Patient demographics and database
Newly diagnosed oral cancer patients treated with resection of the
primary oral tumor and neck dissection with or without adjuvant therapy
were identified from our hospital Cancer Registry Database within the
years 2009 to 2018. Patients without complete treatment, electric
medical records, regular follow up, and distant metastasis on diagnosis
were excluded in our study. The type of neck dissection included
elective neck dissection (END) for clinical N0 disease and radical neck
dissection (RND) for clinical N1-3 disease. Patients with bilateral neck
dissection and those with retrieved LN less than 10 were also excluded.
The variables collected from the Cancer Registry Database included
patient demographic data, such as age, gender, tumor status such as
clinico-pathological TNM stage, and pathological risk features (such as
margin status, tumor differentiation, perineural
invasion, lympho-vascular permeation, number of retrieved LNs, number of
positive LNs, extranodal extension, adjuvant treatment
modality, radiation dose, and chemotherapy regimen). All staging was
done according to the American Joint Committee on Cancer (AJCC) cancer
staging (7th edition). As shown in figure 1, the study periods were
categorized into two stages: period 1 (before 2015 Oct), and period 2
(after 2015 Oct). In the period 1, the operative procedure of primary
tumor resection and neck dissection was performed as per the guidelines
or literatures in target group and control group.3,14Since period 2, soft but non-mandatory intervention, lymph node
promotion activiy (LNPA) for cervical lymph node improvement was
implemented in target group alone. The LNPA used in the target group
including: (1) promotion of the importance of survival rates and LN
yield in head and neck cancer in weekly meetings, and resident journal
club conferences; (2) surgeons were encouraged to identify important
landmarks, like cervical ansa, internal jugular vein, carotid artery,
spinal accessory nerve, splenius capitus, levator scapula, anterior
scalene muscle, and transverse cervical artery during neck dissection ;
(3) the average number of LN yield was announced per month, but
individual data related to the patients or surgeons were not disclosed
(Figure 1).
The hypothesis was that target group surgeons’ behavior response to the
LNPA could improve LN yield in the period 2. The effect of LNPA on lymph
node yield and regional recurrence was evaluated by
difference-in-differences analysis. For regional recurrence, we used a
concept of regional recurrence density which meant the proportion of
regional recurrence among those treated with neck dissection during a
specific period. This method could provide us a comparison of regional
recurrence between the target group and control groups in different
periods.