FY 2027 MEDESUN®
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Section I.C.2 · C00-D49 · FY 2027

Chapter 2 — Neoplasms

A sequencing chapter. The same patient produces a different principal diagnosis depending on whether the encounter treated the primary, the metastasis, the therapy or a complication.

Dr. Santosh Kumar Guptha, CCS, CPC, COC, CIC, CRC, CPMA, CDIP, CPCO — AHIMA-Approved ICD-10-CM/PCS Trainer, MEDESUN® Medical Coding Academy Guidelines effective October 1, 2026 – September 30, 2027

Chapter 2 is a sequencing chapter. The codes themselves come from the Neoplasm Table and the Tabular List; what the guidelines decide is which code goes first — and that depends entirely on what the encounter was for. Treatment of the primary site, treatment of the metastasis only, chemotherapy administration, a complication, and pain management each produce a different principal diagnosis for the same patient.

What is chiefly responsible for this admission or encounter?Treatment of theprimary siteprimary malignancy isprincipal / first-listedTreatment of thesecondary site onlysecondary neoplasm isprincipal; primary is addedChemo, immunotherapyor external beam RTZ51.0 / Z51.11 / Z51.12 first;the malignancy is secondaryComplication of surgeryfor the neoplasmthe complication isprincipal / first-listedAnemia in neoplastic diseasemalignancy first, then D63.0 — the anemia exceptionAnemia from chemo / immunotherapy / radiotherapyanemia first, then the neoplasm, then T45.1X5- or Y84.2Brachytherapy (insertion of radioactive elements): code the malignancy first — do NOT assign Z51.0
Section I.C.2 — what the encounter was for decides the sequence

How do you find a neoplasm code?#

StepAction
1Determine from the record whether the neoplasm is benign, in situ, malignant or of uncertain histologic behavior. If malignant, determine any secondary (metastatic) sites.
2If a histological term is documented, look that term up in the Alphabetic Index first — not the Neoplasm Table. The Index entry tells you which column of the table applies. For “adenoma”, the Index instructs “see also neoplasm, by site, benign”.
3Use the Neoplasm Table, selecting the column that corresponds to the type of neoplasm.
4Verify in the Tabular List — confirming the code and checking whether a more specific site code exists.
Special situationRule
Primary malignancy overlapping site boundariesClassify to the .8 subcategory or code — overlapping lesion — unless the combination is specifically indexed elsewhere
Multiple neoplasms of the same site that are not contiguous, such as tumours in different quadrants of the same breastAssign codes for each site
Malignant neoplasm of ectopic tissueCode to the site of origin mentioned — an ectopic pancreatic malignant neoplasm involving the stomach is coded to malignant neoplasm of pancreas, C25.9
Benign neoplasms in other chaptersCertain benign neoplasms such as prostatic adenomas are found in the specific body system chapters, not Chapter 2

Which code is the principal diagnosis?#

Reason for the admission or encounterSequence
Treatment directed at the primary site, and the malignancy is chiefly responsible for the encounterThe primary malignancy is the principal / first-listed diagnosis
EXCEPTION Administration of chemotherapy, immunotherapy or external beam radiation therapy is chiefly responsible1Z51.0, Z51.11 or Z51.122the underlying diagnosis or problem for which the service is performed, as a secondary diagnosis
Treatment directed toward the secondary site only, with the primary still present1the secondary neoplasm as principal diagnosis2the primary malignancy as an additional code
Determining the extent of the malignancy, or a procedure such as paracentesis or thoracentesisThe primary malignancy or the appropriate metastatic site is principal — even though chemotherapy or radiotherapy is administered
An episode of care involving surgical removal of a neoplasm followed by adjunct chemotherapy or radiation during the same episodeThe neoplasm code is the principal / first-listed diagnosis
Insertion or implantation of radioactive elements, for example brachytherapyThe malignancy is principal / first-listed. NEVER Z51.0 is not assigned
Admitted for external beam radiotherapy, immunotherapy or chemotherapy and develops complications such as uncontrolled nausea and vomiting or dehydration1Z51.0, Z51.11 or Z51.122codes for the complications
Admitted for brachytherapy and develops such complications1the appropriate code for the malignancy2codes for the complications
Treatment of a complication resulting from a surgical procedure, where treatment is directed at resolving the complicationThe complication is the principal / first-listed diagnosis
Management of a complication associated with a neoplasm, such as dehydration, where only the complication is treated1the complication2the neoplasm code(s)
EXCEPTION Management of an anemia associated with the malignancy, where treatment is only for the anemia1the malignancy2D63.0 Anemia in neoplastic disease
Anemia associated with an adverse effect of chemotherapy or immunotherapy, where only the anemia is treated1the anemia2the neoplasm3T45.1X5- adverse effect of antineoplastic and immunosuppressive drugs
Anemia associated with an adverse effect of radiotherapy1the anemia2the neoplasm3Y84.2
Management of dehydration due to the malignancy, where only the dehydration is treated (intravenous rehydration)1the dehydration2the malignancy code(s)
Pathologic fracture due to a neoplasm, focus of treatment is the fracture1M84.5- Pathological fracture in neoplastic disease2the neoplasm
Pathologic fracture due to a neoplasm, focus of treatment is the neoplasm1the neoplasm2M84.5-
Malignant neoplasm in a pregnant patient1O9A.1-2the appropriate Chapter 2 code for the type of neoplasm
Pain control or pain managementSee Section I.C.6 — G89.3 may be principal when pain management is the stated reason, with the neoplasm reported additionally

Current malignancy or personal history?#

Use the malignancy code until treatment directed at that site is complete; after that, use a Z85 personal history code. The test is not time elapsed — it is whether further treatment is directed to the site and whether any existing primary malignancy remains there.

