2026 ICD-10-CM Diagnosis Code T66.XXXSRadiation sickness, unspecified, sequela
T66.XXXS is a billable ICD-10-CM diagnosis code for radiation sickness, unspecified, sequela. The 7th character S marks it as a sequela code, which reports a problem that remains after the original condition has resolved. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 922 through 923. The code is exempt from POA reporting. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Effect of other external causes, sequela.
Code Identity
Code Classification
Present on Admission (POA)Billing
T66.XXXS is exempt from POA reporting on inpatient claims to general acute care hospitals. Review other POA exempt codes.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Acquired pancytopenia
- Acute radiation disease
- Acute radiation nephritis
- Acute radiation otitis externa
- Acute radiation syndrome
- Acute vomiting
- Anemia caused by physical agent
- Anemia due to radiation
- Aplastic anemia due to radiation
- Cancer dataset administrative items
- Chronic radiation nephritis
- Chronic radiation sickness
- Coronary arteriosclerosis caused by radiation
- Disorder caused by radiation
- Disorder due to radiotherapy
- Disorder of artery caused by ionizing radiation
- Disorder of bone marrow caused by ionizing radiation
- Disorder of conjunctiva caused by radiation
- Disorder of eyelid caused by radiation
- Disorder of macula of retina caused by radiation
- Disorder of optic nerve caused by radiation
- Disorder of retina caused by radiation
- Disorder of sclera caused by radiation
- Effect of ionizing radiation
- Effect of radiation
- Effects of occupational exposure to radiation
- Effects of overexposure to electromagnetic radiation
- Heart disease caused by ionizing radiation
- Hematopoietic subsyndrome of acute radiation syndrome
- Infertility caused by radiation
- Late effect of radiation
- Late effect of radiation therapy
- Nausea and vomiting
- Nausea and vomiting caused by radiation
- Occupational injury
- Radiation agranulocytosis
- Radiation nephritis
- Radiation sickness
- Radiation sickness following radiation therapy
- Radiation-induced male infertility
- Radiotherapy course change due to acute radiotherapy toxicity
- Reason for change in radiotherapy course
- Subacute radiation sickness
- Toxicity due to radiotherapy
Coding GuidelinesGuidance
The appropriate 7th character is to be added to each code from block Radiation sickness, unspecified (T66). Use the following options for the applicable episode of care:
- A - initial encounter
- D - subsequent encounter
- S - sequela
Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026, published by CMS and the National Center for Health Statistics.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Acute Radiation Syndrome
a condition caused by a brief whole body exposure to more than one sievert dose equivalent of radiation. acute radiation syndrome is initially characterized by anorexia; nausea; vomiting; but can progress to hematological, gastrointestinal, neurological, pulmonary, and other major organ dysfunction.Aplastic Anemia due to Radiation
aplastic anemia caused by excessive absorption of radiation by the bone marrow.
Patient EducationClinical
Radiation Exposure
Radiation is energy. It travels in the form of energy waves or high-speed particles. Radiation can occur naturally or be man-made. There are two types:
The full article covers:
- What is radiation?
- What are the sources of radiation exposure?
- What are the health effects of radiation exposure?
- What are the treatments for acute radiation sickness?
- How can radiation exposure be prevented?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T66.XXXS to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T66.XXXSOverview
Is T66.XXXS (Radiation sickness, unspecified) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report radiation sickness, unspecified, sequela on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character S in T66.XXXS mean?
The final character S makes this a sequela code: it reports a lingering problem that remains after the radiation sickness, unspecified itself has resolved, not the original event.
What MS-DRG does T66.XXXS group to?
When radiation sickness, unspecified, sequela is the principal diagnosis on an inpatient stay, it groups to MS-DRG 922, 923, with relative weights from 1.0177 to 1.7494 depending on complications. Higher weights mean higher Medicare reimbursement.
Is T66.XXXS exempt from POA reporting?
Yes. CMS lists this code among those exempt from present on admission reporting, so hospitals do not assign a POA indicator for radiation sickness, unspecified, sequela on inpatient claims.
What is the ICD-9 equivalent of T66.XXXS?
Under the General Equivalence Mappings, radiation sickness, unspecified, sequela converts to ICD-9-CM 909.4 (late eff cert ext cause). The mapping is approximate, so confirm the match fits the documentation.
Footnotes
[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.
