Discussion

Neck dissection for head and neck malignancies is one of the most important aspects of surgical oncology in the head and neck. It not only serves a therapeutic purpose but can also be a diagnostic and staging procedure. Lymph node yield is an important metric in head and neck surgical oncological quality of care, and knowledge of the factors that may influence final lymph node yield is imperative.

Although there is consensus on the lymph node levels to be removed during a neck dissection depending on the primary site, no clear guidance exists for the lymph node yield.^{Reference Lemieux, Kedarisetty, Raju, Orosco and Coffey6} Multiple organisations, including the Radiation Therapy Oncology Group, recommend removal of at least 18 lymph nodes during a standard neck dissection to ensure proper oncological representation, and recent reports by Ebrahimi et al. and Divi et al. found reduced overall and disease-specific survival in oral cavity SCCs when fewer than 18 lymph nodes were removed.^{Reference Ebrahimi, Clark, Amit, Yen, Liao and Kowalski2,Reference Divi, Harris, Harari, Cooper, McHugh and Bell7} In this multicentre analysis, they found an 88 per cent increased risk of death from oral cavity SCC in patients with a nodal yield less than 18 when compared with those having a yield over 18, with a higher risk of locoregional recurrence in the former group as well. They also found no difference in lymph node yield depending on node stage, meaning that results can be reasonably applied to both node negative and positive patients.^{Reference Ebrahimi, Clark, Amit, Yen, Liao and Kowalski2} Recent evidence also points towards a survival benefit in patients with limited nodal disease in HPV-associated oropharyngeal SCC when more than 26 lymph nodes were removed.^{Reference Zenga, Stadler, Massey, Campbell, Shukla and Awan8}

Various different factors can affect the final number of lymph nodes present in the histological specimen. These can be divided into patient and physician factors. Patient factors include age of the patient, previous surgery or radiation to the neck, primary site of the malignancy (including HPV status in oropharyngeal SCC), and body mass index. Physician factors include volume of neck dissections performed and expertise of the operating surgeon, level of neck dissection and number of levels removed (which is also closely linked to the primary site involved), and expertise of the pathologist in identifying lymph nodes in the histological specimen.

Radiotherapy has been suggested as a factor in reducing the lymph node yield in neck dissection specimens. Data on this are limited to only a few small studies, with conflicting results reported. Moore and Bhattacharyya found no statistical difference in lymph node yield between irradiated (mean lymph nodes 23.5) and non-irradiated necks (mean lymph nodes 23),^{Reference Moore and Bhattacharyya9} whereas Johnstone et al. found a statistically significant reduction in lymph node yield in patients with a history of radiotherapy.^{Reference Johnstone, Miller and Moore4} Reyes et al. examined 98 neck dissections in patients with laryngeal SCC and found no difference between those who had received previous radiotherapy and those who had not.^{Reference Reyes, Rios, Groves, Solares, Jackson and Byrd5}

In the present study, there was a statistically significant reduction in the total number of lymph nodes identified in patients who had undergone previous radiotherapy. This is in concordance with anecdotal observations intra-operatively, where marked fibrosis and lymph node atrophy were noted. The mean reduction in the number of lymph nodes was 14. The possible mechanism for the reduction in lymph nodes in irradiated necks is the fact that radiotherapy leads to fibrosis of tissues and fatty replacement of lymph nodes. Fibrosis following radiotherapy usually starts to develop 8 to 12 weeks later.^{Reference Brook10} These changes also make histological identification of lymph nodes more difficult.^{Reference Shvero, Koren, Marshak, Sadov, Hadar and Yaniv11} However, multiple other mechanisms for variation in lymph node counts may also be responsible: surgeon technique may play a role in the final lymph node count,^{Reference Divi, Harris, Harari, Cooper, McHugh and Bell7} and this was not examined in the present study. Seniority of the pathologists examining the tissue has also been suggested as a factor influencing lymph node yield in neck dissection specimens.^{Reference Divi, Harris, Harari, Cooper, McHugh and Bell7,Reference Ostadi, Harnish, Stegienko and Urbach12} We found no difference in the lymph node count depending on the seniority of the pathologist examining the gross specimen. Tissue fixation can also affect the number of lymph nodes found in the pathological specimen.^{Reference Lemmens, van Lijnschoten, Janssen-Heijnen, Rutten, Verheij and Coebergh13} The policy at our centre is to send all specimens to the histopathology laboratory fresh and pinned to a cork board, followed by fixation in formalin after reception at the laboratory.

Recent reports by Schoppy et al. and Divi et al. suggested using a lymph node count of greater than 18 in neck dissections as a potential measure of quality in head and neck surgery because of the associated improved survival and lower rate of locoregional failure.^{Reference Divi, Harris, Harari, Cooper, McHugh and Bell7,Reference Schoppy, Rhoads, Ma, Chen, Nussenbaum and Orosco14} There has to be a distinction on what levels have been removed, and this will depend on the indication for surgery and the primary site involved, as a greater number of lymph node levels removed will lead to a greater number of lymph nodes retrieved. In order to control for this, we divided the number of total lymph nodes by the number of lymph node levels removed.

The main limiting factor of the present study was the lack of long-term outcome data. This limits our ability to investigate the effect of nodal yield on disease recurrence and disease-specific and overall survival. We were also unable to identify what effect the surgeon's level of training had on the nodal yield. Gathering this information is exceedingly difficult, as the amount of involvement of the trainee in any part of the operation can be highly variable. Nonetheless, we feel that the present data provide useful information on the factors that influence nodal yield and will help guide surgical oncological quality.