An allergic reaction to chemotherapy, more accurately termed a hypersensitivity reaction (HSR), occurs when the immune system mistakenly views the drug as a threat, triggering a defensive response. This reaction can range from mild to life-threatening. While most chemotherapy drugs carry a small risk, HSRs are more common with certain classes, such as platinum compounds like carboplatin and taxanes like paclitaxel. The medical team is prepared for the possibility of a reaction, and standardized protocols ensure these events are managed immediately and effectively.
Recognizing the Signs of a Reaction
Hypersensitivity reactions often occur during the drug infusion itself or within a few minutes to hours after the infusion is complete. Symptoms vary widely in severity, making patient self-reporting and close monitoring by the clinical staff important.
Mild reactions frequently involve only the skin, presenting as flushing, itching (pruritus), or a rash such as hives (urticaria). These symptoms require immediate attention to prevent progression to a more serious event.
Moderate reactions may involve symptoms across multiple body systems, including shortness of breath, chest tightness, or a sudden onset of nausea and vomiting. Some patients also report back or abdominal pain, which can be an early indicator of a systemic reaction.
Severe reactions, known as anaphylaxis, are rapid, systemic, and potentially life-threatening events that affect breathing and circulation. Signs include severe wheezing, difficulty breathing due to airway swelling, a sudden drop in blood pressure (hypotension), and collapse. Patients may also report a feeling of impending doom.
Immediate Medical Management
The first step when a patient shows signs of a hypersensitivity reaction is the immediate cessation of the chemotherapy infusion. The intravenous access line is kept in place and flushed with a non-reactive solution like normal saline, which provides a route for administering emergency medications.
Next, the clinical team performs a rapid assessment of the patient’s airway, breathing, and circulation, checking vital signs such as heart rate, blood pressure, and oxygen saturation. This initial assessment grades the severity of the reaction and determines the appropriate intervention.
Rescue medications are administered based on symptom severity. For milder symptoms, intravenous antihistamines (H1 and H2 blockers) and corticosteroids are typically given to block the effects of released histamine and modulate the immune response.
For severe reactions or full anaphylaxis, epinephrine is the standard emergency medication. Epinephrine works quickly to increase blood pressure, relax the smooth muscles in the lungs to ease breathing, and reduce swelling. Once the patient is stabilized, they are monitored closely to ensure symptoms do not recur in a biphasic reaction.
Strategies for Continuing Treatment
After an acute reaction is managed, the focus shifts to safely continuing the necessary cancer treatment, which often involves the drug that caused the reaction. The treatment plan is adjusted based on the initial reaction’s severity and the availability of equally effective alternative drugs.
Following a mild reaction, the treatment team may opt for enhanced premedication before the next cycle. This involves increasing the doses of corticosteroids and antihistamines given prior to the infusion, which can successfully prevent a reaction upon re-exposure, particularly with taxane-based drugs.
If an alternative chemotherapy drug exists that is equally effective, substitution may be the simplest solution. However, for many cancers, the drug that caused the reaction, such as a platinum agent, is considered the most effective or standard-of-care option, making substitution undesirable.
When the culprit drug is the best available option, drug desensitization is often used to safely reintroduce the medication. Desensitization involves administering the chemotherapy drug in highly diluted, gradually increasing concentrations over several hours. This slow, controlled exposure temporarily saturates the immune cells without triggering a full hypersensitivity response.
The desensitization process is performed in a closely monitored setting with emergency staff and equipment available. A common approach uses multiple bags of the drug, each with an exponentially increasing concentration, which are infused sequentially. This procedure must be repeated before every subsequent cycle, but it allows patients to continue receiving the most effective therapy, with reported success rates for completing the full dose near 99%.