Neck and head cancer, including cancers of the buccal cavity in addition to salivary and thyroid glands is the sixth most common malignancy worldwide. The rate of death reaches 350,000 cases out of approximately 600,000 new cases yearly. Radiotherapy is an essential method of cancer therapy either alone or combined with chemotherapy and/or surgery. During RT for malignancies of the neck and head, salivary glands may become unintentionally exposed to radiation, which causes damage, especially if a high radiation dose is used to control cancer[23]. Parotid glands have been suggested to be more[24] or equally[25] vulnerable to radiotherapy compared to submandibular glands, as serous cells show more damage in the form of degranulation, atrophy and pyknosis and at lower doses than mucous and ductal cells[24, 25]. Since parotid gland mainly consists of serous cells, the submandibular gland is a mixed serous/mucous gland, and it might be that the parotid gland is more radiosensitive in terms of saliva production[26].
Our examination of parotid glands 28 days post irradiation revealed loss of acinar outline, severe intracytoplasmic vacuolization of acinar cells, interacinar shrinkage and duct degeneration in addition to fibrosis around ducts. Similar results were reported in the parotid glands 8 hours and 30 days following 15 Gy irradiation[27] and submandibular glands 28 days after 18 Gy irradiation[28]. They reported strong vacuolization, many pyknotic nuclei, lysis of acinar and duct cells, degeneration of many granular convoluted tubules and striated ducts epithelial cells, and interstitial edema and fibrotic tissue. Moreover, the number of granular convoluted tubules and acinar cells decreased[27, 28]. In addition, Birer et al. [29] found that radiation treatment (9 Gy) resulted in a significant increase in fibrosis in both salivary glands and tongue. In the same context, irradiated submandibular glands (15 Gy) showed marked acinar cell vacuolization, condensed nuclei, mononuclear infiltration and focal fibrosis 4 weeks post irradiation, in addition to increased collagen deposition[30]. Irradiated human submandibular glands represented similar changes as shown by Nam et al.[31], who reported disorganized acinar and ductal structures, lymphocytic infiltration and high collagen deposition after irradiation.
Morphometric analysis of TGF-β expression revealed a significant increase in irradiated parotid glands in comparison to control and treatment groups. Similarly, Kim et al.[28] found a marked elevation in TGF-β1 protein in submandibular glands, with peak expression on day 28 after 18 Gy irradiation. Additionally, Xu et al.[9] found a strong TGF-β1 expression in the cytoplasm of ductal and some acinar cells of submandibular glands 8 hours and 30 days after 15 Gy irradiation with extracellular TGF-β1 expression on the 30th day post irradiation.
Oral administration of NSO prior to gamma irradiation provided a high degree of protection for parotid glands, as evidenced by the absence of acinar vacuolization and duct degeneration in addition to decreased fibrosis. Moreover, it significantly decreased TGF-β expression compared to irradiated glands. These results were parallel to those obtained by Nor-Eldin and Elsayed[32], where administration of NSO improved the cerebral and cerebellar changes induced by exposure to X-ray (8 Gy) with a significant decrease in the number of GFAP-positive astrocytes. Likewise, administration of NSO provided partial protection and improvement of histopathological alterations of parotid gland subjected to fenitrothion intoxication[33]. Mouket et al.[34] demonstrated the radio-protective effect of NSO on salivary glands exposed to total cranial irradiation in terms of a significant reduction in oxidative stress markers and strong antioxidant effects. Additionally, the radio-protective effect of NSO on the irradiated heart was confirmed by Kaplan et al.[35], where NSO significantly decreased TOS and OSI levels in the heart tissue, while TAS levels significantly increased. Moreover, Shahzad et al.[36] found that NSO administration reduced inflammation and collagen deposition in the lungs of ovalbumin exposed rats with a marked reduction in transforming growth factor beta mRNA expression. Furthermore, Majdalawieh et al.[37] found that aqueous extract of NS significantly decreased the levels of the proinflammatory mediators IL-6, TNF-α, and NO by primary macrophages during splenocyte proliferation. Similarly, oral administration of thymoquinone, and NSO component, significantly suppressed the levels of IL-1β, IL-6, TNF-α, IFN-γ and PGE2 in an arthritis model in wistar rats[38]. The radio-protective effect of orally administered NSO against whole body gamma irradiation was reported by Assayed[16] on the spleen and thymus and Amin et al.[39] on liver and blood components and they found noticeable regeneration in spleen and thymus lymphoid follicles. NSO administration reduced the hepatic histopathological changes induced by irradiation, where moderate vacuolar degeneration and a clear and non-congested central vein were found. Additionally, it significantly decreased liver MDA, ALT and AST levels in response to irradiation and inhibited the reduction in RBC count and hemoglobin content induced by irradiation[16, 39].