The sense of smell (olfaction) is a complex chemical process that significantly influences appetite, safety, and overall quality of life. Research confirms that both cancer and its treatments can profoundly alter a person’s ability to smell. These sensory changes are a frequent side effect, yet they remain under-recognized compared to other treatment toxicities. Understanding these alterations is important for managing the nutritional and emotional impacts experienced by patients.
Direct Impact of Cancer Location on Olfaction
A tumor can directly compromise the sense of smell through physical obstruction or by invading neurological pathways. Cancers originating in the nasal cavity, paranasal sinuses, or nasopharynx commonly cause a physical blockage of airflow. This prevents odor molecules from reaching the olfactory epithelium, the specialized tissue high in the nasal passages responsible for detecting scents.
Tumors situated near the brain’s base, or those that spread to the brain, can cause sensory loss by compressing or destroying the olfactory nerves and related structures. For instance, a tumor located at the front of the skull base, such as an olfactory groove meningioma, may gradually press on the olfactory bulb or tracts, leading to a diminished or lost sense of smell.
Less commonly, systemic effects of cancer can indirectly affect olfaction through paraneoplastic syndromes. These syndromes occur when cancer cells release substances that trigger an immune response, mistakenly attacking healthy tissues, including components of the nervous system. Although rare, they represent a mechanism where a tumor distant from the head, such as in the lung, can still interfere with sensory perception.
Olfactory Changes Caused by Cancer Treatments
Therapies designed to eliminate cancer cells often affect rapidly dividing healthy cells throughout the body, including the delicate sensory cells responsible for smell. Chemotherapy drugs are systemic treatments that can damage the olfactory receptor neurons, which have a high turnover rate. This damage disrupts the chemical signals sent to the brain, leading to an altered or reduced sense of smell for up to 60% of patients receiving certain regimens.
For many patients, these chemotherapy-induced changes are temporary, with olfaction gradually returning to normal within a few weeks or months after treatment completion. However, the effect of radiation therapy, particularly when aimed at the head and neck region, can be more permanent. Radiation can directly destroy the olfactory epithelium and the surrounding supportive cells, and the severity of the loss is often dose-related.
Radiation also dries out the nasal passages by damaging the mucus-producing glands, which are necessary for dissolving odor molecules to be detected by the receptors. Surgical procedures to remove tumors in the head and neck may also involve cutting or damaging the olfactory nerve bundles, leading to immediate and sometimes permanent sensory loss. The combination of multiple treatments, such as surgery followed by radiation and chemotherapy, increases the likelihood and severity of olfactory dysfunction.
Specific Types of Smell Alterations
Olfactory dysfunction manifests in distinct ways, ranging from a complete lack of scent perception to distorted or phantom smells. Anosmia refers to the total inability to detect odors. A less severe form is hyposmia, characterized by a reduced ability to smell, where odors are detected but seem much fainter than usual.
A more challenging experience is parosmia, where familiar odors become distorted, often smelling unpleasant, rancid, or chemical instead of their true scent. For instance, the pleasant smell of coffee might be perceived as a foul, burning odor. Patients may also experience phantosmia, which involves perceiving odors that are not actually present, commonly described as phantom smells of smoke or burning rubber.
These qualitative changes, like parosmia and phantosmia, can be highly distressing because they interfere with the brain’s interpretation of the world. While anosmia is a quantitative loss of function, parosmia involves a misfiring of the olfactory signal, causing everyday smells to trigger a negative physical or emotional reaction. Patients may experience one or a combination of these alterations, greatly affecting their relationship with food and their environment.
Managing Smell Changes and Nutritional Support
The impact of altered olfaction extends beyond inconvenience, directly influencing appetite and leading to weight loss and malnutrition. Since the majority of flavor perception comes from smell, food often becomes unappealing or tastes bland, reducing the motivation to eat. This sensory loss also creates safety hazards, as the ability to detect warning signs like smoke, gas leaks, or spoiled food is diminished.
To manage these issues, patients can focus on modifying the sensory experience of food to compensate for the lack of aroma. Flavor enhancement strategies involve using strong, non-olfactory sensations, such as tart, sour, spicy, or cooling flavors. Adding lemon juice, vinegar, horseradish, or strong herbs and spices can stimulate the trigeminal nerve, providing a sensation that bypasses the olfactory system.
Eating cold or room-temperature foods can also help, as they typically release fewer volatile odor compounds than hot foods, which may minimize unpleasant smells. Another proactive strategy is olfactory training, which involves regularly and intentionally smelling a set of strong odors, such as essential oils, to stimulate the regeneration of olfactory neurons and potentially restore function. Consulting with an oncology-focused dietitian can provide individualized nutritional support to ensure adequate calorie and protein intake despite the sensory changes.