Gastric cancer is one of the most common malignancies worldwide and poses a serious threat to human health. As human societies develop and people's lifestyles change, the prevalence of gastric cancer increases in young adults. Gastric cancer is the second leading cause of death in cancer patients worldwide 1. In Iran, gastric cancer has the highest incidence and mortality rates among all malignancies in men 2. Moreover, this type of cancer has the third incidence and the first mortality rate rank among Iranian women 3. Currently, clinical treatment for gastric cancer mainly includes surgery, chemotherapy, and radiotherapy 4. A course of chemotherapy can be associated with a variety of side effects, including hair loss, muscle aches, mood disorders, weight changes, and gastrointestinal complications 5.

Chemotherapy-Induced Nausea, Vomiting, and Retching (CINVR) are one of the common side effects and fears encountered by patients undergoing chemotherapy 6,7. Generally, CINVR persists for approximately 5 days after receiving chemotherapy. Nausea and vomiting that occur in the first 24 hours after chemotherapy administration are defined as acute CINVR and those occurring between 1 to 5 days after chemotherapy administration are called delayed CINVR 8. Despite the use of antiemetics, the incidence rate of nausea and vomiting is still high (about 20-70%) 9. Moreover, despite the widespread use of antiemetics such as serotonin-receptor antagonists and neurokinin receptor antagonists, about 48-67% of patients undergoing chemotherapy suffer from acute nausea 10,11.

Hypoxia is one of the leading causes of nausea and vomiting that has recently been considered important. Hypoxia considerably affects the chemical release of serotonin from the intestines. The exact mechanism for the effect of oxygen has not been discovered, although intestinal ischemia and the prevention of the release of mediators such as serotonin from the intestine may lead to nausea and vomiting. Hypotension causes brainstem ischemia and ultimately stimulates the vomiting center in the brainstem. Hypotension also leads to intestinal ischemia and the release of nausea-inducing agents such as serotonin 12,13. The use of high concentrations of oxygen can reduce nausea and vomiting through the aforementioned mechanism 14.

In a study by Heidari et al. (2018) about the effect of high concentrations of oxygen on postoperative nausea and vomiting, it was found that high oxygen concentrations had reduced the level of postoperative nausea and vomiting, but patients still needed metoclopramide administration. They suggested that a combination of five Complementary and Alternative Medicine (CAM) therapies should be used to reach better outcomes 15. Meditation methods, especially the Benson relaxation technique, are another group of CAM therapy for preventing CINVR. Benson's relaxation technique, as one of the methods of concentration on the senses, was first developed by Herbert Benson (1970) and is so popular because of its ease of learning and teaching 16.

Benson believed that stress relief was a key part of meditation. In a study on various methods of de-stressing, Benson concluded that four basic elements improve de-stressing effects, including a calm environment, a comfortable state, mental devices such as a term used for it, and a passive attitude 6. Benson's relaxation method includes the following steps: (a) being in a comfortable position, (b) closing the eyes slowly, (c) relaxing all the muscles of the body from the soles towards the face slowly and gradually, and keeping calm, (d) breathing comfortably and normally through the nose and being aware of how to do it (breathing through the mouth slowly and repeating a number or a word that evokes relaxation for the patient when letting it out), (e) performing the above for 15 minutes and trying to relax the muscles in the meantime and then opening your eyes and standing up for a few minutes, (f) not being worried about achieving a deep level of relaxation and allowing relaxation to happen with its own rhythm, and (g) ignoring disturbing thoughts when they occur and being indifferent to them 6,17.

The effect of the Benson relaxation technique on nausea and vomiting has been investigated in numerous studies, most of which have shown the positive effect of this technique only on CIN 18,19. Mustian et al. (2011) also emphasized the combined use of the Benson relaxation technique and other cognitive-therapeutic methods to achieve better results 20. Therefore, considering that few studies have shown the antiemetic effects of oxygen therapy and the Benson relaxation technique separately, it is necessary to conduct a comparative study on the effect of single and combined use of these methods on CINVR in patients with gastric cancer. Accordingly, we would be able to examine the effectiveness of these methods comprehensively. The present study aimed to investigate the effect of single and combined use of the Benson relaxation technique and oxygen therapy on CINVR in patients with gastric cancer.