Toxic pulmonary damage
Radiation pneumonitis is a classic complication requiring mandatory correlation of radiological findings with clinical symptoms. Histologically, the process manifests as diffuse alveolar damage, which appears nonspecific on imaging. However, the main distinguishing pattern on computed tomography is the "straight edge" sign (straight edge), where the zone of changes abruptly terminates, geometrically matching the contours of the irradiated target.
The timing of manifestation directly depends on the dose received. Under standard fractionation regimens (35–40 Gy), pneumonitis develops in 6–8 weeks, reaching maximum extent by 3–4 months. Doses exceeding 40 Gy lead to an earlier onset, while doses below 30 Gy rarely cause pneumonitis. In the long-term period (9–12 months later), persistent organizing fibrosis develops, accompanied by lung volume loss, traction effects, and bronchiectasis.
Cytotoxic chemotherapy acts as a synergist in radiation injury. Drugs such as doxorubicin, bleomycin, cyclophosphamide, mitomycin, and vincristine exacerbate fibrosis. Isolated administration of bleomycin causes dose-dependent basal interstitial changes (reticulonodular pattern on CT). Methotrexate therapy can provoke a hypersensitivity reaction manifesting as alveolar opacities.
Bone and bone marrow changes
Radiodiagnostics detect the effects on bone structures in both acute and late periods. In the early stages, standard radiography may reveal only local demineralization and osteopenia. Late effects (two years or more) are characterized by coarse trabecular architecture and mixed lytic-sclerotic lesions. Modern megavoltage therapy has reduced the risk of complications, but spontaneous fractures and avascular necrosis are still possible.
The MRI appearance is of particular importance. Already at doses of 15–20 Gy, hematopoietic bone marrow transforms into fatty marrow. On tomograms, this produces a diffuse high signal on T1-weighted images.
Interpretation of post-treatment changes can be extremely challenging. For example, following chemotherapy for spinal lymphoma, a "bizarre" signal elevation on T1 and T2 may be observed in the areas of the former tumor. However, metabolic activity on 18F-FDG PET will be normal. This proves that an abnormal MR signal can persist even in complete remission and with no residual disease.
Neurotoxicity and nervous system damage
Toxic nervous system damage occurs both as a consequence of irradiation and against the background of chemotherapy. Radiation therapy of the brain or spinal cord provokes microvascular ischemic disorders. On MRI, this manifests as a diffuse high signal from the deep white matter on T2 and FLAIR sequences (post-radiation leukoencephalopathy or radiation myelitis). A similar picture can be caused by cytotoxic drugs: 5-fluorouracil, levamisole, and intrathecal methotrexate.
A specific condition is posterior reversible encephalopathy syndrome (RPLS). It is typical for patients after bone marrow transplantation taking immunosuppressants (e.g., cyclosporine) and features a specific pattern of gray and white matter involvement.
At the peripheral level, radiation brachial plexopathy occurs (often after breast cancer treatment). Its substrate is vasa nervorum ischemia. MRI reveals thickening of the plexus elements. Importantly, unlike tumor recurrence, radiation injury lacks a mass lesion. Meanwhile, peripheral neuropathy caused by vincristine generally has no imaging signs on MRI.
Immunotherapy toxicity and monitoring methods
Immunotherapy has high toxicity: severe grade 3–4 reactions occur in more than 50% of cases. Inflammatory skin diseases, vitiligo, colitis, hepatitis, pneumonitis, and sarcoidosis-like reactions develop. Structural changes detected on imaging do not always have clinical significance, making it critically important for the radiologist to know the treatment timelines and regimens.
Hybrid technologies are used to assess complications and treatment efficacy:
- 18F-FDG PET/CT helps differentiate active recurrence from post-radiation fibrosis (metabolically active foci indicate continued growth).
- Dual-energy CT allows for more detailed characterization of tissue structure.
- Whole-body MRI is used for multiorgan staging, including in advanced prostate cancer, for which the METastasis reporting and data system standardization system has been implemented.
For radiation pneumonitis: The "straight edge"—the boundary of fibrosis runs as if drawn by a ruler, repeating the contour of the beam aperture. This is the main sign distinguishing it from infectious pneumonia.
Class and exam questions
How to differentiate radiation plexopathy from tumor recurrence on MRI?
In radiation injury, plexus elements are thickened due to vascular ischemia, but no volumetric soft-tissue component (tumor mass) is visualized.
Does an abnormal bone marrow MR signal after treatment always indicate recurrence?
No. Following chemo- and radiotherapy, the MRI appearance may remain mixed and "bizarre" (with high signal on T1/T2) even in complete remission, which is confirmed by a negative PET scan.
How long after lung irradiation does organizing fibrosis finally form?
Fibrotic changes develop at a late stage and finally form 9–12 months after the completion of radiation therapy.
What else to study on this topic
- Response criteria for immunotherapy in solid tumors
- Assessment of multiorgan involvement in prostate cancer (MET-RADS)
- CT semiotics of cytotoxic chemotherapy hepatotoxicity
- Posterior reversible encephalopathy syndrome (RPLS): pathogenesis and MRI patterns
- Non-atheromatous arteritis and indications for renal artery stenting
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