SituationCode
Primary malignancy excised, but further treatment — additional surgery, radiation therapy or chemotherapy — is directed to that siteThe primary malignancy code, until treatment is completed
Primary malignancy previously excised or eradicated, no further treatment directed to the site, and no evidence of any existing primary malignancy at that siteA code from category Z85 Personal history of malignant neoplasm
Any mention of extension, invasion or metastasis to another siteCoded as a secondary malignant neoplasm to that site. The secondary site may be the principal or first-listed diagnosis, with the Z85 code as a secondary code
Former site of a primary malignancyZ85.0–Z85.85 only
Former site of either a primary or a secondary malignancyZ85.89
Leukemia or category C90 multiple myeloma and malignant plasma cell neoplasms in remissionThe category code with the remission value — not Z85.6 or Z85.79. QUERY If the documentation is unclear whether remission has been achieved, query the provider

The special neoplasm codes#

CodeUseConstraint
C80.0 Disseminated malignant neoplasm, unspecifiedAdvanced metastatic disease where no known primary or secondary sites are specifiedNEVER Not used in place of coding the primary site and all known secondary sites
C80.1 Malignant (primary) neoplasm, unspecifiedEquates to “cancer, unspecified”; used only when no determination can be made as to the primary siteShould rarely be used in the inpatient setting
C80.2 Malignant neoplasm associated with transplanted organ1T86.- Complications of transplanted organs and tissue2C80.23an additional code for the specific malignancyA malignant neoplasm of a transplanted organ is coded as a transplant complication
C84.7A / C84.7BBreast implant associated anaplastic large cell lymphoma (BIA-ALCL); C84.7B for BIA-ALCL in remissionNEVER Do not assign a complication code from Chapter 19
C81–C85 with a final character for extranodal and solid organ sitesWhen a malignant neoplasm of lymphoid tissue metastasises beyond the lymph nodes — for example diffuse large B-cell lymphoma spreading to lung, brain and left adrenal gland is C83.398Assigned instead of a secondary neoplasm code for the affected solid organ

Practice questions#

Q1A patient with carcinoma of the sigmoid colon and known liver metastases is admitted for radiofrequency ablation of the liver lesions only. The colon primary remains in place and is not treated at this admission. What is the principal diagnosis?

  1. AThe malignant neoplasm of the sigmoid colon
  2. BThe secondary malignant neoplasm of the liver
  3. CC80.0
  4. DZ51.0
Show answer & rationale

Correct answer: B. The secondary malignant neoplasm of the liver

Rationale. Guideline I.C.2.b states that when a patient is admitted because of a primary neoplasm with metastasis and treatment is directed toward the secondary site only, the secondary neoplasm is designated as the principal diagnosis even though the primary malignancy is still present. Guideline I.C.2.l.2 repeats the rule and adds that the primary malignancy is coded as an additional code.

Q2A patient is admitted specifically for insertion of radioactive seeds (brachytherapy) for prostate cancer. What is the principal diagnosis?

  1. AZ51.0
  2. BThe malignant neoplasm of the prostate
  3. CZ51.11
  4. DZ51.0 with the prostate malignancy as secondary
Show answer & rationale

Correct answer: B. The malignant neoplasm of the prostate

Rationale. Guideline I.C.2.e.2 is explicit: if the admission or encounter is for the insertion or implantation of radioactive elements, for example brachytherapy, the appropriate code for the malignancy is sequenced as the principal or first-listed diagnosis and Z51.0 should not be assigned. Z51.0 is reserved for external beam radiation therapy.

Q3A patient receiving chemotherapy for lymphoma is admitted for transfusion of anemia documented as an adverse effect of the chemotherapy. Only the anemia is treated. What is the correct sequence?

  1. AThe lymphoma, then D63.0
  2. BThe anemia, then the lymphoma, then T45.1X5-
  3. CZ51.11, then the anemia, then the lymphoma
  4. DThe anemia, then D63.0, then the lymphoma
Show answer & rationale

Correct answer: B. The anemia, then the lymphoma, then T45.1X5-

Rationale. Guideline I.C.2.c.2 states that when the encounter is for management of an anemia associated with an adverse effect of the administration of chemotherapy or immunotherapy and the only treatment is for the anemia, the anemia code is sequenced first, followed by the appropriate codes for the neoplasm and the adverse effect, T45.1X5-. This is the mirror image of guideline I.C.2.c.1, where the anemia is associated with the malignancy itself and the malignancy is sequenced first with D63.0.

