proportion with metastases to the contralateral lung that were not seen on an ordinary CT was 10/32 (30 %). In our view, this strengthens the indication for using PET-CT for assessing lung cancer.
Because the population in our study is not a representative selection of all lung cancer patients in a population, this limits the possibility of generalising our results to a random selection of the general population. We wanted to carry out a PET-CT on all potentially operable patients, but for reasons of capacity had to prioritise. About half of the group who underwent surgery without a prior PET-CT consisted of patients with stage 1A lung cancer. It is therefore reasonable to believe that the prevalence of operable patients in this group was higher than in the one examined with PET-CT. Since calculations of positive and negative predictive value depend on prevalence, we cannot generalise the results to the entire group of potentially operable patients.
Earlier studies have shown that PET-CT may be false positive for spreading to mediastinal lymph nodes in 3 – 16 % of scans (12). In our survey, 9 % of findings were false positives. The reasons for the false positive findings may be inflammation (infectious and non-infectious), granulomas, or they may be iatrogenic, in connection with biopsy-taking. It is therefore important to specify that PET-CT cannot replace invasive examination where the latter is indicated (20). However, the examination can increase the accuracy of further assessment. In practice, we have seen few iatrogenic causes of false positive scans.
PET-CT in combination with diagnostic CT with intravenous contrast yields an accurate assessment of primary tumour, mediastinal lymph nodes and remote metastases. If these scans do not arouse suspicion of spreading to either mediastinal lymph nodes or other organs, the patient can with few exceptions be referred directly for surgery without further scans such as endobronchial ultrasound, endoscopic ultrasound, MRI or scintigraphy (6). This can help to shorten the assessment time. Using PET-CT to detect remote metastases in patients with unknown tumour histology will also enable a biopsy to be taken from the most superficial and readily accessible lesion. Taking a biopsy sample from such a lesion may often be a less risky procedure than taking a sample from a primary tumour in a lung or from lymph nodes in the mediastinum. Mediastinoscopy was not needed in our examination in order to reach a definite diagnosis. In modern lung cancer assessment, the use of PET-CT and endoscopic ultrasound-guided fine-needle aspiration has now largely replaced mediastinoscopy. We found that PET-CT had a 94 % negative predictive value in our population. This indicates that it may be justified to refrain from further assessment if the PET-CT is negative in the mediastinum or other organs; this is also recommended in the guidelines (6). Our results are supported by other, similar studies (21). A Danish study has shown that systematic use of PET-CT is more cost-effective than conventional assessment alone for avoiding futile thoracotomies (22).
However, some lung cancer patients still have such extensive comorbidity that any kind of invasive assessment is contraindicated and a histological diagnosis cannot be made (23). Stereotactic radiotherapy may offer an alternative type of treatment for some of these patients, and they, too, should be assessed with PET-CT to enable optimal planning of the treatment (6). Individual stage IIIA and IIIB patients with certain positive prognostic factors may also be candidates for curative radiotherapy even if from a technical point of view they are not operable, as also indicated in the Norwegian guidelines (6).
We found a kappa agreement of 0.61. Kappa agreement is a measure of the reliability of the examination and reflects the extent to which there will be consistency between two examinations if they are repeated under identical conditions. Kappa gradation tables have been developed, in which agreement of over 0.6 is graded as good (i.e. > 60 % better agreement in the event of examination than if the result of the examination had been entirely random). We also found a diagnostic accuracy of 86 % (95 % CI: 83 – 89). This represents the sum of all positive and negative examinations that were true positive or true negative, divided by the total number of scans carried out.