Liao et al. BMC Cancer 2012, 12:236 http://www.biomedcentral.com/1471-2407/12/236 predictive rate with negative CT, MRI, PET, and US results increased to 82%, 84%, 86% and 84%, respectively (with a positive neck lymph node metastasis rate below 20%), meaning a “watchful waiting policy” is possibly jus- tified in these cases. In a recent report [8], the threshold was estimated even higher (44.4%) for oral tongue can- cer. The occult cervical lymph node metastasis rate has been estimated at 25% ~ 35%, except for the glottic lar- ynx, by palpation [9]. Therefore, a “watchful waiting pol- icy” is feasible for some low pre-test occult metastases in cN0 neck patients, such as clinically T1 ~ 2N0M0 lip cancer patients. However, some clinically T4N0 tongue or tongue base cancers may have pre-test probability >60%. Even with a negative imaging result, the post-test probability is still approximately 20% (Figure 2), and elected neck dissection is still necessary for these patients. In patients with positive imaging results, even with a very low pretest possibility set at 10%, the positive nodal metastasis probabilities were all above 20% (47%, 27%, 36% and 25% for CT, MRI, PET and US, respect- ively). Elective neck dissection should be performed for all patients with positive pre-op diagnostic results. According to our results, the pooled estimates for sen- sitivity were 52% (95% confidence interval [CI], 39% ~ 65%), 65% (34 ~ 87%), 66% (47 ~ 80%) and 66% (54 ~ 77%) on a per-neck basis for CT, MRI, PET and US, respectively. The pooled estimates for specificity were 93% (87% ~ 97%), 81% (64 ~ 91%), 87% (77 ~ 93%) and 78% (71 ~ 83%) for CT, MRI, PET and US, respect- ively. Our results are similar to a previous meta-analysis [5], which compared PET to other traditional imaging modalities (including CT, MRI and US-guided fine- needle aspiration). This previous study concluded that PET was not superior to other imaging modalities in a cN0 neck work-up. However, the study was focused on all nodal statuses and subgroup analysis for N0 patients combined CT, MRI and US-guided fine-needle aspiration (US-FNA) in the same group. PET exam is the more expensive imaging option for nodal surveillance; however, it did not provide better sensitivity and specificity. Therefore, it should not be routinely used in neck nodal status work-ups. In our opinion, CT or MRI is preferred for cN0 neck pre-op evaluation because CT and MRI had similar diagnostic sensitivities to PET and US. Furthermore, CT and MRI can evaluate the status of primary tumor at the same time. The US is an inexpensive and convenient tool to monitor nodal status and can be used with real-time guided fine-needle aspiration. However, the primary tumor lesion and some deep-seated lymph nodes, such as retropharyngeal nodes, cannot be assessed [10]. The alignment of the results between preoperative im- aging and histologic specimen after neck dissection should be taken into consideration. According to Page 6 of 7 previous reported literatures [11,12], the rate of regional recurrence in pN0 patients varying from 3% to 10%. Ap- plying only the histopathological results as reference standard, one could underestimate the real occult metas- tasis rate. Therefore, we included studies using either pathological examinations, or clinical follow-up results, or both as references. Besides, the criteria for positive results in pre-operative diagnostic imaging were not uni- form in different institutions, and may be operator- dependent. These variations all leaded to heterogeneity in this meta-analysis, and this was the reason to adapt a random effect model for data pooling. During this review, we found that US-FNA had a very high specificity; if the US-FNA cytology had a positive result, almost all of the histology specimen results also proved positive [1,13-15]. Therefore, in our opinion, US is preferred for neck status follow-up in “watchful wait- ing” patients, and US-FNA can be performed if nodal metastasis is suspected. In our review, we did not include US-FNA because US-FNA cytology examination required a cytologist’s as- sistance and was used after US examination. However, it must be noted that US-FNA had 100% specificity be- cause there were no false-positive cases. Therefore, we did not believe that the comparison of sensitivity, specifi- city and summary ROC curves were justified between US-FNA and other imaging modalities because the lack of false-positive cases spuriously inflates the value of the area under the ROC curve. In our review and subsequent meta-analysis, we found modern imaging modalities had fair diagnostic perform- ance in cN0 neck patients. For positive imaging results, elective neck dissection is indicated; for some select low- risk patients with pre-test probability below 30% of nodal metastasis, a “watchful waiting policy” may be an accept- able alternative to neck dissection if strict adherence to a cancer surveillance protocol is followed. Conclusions Modern imaging modalities offer similar diagnostic ac- curacy to define and diagnose cN0 neck. Minimizing morbidity and avoiding elective neck dissection is ac- ceptable in some select cases. Competing interests We declare no conflicts of interest. Authors' contributions LJL: conception and design, WCL: acquisition of data (reviewer 1), WLH: statistical analysis, CTW: acquisition of data (reviewer 2), MSL: Instructor (revising). All authors read and approved the final manuscript. Acknowledgements This work was supported by the National Science Council of the Republic of China (Grant NSC-100-2314-B418-005) and grants from the Far Eastern Memorial Hospital (FEMH - 100-2314-B418-005).