His Immunotherapy Runs About $12,000 a Round. Original Medicare’s Uncapped 20% Rides Along Every Time.

A 66-year-old newly diagnosed with non-small cell lung cancer starts pembrolizumab during his third month on Medicare. His oncologist schedules an infusion every three weeks. The manufacturer lists the standard three-week dose at $12,272, although Medicare’s approved amount can differ. He has Original Medicare, no Medigap policy, and no idea that the 20% coinsurance attached to a Part B drug has no annual ceiling. The first bill runs into four figures. So can the next one, three weeks later.

Chemotherapy infusions, immunotherapies such as pembrolizumab and nivolumab, injections for macular degeneration, and certain rheumatology drugs can all fall under Medicare Part B. For someone without supplemental coverage, the same percentage follows every covered treatment.

Part B’s 20% Has No Off Switch

After the $283 annual Part B deductible in 2026, Original Medicare generally pays 80% of the Medicare-approved amount for covered outpatient services. The patient owes the remaining 20%. Unlike Medicare Advantage, Original Medicare has no annual out-of-pocket maximum. A $50 service might leave a $10 bill. An infusion with a Medicare-approved amount of $11,000 can leave approximately $2,200. If treatment continues every three weeks, that percentage returns with every dose.

Pembrolizumab given on that schedule can mean approximately 17 treatments over a full year. At an assumed approved amount of $11,000 per treatment, the patient’s share approaches $37,400 before counting oncology visits, scans, administration charges, or other care. The exact bill depends on Medicare’s approved amount, not the manufacturer’s list price. The dangerous part is the percentage, because it has no finish line.

Medigap Changes the Calculation

Medigap Plan G covers Part B coinsurance after the beneficiary pays the annual Part B deductible. Premiums vary considerably by age, location, insurer, and pricing method, but replacing repeated four-figure treatment bills with a predictable monthly premium can change the financial shape of a cancer year. The catch is timing. The federal Medigap Open Enrollment Period lasts six months beginning the first month someone is 65 or older and enrolled in Part B. During that period, insurers cannot deny coverage or charge more because of medical history.

Afterward, insurers in most states can use medical underwriting unless a guaranteed-issue right applies, and active lung cancer can make approval difficult or impossible. Connecticut and New York guarantee year-round access, Massachusetts offers broad year-round enrollment in practice, and Maine guarantees at least Plan A during an annual window. Other states provide narrower exceptions. The coverage chosen while someone is healthy can determine the bills that arrive after a diagnosis.

Medicare Advantage Caps the Exposure, With Trade-Offs

Medicare Advantage plans place an annual limit on what members pay for covered Part A and Part B services. In 2026, the average in-network limit is approximately $5,421, although individual plans can set higher or lower amounts within federal limits. Cost-sharing for an in-network Part B drug counts toward that ceiling.

That protection comes with plan rules. The oncologist, infusion center, and cancer hospital must fit the network, particularly in a health maintenance organization (HMO). A preferred provider organization (PPO) may cover out-of-network treatment at a higher cost. Some cancer drugs and related services may also require prior authorization.

Neither system is automatically better. Original Medicare with Medigap trades a higher monthly premium for wide provider access and predictable cost-sharing. Medicare Advantage trades lower upfront premiums for networks, plan-specific charges, and an annual spending cap.

What to Check Before Treatment Starts

A cancer diagnosis leaves little patience for insurance paperwork, but three calls can expose the real cost before the first infusion:

1. If the six-month Medigap window is still open, compare Plan G and high-deductible Plan G now. Ask what each premium buys against the proposed treatment schedule.
2. If the window has closed, check for state protections or a federal guaranteed-issue event before assuming Medigap is unavailable. Carriers also apply different underwriting standards.
3. If treatment has already begun without supplemental coverage, ask the cancer center for a financial navigator. Check eligibility for the Qualified Medicare Beneficiary (QMB) Program, hospital financial assistance, and independent charitable foundations. The manufacturer’s access program may help identify options, but its commercial copay program is generally limited to privately insured patients.

The cancer drug is pricey enough to command attention. The silent risk is a percentage that never knows when the patient has paid enough. Original Medicare covers most of every infusion. Without supplemental protection, another four-figure bill might still arrive every three weeks.

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