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The top radiation oncology claim denials and how to fix each one

Medical Billing Blog·

Radiation oncology claim denials cluster in about a dozen reason codes, and each one has a right response: correct the claim and resubmit, appeal with the record, or accept the adjustment. The wrong choice delays payment either way: a corrected claim sent as an appeal waits up to 60 days for a decision it did not need, and an appeal sent as a new claim comes back as a duplicate. This is a reference list for a radiation oncology billing office in 2026: the Claim Adjustment Reason Code (CARC) and typical Remittance Advice Remark Code (RARC) on the 835, why each denial happens in this specialty, and the fix.

Most of these are preventable at charge entry, and none of them means the physician did anything wrong.

How to read the denial first

The CARC says what happened; the group code in front of it (CO, PR, OA) says who owes the balance; the RARC says why in more detail. CARC 16 and CARC 96 must be accompanied by at least one RARC, so on those two the remark code carries the detail. For Medicare, a redetermination request is due 120 days from receipt of the initial determination (receipt is presumed five days after the notice date), there is no minimum dollar amount, and the MAC generally decides within 60 days. Two kinds of denial have no appeal rights at all: exact duplicates (chapter 1 of the Claims Processing Manual) and claims returned with RARC MA130 as unprocessable, which are corrected and resubmitted instead. See group and reason codes and remark codes.

The ten denials

Denial CARC Typical RARC Why it happens in radiation oncology Fix
Exact duplicate 18 (group OA) M86, M80 Twenty identical 77407 or 77387-26 lines in a course; a BID day billed as two matching lines; a whole claim resubmitted to fix one line Do not rebill. Check claim status. A real second session on one date goes on a separate line with the modifier that says so; chapter 13 says two sessions pay only “as long as there has been a distinct break.” No appeal rights on exact duplicates.
Bundled 97, 234 M15, N19, M144 77387 technical, 77417 port films, simulation during IMRT, E/M visits during the course (chapter 13 section 70.2 bundles them), 77336 on the same date as planning codes Usually accept. Appeal only where NCCI allows a modifier, for example 77336 with 59 or XE when planning was repeated mid-course.
Missing or invalid information 16 MA130, M51, M20, M53, N29, N382 Missing 26 on 77387, a 2025 code, a bad MBI, no units, a missing report Correct and resubmit. MA130 means no appeal rights.
Timely filing 29 often none Twelve months from the date of service, and the limit counts from the first fraction’s date while the course runs or while you wait for a 77427 count Appeal only with proof of timely acceptance. Otherwise write off. Prevent it by billing as units accrue. See timely filing.
Medical necessity 50 N115, N29 Diagnosis does not support the service under an LCD or payer policy; fraction count above policy; IMRT at a site the policy excludes Appeal with the record: the note, the plan, the policy criterion met. This is what redetermination is for.
No prior authorization 197 N54, N351, N640 Course ran past the approved dates; 77412 billed where 77407 was approved; a 2025 approval never remapped to the 2026 codes Confirm what the payer has on file. Correct the line if the approval was right and the claim wrong; appeal with the approval letter if the reverse. See radiation therapy prior authorization in 2026.
Units above the MUE 151 N362 More than 10 units of 77300 on a date, more than 2 of 77387 or a delivery code, more than 1 of 77427, 77301 or 77338 Depends on the MUE Adjudication Indicator. MAI 3 codes (77300, 77301, 77338, 77387, 77402 to 77412) may be appealed with records showing the units were furnished. MAI 2 codes (77427, 77336, 77417, 77470) cannot be overridden; correct the units. See MUEs.
Modifier or place of service inconsistent 4, 5 N519, N822 26 on 77427 or 77263; TC from a hospital-based physician; 77387 without 26; POS 11 on a hospital department’s claim Correct and resubmit. Fix the site-of-service rule that produced it.
Non-covered 96 (group CO or PR) N130, N30 77417, 77387 technical, nutrition or pain management visits during treatment, statutory exclusions Usually accept. Watch the group code: PR shifts the balance to the patient, which is only right where an ABN or policy supports it.
Eligibility 27, 31, 109 N382, MA61 Coverage changed on January 1 mid-course; a Medicare Advantage enrollment the front desk did not catch; an MBI typed wrong Verify, then bill the correct payer. CARC 109 states it directly: “send the claim/service to the correct payer.”

The 2026 transition denials

The code change on January 1, 2026 added three denial patterns that did not exist before. A fourth looked like a new pattern but was a contractor error.

Pattern What the remit says Fix
A 2025 code on a 2026 date: 77385, 77386, 77014, 77401, G6001 to G6017 CARC 181, “Procedure code was invalid on the date of service,” with M51 or N56 Correct the code and resubmit. There is nothing to appeal. The charge usually came from the treatment system; fix it there (see Record and Verify charge capture after the 2026 code change).
77387 without modifier 26 CARC 4 or 16 with N822, “Missing procedure modifier(s),” or a bundling denial, because the technical component is status B Add 26 and resubmit. Every setting, including freestanding centers.
Level downgrade: 77412 billed, 77407 paid CARC 150, “Payer deems the information submitted does not support this level of service,” or a payer-specific code Appeal with documentation that names the Level 3 criterion: multiple isocenters, active motion management, total skin electrons, or mixed electron and photon fields.
77387-26 denied by a MAC in January and February 2026 A loading error, not a rule No action; the MAC mass adjusted. Confirm the adjustments posted. The companion post on the 2026 codes has the details.

The short version: 4, 5, 16, 181 and a missing modifier are corrected and resubmitted; 50, 150, 197 and 151 on an MAI 3 code are appealed with the record within 120 days; 18, 97, 96 and 234 are usually accepted; 27, 31 and 109 are rebilled to the right payer.

What to do now

  • Post every denial with its CARC and RARC and sort by them monthly. The top three show which process needs fixing.
  • Separate correct-and-resubmit denials from appeals on the day the remit posts, using the table above, so a claim that could be fixed in a day does not wait 60 days.
  • Verify eligibility at the start of every course and again in January. Mid-course plan changes cause most of the 27 and 31 denials.
  • Bill as units accrue, not at the end of the course. It keeps the first fraction inside the 12-month limit and shows code problems in the first week.
  • Write a Level 3 documentation line into the planning template, naming the criterion, so the appeal for a downgrade is already in the chart.
  • Scrub at charge entry, not at claim submission. Iridium Suite’s claim scrubber checks every service line as it is entered against CCI edits, payer rules and radiation oncology frequency rules, warns when a deleted delivery, IMRT, guidance or port film code is billed, and requires the 26 modifier on 77387; see the claim scrubber.
  • Never issue an ABN for an MUE denial. NCCI chapter 1 says MUE denials are coding denials, not medical necessity denials, so liability does not shift to the patient.

Further reading

radiation oncologydenialsCARCRARC2026