Data of the prospective cohort study Netherlands Quality of Life and Biomedical Cohort (Net-Qubic) Study were used . Patients were recruited between 2014 and 2018 and included when they were 18 years or older, diagnosed with oral, oropharyngeal, hypopharyngeal, laryngeal, or unknown primary HNC. Patients with recurrent or residual disease, with cognitive impairments, and having trouble understanding or reading the Dutch language were excluded. The study protocol was approved by the Medical Ethics Committee (NL45051.029.13). In the present study, the study population consisted of patients with data on MAT, WST, and salivary flow test. These tests were only performed in one single center (University Medical Center Utrecht (UMCU)). Sociodemographic and clinical data about age, sex, tumor stage, tumor location, and treatment were collected from medical records. All participants signed informed consent. Data from objective tests and subjective questionnaires were used as collected before primary treatment (baseline, M0), 3 months after treatment (M3), and 6 months after treatment (M6). Patients that did not perform both objective and subjective measures at one time point were excluded. A comparison between objective and subjective data was based on assumptions regarding best fit of subjective data to objective data.
Mixing ability test
The MAT consists of two layers of wax, with the colors red and blue (Plasticine modelling wax, non-toxic DIN EN-71, art. nos. crimson 52,801 and blue 52,809, Stockmar, Kalten Kirchen, Germany) . The total thickness is 3 mm, with a diameter of 30 mm. The outcome variable ranges between 5 and 30, where a lower score implies a better mixed tablet and better masticatory performance. A subject was asked to chew on this tablet 20 times in order to mix the two colors. The tablet is then flattened, pressed to a thickness of 2 mm, and scanned on both sides using a high-quality scanner (Epson® V750, Long Beach, CA, USA). The scanned images are then processed using Adobe Photoshop CS3 extended (Adobe, San Jose, CA, USA). The histograms of both sides of the flattened and scanned wax tablet are added to obtain red and blue intensity distributions. The spread of the color intensities is measured . In previous research, this test has proven to be highly reliable in patients with HNC (ICC = 0.886) .
100-mL water swallow test
During the WST, a subject was asked to drink 100 mL of water as quickly as is comfortably possible. The time to swallow this 100 mL (in seconds) and the number of swallows were counted, both by the subject and the researcher. Timing started when the water touched the bottom lip, and stopped when the larynx came to rest after the last swallow . Persons failed the test when they coughed or choked post swallow, had a wet voice quality post swallow, or were unable to drink the whole 100 mL . When a person was unable to drink the 100 mL, the residual water was measured and noted. In a previous research, this test has proven to be highly reliable in patients with HNC (ICC = 0.923 for number of swallows, and ICC = 0.893 for duration) .
Salivary flow was collected simultaneously from the floor of mouth (mainly submandibular gland) using a pipette, and from the left and right parotid gland using Lashley cups, as first described in 1981 . The cups were placed over the orifice of the Stenson’s duct. Stimulation of the glands was achieved by applying one drop of citric acid to the mobile part of the tongue every minute, and collection was carried out for 10 min. The volume of saliva was measured as collected in tubes by weight, assuming the density of saliva as 1 g/ml. The flow rate was expressed in milliliters per 10 min (ml/10 min) for both parotid glands and the submandibular gland. In the present study, we used the total amount of saliva by adding up the saliva of both parotid glands and the submandibular gland. No oral stimulus was permitted for at least 30 min before saliva collection, including the WST and MAT . In previous research, this test scored an ICC of 0.66 and 0.63 for the left and right parotid flow glands, indicating moderate test–retest reliability .
The EORTC QLQ-H&N35 is an additional questionnaire to the EORTC QLQ-C30 (core instrument), and widely used to measure QoL in patients with HNC . It consists of 7 subscales: pain in the mouth (4 items), problems with swallowing (4 items), senses (2 items), speech (3 items), social eating (4 items), social contact (5 items), sexuality (2 items), and 11 single items which address problems with teeth, opening mouth, dry mouth, sticky saliva, coughing, feeling ill, painkillers, nutritional supplements, feeding tube, weight loss, and weight gain . The scores are transformed to a scale of 0 to 100, with a higher score on the symptom scales implying a higher level of symptoms or problems . In the present study, we used the subscales “pain in mouth” and “social eating,” and the single items “teeth,” “opening mouth,” “weight loss,” and “weight gain” to explore the association between these PROs and the MAT. The subscales “pain in mouth” and “problems with swallowing,” and the single items “dry mouth,” “coughing,” and “feeding tube” were used to explore the association between these PROs and the WST. The single items “dry mouth” and “sticky saliva” were used to explore the association between these PROs and the salivary output. This questionnaire performs well on internal consistency and construct validity, and is able to differentiate between diverse groups of patients regarding treatment, tumor size, time elapsed since treatment, and age . In patients with HNC, Cronbach’s α ranges from 0.75 to 0.93 for most scales, indicating satisfactory internal consistency [20, 31].
The SWAL-QoL-NL consists of 39 items on 9 subscales: general burden, food selection, eating duration, eating desire, fear of eating, mental health, social functioning, and symptoms [21, 29]. After completing, a total SWAL-QoL-NL score could be calculated based on 23 items (item 1–9 and 12–25). The scores range from 0 to 100, with a higher score indicating more impairment . In the present study, we used the subscales “food selection,” “eating duration,” “eating desire,” “fear of eating,” and the total score to explore the association between these PROs and the MAT. We used the subscales “general burden,” “symptoms,” and the total score the explore the association between these PROs and the WST. Cronbach’s α ranges from 0.79 to 0.95 in patients with oropharyngeal dysphagia, and intraclass correlations range from 0.59 to 0.91, indicating excellent scale reliability .
The GRIX consists of 14 questions and four subscales: xerostomia during day and night, and sticky saliva during day and night . The scores were transformed to a scale from 0 to 100, with a higher score indicating more problems regarding xerostomia or sticky saliva. Total xerostomia and sticky saliva were calculated by adding up the day and night scores to get a score from 0 to 200. In the present study, all subscales were used to explore the association between the PROs and salivary flow. Cronbach’s α of these scales ranges between 0.82 and 0.94, and test–retest reliability was between 0.63 and 0.67, indicating moderate correlations .
Data were tested for normality using a Shapiro–Wilk test. The associations between the WST, MAT, and salivary flow versus PROs were tested using Spearman’s rank correlation coefficient. The spearman correlation coefficient was categorized as very weak (0.0 to 0.1), weak (0.1 to 0.39), moderate (0.4 to 0.69), strong (0.7 to 0.89), and very strong (0.9 to 1.0) . Scatterplots were created to visualize the MAT, WST, and salivary flow outcomes that had the highest correlation with one of the PROs. All analyses were performed using Statistical Package for the Social Sciences (SPSS) version 25 (Chicago, IL). A Bonferroni correction was used to account for the number of tests performed, in order to avoid a type Ι error . This correction was calculated by dividing the p-value by the number of tests performed. The corrected p-value was 0.05/12 = 0.004 for the MAT, 0.05/8 = 0.006 for the WST, and 0.05/8 = 0.006 for the salivary flow. A p-value ≤ 0.004 or 0.006 was considered statistically significant.