Axillary Lymph Node Dose With Modern Tangential Breast Radiation

Reviewer: Roberto Santiago, MD
OncoLink
Last Modified: October 7, 2002

Presenter: Daniel Reed
Presenter's Affiliation: Dept of Radiation Oncology, Univ of Washington Medical Center, Seattle, WA
Type of Session: Scientific

Background

The computerized tomography (CT) scan has become a valuable tool in defining the location of internal mammary nodes, in the development of dose?volume histograms relating to the radiation dose to vital organs such as the heart and lungs, and more recently, in defining the depths of supraclavicular and axillary nodes in patients undergoing definitive breast irradiation.
Knowing the anatomical location of these nodes is critical if they are to be effectively targeted.
A level I/II axillary lymph node dissection has been the standard treatment for this nodal area. However, the use of axillary sentinel lymph node mapping has become popular and there is renewed interest in the value and technical aspects of nodal irradiation in these patients.
The study evaluates the radiation dose coverage of lymph nodes in levels I and II of the axilla during breast conservation therapy (BCT) employing conventional tangential radiation fields and CT based three-dimensional (3D) treatment planning.

Materials and Methods

The 3D dose distribution of tangential breast irradiation fields using CT based 3D treatment planning in 50 consecutive women undergoing BCT for early stage breast cancer was evaluated.
The dose distributions were compared to the anatomical axillary level I and II lymph node volumes as defined on CT by a radiation oncologist with collaboration from Diagnostic Radiology and Surgical Oncology.
A volume defined by the axillary surgical clips was also determined by contouring the axillary nodal volume encompassed by the location of the surgical clips in patients who had undergone level I/II lymph node dissection.
All patients had breast tangential fields designed to encompass the entire breast parenchyma based on 3D treatment planning.

Results

In the 50 patients evaluated, the 95% isodose line encompassed all (100%) of the breast parenchyma volume.
However, only an average of 55% (median, 53%) of the anatomic axillary level I/II lymph node volume was encompassed by the 95% isodose line.
The mean anatomic axillary level I/II volume was 146.3cc (83.1cc?313.0cc) of which a mean of 84.9cc (25.1cc?219.0cc) was encompassed by the 95% isodose line.
Axillary surgical clip volumes grossly underestimated the specified ?anatomical? axillary level I/II lymph node volumes.

Author's Conclusions

Axillary level I and II lymph node volumes, as defined in the study, did not receive a therapeutic dose with the use of standard tangential breast radiation fields.
None of the patients evaluated had their entire axillary level I/II lymph node volume covered by the 95% isodose line.
The data gathered in this study should question the approach of ongoing national prospective clinical trials.
Complete level I/II lymph node dissection remains the standard of care in patients with positive axillary lymph nodes despite advances in sentinel lymph node biopsy.
Definitive irradiation of level I and level II axillary lymph node requires significant modification of standard tangential fields that require the input from 3-dimensional treatment planning with specific targeting of anatomically defined axillary lymph node volumes.

Clinical/Scientific Implications

Recent data from Goodman et al. suggests that the vast majority of patients with early stage breast cancer treated with BCT had level I nodes covered by the 90% isodose line.
The authors of the present study did not effectively define the superior border of the tangential fields.
There is considerable variation both in the depth of supraclavicular and axillary lymph nodes and the fields in which these nodal groups appear.
To be certain that nodal groups which one plans to treat are actually treated, as well as to minimize nodal treatment when such treatment is not planned, it is recommended that before the placement of radiation fields, the nodal groups be carefully outlined on a CT scan much as one would outline a tumor volume in other disease sites.

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