Q4Diffuse large B-cell lymphoma has metastasised to the lung, the brain and the left adrenal gland. How is the metastatic disease reported?

  1. ASecondary neoplasm codes for the lung, brain and adrenal gland
  2. BA code from C81-C85 with a final character identifying extranodal and solid organ sites
  3. CC80.0
  4. DThe lymphoma code plus C79.9
Show answer & rationale

Correct answer: B. A code from C81-C85 with a final character identifying extranodal and solid organ sites

Rationale. Guideline I.C.2.t states that when a malignant neoplasm of lymphoid tissue metastasises beyond the lymph nodes, a code from categories C81-C85 with a final character identifying extranodal and solid organ sites is assigned rather than a code for the secondary neoplasm of the affected solid organ. The guideline's own worked example is this exact clinical picture, assigned to C83.398.

Q5A patient had a left mastectomy for breast cancer four years ago. No further treatment has been directed to that site and there is no evidence of existing malignancy there. She is now seen for an unrelated complaint. How is the breast cancer reported?

  1. AThe malignant neoplasm of the left breast
  2. BA code from Z85
  3. CZ85.89
  4. DIt is not reported at all
Show answer & rationale

Correct answer: B. A code from Z85

Rationale. Guideline I.C.2.d and I.C.2.m both provide that when a primary malignancy has been previously excised or eradicated from its site, there is no further treatment directed to that site, and there is no evidence of any existing primary malignancy at that site, a code from category Z85 personal history of malignant neoplasm is used. Z85.3 is the breast subcategory; Z85.89 is reserved for the former site of either a primary or a secondary malignancy where the specific subcategories do not apply.

Scenarios#

Scenario 1Chemotherapy admission that turns into a complication admission

A patient with non-small cell lung cancer is admitted for a scheduled cycle of intravenous chemotherapy. During the stay she develops intractable nausea and vomiting and dehydration requiring intravenous rehydration.

Show coded answer & rationale
SeqCodeDescription
1Z51.11Encounter for antineoplastic chemotherapy — principal diagnosis
2C34.9-Malignant neoplasm of the bronchus or lung, the applicable site code
3R11.2Nausea with vomiting, unspecified
4E86.0Dehydration

Rationale. Guideline I.C.2.e.3 states that when a patient is admitted for the purpose of external beam radiotherapy, immunotherapy or chemotherapy and develops complications such as uncontrolled nausea and vomiting or dehydration, the principal diagnosis is Z51.0, Z51.11 or Z51.12, followed by codes for the complications. Note how different the answer would be for brachytherapy: the same guideline directs that the malignancy is then the principal diagnosis.

Guideline: Section I.C.2.e.2, I.C.2.e.3

Scenario 2Pathological fracture in a patient with metastatic disease

A patient with metastatic prostate cancer to bone presents with a pathological fracture of the right femur. Surgical fixation of the femur is performed. The focus of treatment at this admission is the fracture.

Show coded answer & rationale
SeqCodeDescription
1M84.551-Pathological fracture in neoplastic disease, right femur, with the appropriate 7th character — principal diagnosis
2C79.51Secondary malignant neoplasm of bone
3C61Malignant neoplasm of prostate

Rationale. Guideline I.C.2.l.6 states that when an encounter is for a pathological fracture due to a neoplasm and the focus of treatment is the fracture, a code from subcategory M84.5 is sequenced first, followed by the code for the neoplasm. Had the focus of treatment been the neoplasm with an associated pathological fracture, the same guideline would reverse the order. Guideline I.C.13.c governs the 7th character on the M84.5- code.

Guideline: Section I.C.2.l.6, I.C.13.c

Scenario 3A symptom that cannot become the principal diagnosis

A patient with known, active metastatic pancreatic cancer is admitted for the fourth time this year with intractable ascites requiring therapeutic paracentesis. The ascites is a recognised consequence of the malignancy.

Show coded answer & rationale
SeqCodeDescription
1C25.9Malignant neoplasm of pancreas, unspecified — or the specific site documented; principal diagnosis
2C78.6 / C79.-Secondary malignant neoplasm of the applicable site(s)
3R18.0Malignant ascites — verify in the Tabular List; reported additionally, not as principal

Rationale. Two guidelines close this off. I.C.2.g states that symptoms, signs and ill-defined conditions listed in Chapter 18 that are characteristic of, or associated with, an existing primary or secondary malignancy cannot be used to replace the malignancy as the principal or first-listed diagnosis, regardless of the number of admissions or encounters for treatment and care of the neoplasm. I.C.2.f adds that where the encounter is for a procedure such as paracentesis or thoracentesis, the primary malignancy or appropriate metastatic site is the principal diagnosis. Repetition of admissions is explicitly not a reason to change the sequence.

Guideline: Section I.C.2.f, I.C.2.g

Primary sources#