1.12 Pharmacology & Contrast Media in Radiation Therapy

Why Contrast Media Are Used in CT Simulation

Iodinated contrast media are administered during CT simulation to opacify blood vessels, highly vascular tumors, and certain organs (kidneys, bladder, GI tract), improving the physician's ability to distinguish tumor from adjacent normal structures and vasculature (e.g., separating a pancreatic head mass from the SMA/SMV, or a nodal mass from a great vessel) when contouring target volumes and organs at risk.

Ionic vs. Non-Ionic Contrast

Non-ionic, low-osmolar or iso-osmolar contrast is now the standard for intravenous use because it produces substantially fewer adverse reactions than older ionic, high-osmolar agents, even though it costs more per dose.

Renal Function Screening Before Contrast

Because iodinated contrast is cleared by the kidneys and can precipitate contrast-induced nephropathy (CIN) in patients with impaired renal function, departments screen renal function before administering IV contrast:

• BUN (blood urea nitrogen) — reflects nitrogenous waste clearance; elevated in renal impairment but also affected by dehydration, diet, and other non-renal factors, so it is a less specific marker than creatinine.
• Creatinine — a muscle-derived waste product cleared almost entirely by the kidneys; a more specific marker of renal function than BUN, though it is affected by muscle mass.
• eGFR (estimated glomerular filtration rate) — calculated from creatinine, age, sex, and sometimes race; the preferred single value for screening because it estimates actual filtering capacity rather than relying on a raw lab value alone. Many departments set an eGFR threshold (commonly around 30 mL/min/1.73m², though institutional policy varies) below which contrast is avoided or requires nephrology consultation/hydration protocols.
• Metformin — patients taking this diabetes medication are typically instructed to hold it around the time of contrast administration per institutional protocol, because impaired renal clearance of metformin in the rare setting of contrast-induced renal injury can precipitate lactic acidosis.

Extravasation

Extravasation occurs when contrast (or any IV medication) leaks from the vein into surrounding soft tissue during injection, most often at a power-injector site used for CT contrast boluses. Signs include localized swelling, pain, burning, and skin tightness/discoloration at the injection site during or immediately after injection. If the patient reports these symptoms during injection, the injection must be stopped immediately; the therapist should notify the radiologist/physician, elevate the affected limb, apply cold or warm compresses per protocol, and document the incident, volume, and contrast type per department policy. Most extravasations are managed conservatively, but large-volume extravasations can require surgical consultation, particularly with high-osmolar agents that are more tissue-damaging.

Grading and Managing Contrast Reactions

Contrast reactions are classified by severity, and the therapist's response escalates accordingly:

Regardless of severity, the therapist's core responsibilities are the same: recognize the reaction promptly, stop the contrast administration, notify the ordering/supervising physician without delay, and stay with the patient to monitor airway, breathing, and circulation until qualified help arrives. Contrast reaction risk is higher in patients with prior contrast reactions, asthma, and multiple drug allergies, so a pre-procedure allergy history is part of routine screening, and pre-medication protocols (e.g., corticosteroids and antihistamines given in advance) may be used for at-risk patients who still require contrast.

Other Medications Encountered in Radiation Therapy Practice

Radiation therapists frequently encounter medications outside of contrast media in day-to-day patient care:

• Antiemetics (e.g., ondansetron) — manage nausea from radiation to the abdomen/pelvis or brain, or from concurrent chemotherapy.
• Analgesics — manage pain from tumor burden or treatment-related mucositis/skin reactions; the therapist should be alert to signs of inadequate pain control and report them.
• Topical skin agents — used for acute radiation dermatitis; the therapist should know department policy on what may be applied before daily treatment (some topical agents increase surface dose via a bolus effect).
• Sedatives — occasionally used for anxious or pediatric patients requiring immobility during simulation/treatment; requires monitoring per department sedation protocol.

Hazardous Materials and Safety Data Sheets (SDS)

Radiation oncology departments handle hazardous materials beyond contrast media, including chemotherapy agents (for concurrent chemoradiation patients) and cleaning/disinfection chemicals. Every hazardous substance in the workplace has an associated Safety Data Sheet (SDS), which documents composition, physical/health hazards, safe handling, storage, and emergency spill/exposure procedures — the therapist should know where SDS binders/database access is located and how to respond to an exposure or spill. Hazardous drug handling (including certain chemotherapy agents) follows USP <800>-aligned practices: designated personal protective equipment, closed-system transfer devices where applicable, and specific disposal pathways (separate hazardous waste containers) rather than routine trash or sharps disposal. Even therapists who do not compound or administer chemotherapy directly must know how to safely handle a patient's bodily fluids or linens shortly after chemotherapy administration and how to respond to a spill, since improper handling of hazardous drug residue poses a real occupational exposure risk.