No, Do Not Hold Arimidex During Radiation Therapy
Arimidex (anastrozole) can and should be administered concurrently with radiation therapy in postmenopausal women with hormone receptor-positive breast cancer. There is no need to delay or hold endocrine therapy until radiation is completed.
Evidence Supporting Concurrent Administration
The NCCN guidelines explicitly state that "endocrine therapy and trastuzumab can be administered concurrent with radiation therapy if indicated" 1, 2, 3. This recommendation appears consistently across multiple guideline iterations, establishing concurrent administration as standard practice.
The FDA label for Arimidex provides no contraindications or warnings against concurrent radiation therapy 4. The drug can be taken continuously at 1 mg daily regardless of other treatment modalities.
Clinical Trial Evidence
A prospective study directly addressed this question by comparing 57 patients who received concurrent anastrozole with radiation versus 126 patients who received sequential therapy 5. The results showed:
- No increase in acute grade 2 radiation dermatitis (24.6% concurrent vs 20.6% sequential; P = 0.55)
- No increase in grade 3 dermatitis (8.8% vs 7.1%; P = 0.77)
- No difference in treatment interruptions due to skin reactions (14.0% vs 11.2%; P = 0.69)
- No difference in breast fibrosis rates (24.2% vs 24.7%; P = 0.97)
- No compromise in local control (1 local failure concurrent vs 5 sequential)
This study definitively demonstrates that concurrent anastrozole does not increase acute or late radiation toxicity.
Clinical Algorithm
For your 64-year-old patient:
- Start or continue Arimidex 1 mg daily without interruption
- Proceed with planned radiation therapy on schedule
- Continue Arimidex throughout radiation and afterward
- Monitor for standard radiation side effects (dermatitis, fatigue) - these are not increased by concurrent anastrozole
- Continue Arimidex for 5-10 years total depending on risk factors 6, 7
Important Caveats
- Ensure the patient is truly postmenopausal - aromatase inhibitors are ineffective and inappropriate in premenopausal women 8
- Do not combine with tamoxifen - concurrent use reduces anastrozole efficacy 4
- Monitor bone health with baseline and periodic bone density testing, as both anastrozole and radiation can affect bone 8, 4
- Watch for cardiovascular symptoms in patients with pre-existing heart disease, though this is unrelated to radiation 4
The sequential approach (holding anastrozole until after radiation) has no evidence-based rationale and unnecessarily delays systemic therapy that reduces recurrence risk. Starting endocrine therapy promptly after surgery is the standard of care, with radiation administered concurrently as needed.