Discussion
The aim of this study was to compare outcomes of minimal tracheal invasion to the other forms of ETE, namely, strap muscle, RLN, esophagus and larynx invasion. Our results demonstrated that minimal tracheal invasion undergoing shave resection, has similar outcomes to strap muscle invasion as opposed to the other forms of ETE. The analyses of the various outcomes according to the updated AJCC, revealed that minimal tracheal invasion may be more accurately staged as T3b rather than T4a.
The conflicting results of studies on shave resection outcomes might be attributed to inconsistencies of the study designs, such as varying definitions of invasion, completeness of resection, various histology and adjuvant treatments, and, most importantly, the lack of clear patient selection criteria for shave excision.[18] Shave excision for extensive tumors is likely to result in incomplete resection and should be rejected outright. In cases of minimal tracheal invasion (defined as Shin I and II), however, shave resection is a reasonable option and one that apparently has good outcomes. We found a 14% biochemical and structural recurrence rate with shave resection of the trachea. The OS in the shave group with a median follow-up of 54.4±35.8 months was 100%. These results are comparable to those in papers that showed recurrence rates ranging from 5-17%.[19, 20] In fact, the latest (2014) American Head and Neck Society (AHNS) consensus statement stipulates that a tracheal shave excision is appropriate if a short segment of the trachea is invaded and there is minimal cartilage invasion (section 6A).[4] Judicious management of the patients, including appropriate postoperative RAI and intense follow-up, is essential to achieve such excellent results.
One of the endpoints of this study was to analyze the new AJCC staging system for thyroid cancer. We aimed to determine if all forms of tracheal invasion should be automatically categorized as T4a, or if the degree of visceral invasion should influence stage definition. With the caveat of not having reached a level of statistical significance, most probably as the result of the under-power of this study, the trend of minimal tracheal invasion to “behave” more like a T3b disease warrants further investigation. This trend (Fig. 1) yielded surprising results by demonstrating that patients with minimal tracheal invasion whom are considered T4a, have as good as, if not better, prognosis compared to T3b patients. Since extrapolation of this trend seemed promising, we performed another subgroup analysis in which we combined the patients with T3b and T4a shave. The results were significant and persuasive; they showed that patients with minimal tracheal invasion and/or strap muscle involvement have a better prognosis compared to patients with other structural involvement in the neck (Fig. 1). These results were robust for the all ages groups as well as for the 55-year and over age group.
In the international multi-institutional validation of age 55 years as a cutoff for risk stratification in the AJCC/UICC staging system for WDTC,[21] the authors concluded that 55 years (compared to 45 years in the older version) improved the prognostic accuracy of the TNM staging model. This conclusion was valid for the various T and N stages, and it improved the distribution of outcomes between stages I and IV disease and led to a down-staging of a significant percentage (12%) of patients. If, indeed, such be the case, and minimal tracheal invasion has the same prognosis as strap muscle involvement, patients older than 55 years might be more correctly staged as stage 2 rather than stage 3 in the updated AJCC.
The precise biological basis for these results are not within the scope of this study, but we can speculate that very low volume residual disease in the minimal tracheal invasion group combined with proper postoperative treatment plays a major role. The fact that the other ETEs had higher rates of R1 resections compared to the shave group (p= 0.022) also serves to explain their worse outcomes. The better outcomes of the shave group correspond with earlier reports that patients with ETE to the trachea with R1 resections have essentially the same prognosis as after R0 resections.[22]
The other end result of the current study is the relatively good DFS of the entire cohort, despite achieving a “non-oncological” surgical resection. Adjuvant RAI has been shown to decrease recurrence rates in WDTC.[23-25] Tuttle et al .[26] reported that even when post-RAI whole-body scans showed residual disease in the neck, RAI treatment resulted in a 70% loco-regional control rate. Furthermore, the findings of several reports suggested that RAI treatment after primary surgery may also improve OS in patients with high-risk features, such as ETE.[27, 28]
There are obvious limitations to this study that bear mention. The most important shortcoming is the fact that we did not reach a level of statistical significance to prove that patients with minimal tracheal invasion have a more favorable outcome compared to other T4a patients among various age groups. The retrospective nature of this study and the relatively short follow-up time for WDTC are additional drawbacks. We were, however, able to show that patients with minimal tracheal invasion combined with strap muscle involvement had a significantly better outcome compared to other forms of ETE. We believe that these results warrant further study to elucidate whether this trend will prove valid in an appropriately powered investigation and that they may serve to downstage thousands of patients worldwide.
In summary, patients with minimal tracheal invasion who undergo shave resection had outcomes similar to patients with strap muscle invasion. They may have a better prognosis than patients with other forms of ETE. If that proves to be correct, patients with minimal tracheal invasion should be staged as T3b rather than T4a (AJCC staging system).
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