What is the least likely side effect, RTx (Radiation Therapy) myelopathy or frozen shoulder, post RTx (Radiation Therapy) for unknown primary cancer?

Radiation Therapy Myelopathy is the Least Likely Side Effect

Radiation therapy myelopathy is significantly less likely than frozen shoulder following radiation therapy for unknown primary cancer, with myelopathy occurring in less than 2% of cases compared to frozen shoulder which can affect up to 72% of patients in high-risk populations.

Comparative Incidence Rates

Radiation Myelopathy - Rare Complication

• The frequency of radiation-induced myelopathy following spine SBRT is extremely low at less than 2% 1
• Even with reirradiation to previously treated spinal areas, the crude risk of radiation myelopathy remains only 1.2% 1
• Modern dose constraints and fractionation modeling maintain myelopathy risk under 5% when proper spinal cord dose limits are followed 1
• Historical data from patients receiving cumulative doses of 36.5 to 81 Gy (median 54 Gy) with overlapping spinal cord exposure showed only one episode of radiation myelopathy over an 18-year period 1

Frozen Shoulder - Common Complication

• Frozen shoulder has a dramatically higher incidence, reaching up to 72% in certain post-treatment populations such as stroke patients 2
• Breast cancer patients post-axillary dissection demonstrate shoulder dysfunction prevalence rates ranging from 1.5-50% 3
• Head and neck cancer patients frequently develop shoulder range of motion restrictions following radiation therapy 3
• The condition develops particularly when shoulder immobilization occurs or when formal physical therapy is not initiated within 6-8 weeks post-treatment 4, 3

Pathophysiologic Context

Why Myelopathy is Rare

• Modern radiation planning with pre-treatment MRI allows precise spinal cord delineation 1
• Maximum point dose constraints based on number of fractions are well-established and routinely followed 1
• Radiation to fields encompassing the spinal cord poses myelopathy risk primarily when combined with radiosensitizing agents like busulfan that cross the blood-brain barrier 1

Why Frozen Shoulder is Common

• Shoulder immobilization directly promotes frozen shoulder development, and radiation therapy often necessitates positioning that limits shoulder mobility 4, 3
• The pathophysiology involves synovial inflammation followed by capsular fibrosis, which can be triggered by immobilization during treatment 5, 6
• External rotation limitation is the earliest and most significant finding, and radiation positioning often restricts this critical movement 2, 3

Clinical Recognition Patterns

Myelopathy Presentation

• Radiation myelopathy typically presents months to years after treatment with progressive neurologic deficits 1
• The risk is dose-dependent and related to cumulative spinal cord exposure 1

Frozen Shoulder Presentation

• Frozen shoulder presents with gradual onset of pain and progressive loss of glenohumeral range of motion, with greatest limitation in external rotation, followed by abduction, then internal rotation 2, 5
• Unlike rotator cuff pathology, frozen shoulder shows equal restriction in both active and passive motion without focal weakness 2
• The condition progresses through three stages: freezing (pain predominant), frozen (stiffness predominant), and thawing (gradual resolution) 5, 7

Critical Clinical Caveat

The question context mentions "scarring" visible on imaging, which is more consistent with post-radiation fibrosis affecting soft tissues (including shoulder capsule) rather than spinal cord injury. Radiation-induced capsular fibrosis can directly contribute to frozen shoulder development, while radiation myelopathy would present with neurologic deficits rather than visible scarring 6, 8.