Does the incidental coverage of axillary nodes by either standard breast or  high tangential radiotherapy fields explain Z0011 trial results?


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Marjory Maclennan1, Sanjana Masinghe1, Barbara Cadwallader1, Josie Cameron1, Avril Middleton1, Carolyn Bedi1, Tamasin Evans1
1Edinburgh Cancer Centre, Edinburgh, UK

Background

In early breast cancer, axillary node clearance (ANC) or nodal irradiation following positive sentinel lymph node (SLN) biopsy provide equivalent local control. The ACOSOG Z00111 trial demonstrated equivalent survival and locoregional recurrence in patients with positive SLNs offered either ANC or no further axillary treatment. Incidental irradiation of axillary nodal levels by breast tangential fields (TF) may explain these results. Oncologists in Z0011 may have used ‘high tangent s (HTs)', planned by altering the superior field border, with the intention of including the lower nodal levels. These details are unpublished.

Our purpose was to evaluate the volume of axillary nodes encompassed and the dose delivered to each level using both our standard and a high tangential field set up.

Method

Levels 1, 2 and 3 axillary nodes were retrospectively outlined on axial CT simulation images of 50 pts who received standard whole breast irradiation alone following breast conserving surgery. Each patient was planned using both standard TFs and HTs. Dose-volume histograms were used to calculate the percentage volume receiving 95% of the dose (V95%) and the mean dose [D mean (Gy)] of each nodal level.

Results

  • Mean dose to each axillary node level was substantially lower than that considered therapeutic even using a modified high tangential field set up (Dmean level 1 = 27.6Gy using standard fields, 35.5Gy with HTs).
  • Levels 1 and 2 were only partially included in standard and high tangential field set up. Level 1 V95%=16% using standard fields and 29% with HTs. Level 2 V95%=2.91% with standard fields and 8.38% using HTs.
  • Large interpatient variation in V95%.
  • Level 3 V95% was 0% for all patients.

Conclusion

We suggest the use of ‘HTs' cannot adequately explain the results of Z0011. Until more information is available on the radiotherapy delivered we cannot assume its impact upon the results.