Unintended coverage: how standard breast radiotherapy irradiates the internal mammary nodes
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Published on: February 6, 2019
Rishi P Nair1, Depanshu Aggarwal1, Puneet Pareek1
1Department of Radiation Oncology, All India Institute of Medical Sciences Jodhpur, Jodhpur, India.
Background:
Breast cancer is the most common malignancy in women worldwide. Irradiation of regional nodes, including the internal mammary nodes (IMNs), may improve outcomes, but routine IMN irradiation remains controversial because of toxicity. Modern breast radiotherapy may incidentally irradiate the ipsilateral IMNs. We quantified incidental IMN doses and factors affecting coverage in patients without targeted IMN irradiation.
Materials And Methods:
In this retrospective dosimetric study, 104 women with clinical stage I-III disease were analysed. The ipsilateral IMNs (levels I-III) were contoured on planning CT scans according to the RTOG guidelines. Dose-volume histogram (DVH) metrics (minimum, mean, and maximum doses; V95%, V90%, and V50%) were recorded. Group comparisons [mastectomy vs. breast-conserving surgery (BCS) and 3DCRT vs. VMAT)] were performed using statistical tests, with p < 0.05 considered statistically significant.
Results:
The median age was 52 years; 56.7% of patients were aged > 50 years; left- and right-sided tumours were equally represented. Seventy-five per cent underwent mastectomy and 25% underwent BCS. Eighty-one per cent received 3DCRT; 46% received supraclavicular/axillary irradiation; the median follow-up was 40 months. The mean IMN volume was 7.30 ± 1.76 cm3. The mean IMN dose was 35.53 ± 6.20 Gy, and the minimum dose was 7.73 ± 6.37 Gy. On average, 47.3% of the IMN volume received ≥ 95% of the prescribed dose. Mastectomy plans delivered a higher mean IMN dose than BCS plans (36.55 vs. 32.48 Gy, p = 0.0046). 3DCRT plans provided better IMN coverage than VMAT plans (mean V95%, 51.4% vs. 29.1%; p = 0.0012). Radiotherapy modality and type of surgery were significant in the multivariate analysis.
Conclusions:
Standard tangential fields provide only partial IMN coverage with wide variability. Most patients did not reach therapeutic IMN doses, highlighting the need for explicit contouring and dedicated planning when IMN irradiation is indicated.
