Boost delineation in breast radiation therapy: Isotropic versus anisotropic margin expansion.
OBJECTIVE
The purpose of this article is to compare isotropic and anisotropic margin expansion with regard to the size of the clinical target volume boost (CTVboost) and the interobserver variability (IOV).


METHODS
Lumpectomy cavities marked with 3 or more surgical clips were delineated by 6 radiation oncologists who specialized in breast radiation therapy. CTVboost anisotropicwas created by manually expanding the tumor bed with an anisotropic margin of 15 mm (20 mm in case of extensive intraductal component) minus the surgical free margins in 6 directions (anteroposterior, craniocaudal, and superoinferior). For the CTVboost isotropic, the tumor bed was enlarged with an isotropic margin of 15 mm (20 mm in case of extensive intraductal component) minus the minimal surgical free margin. The volumes of the delineated CTVboost(cm3) were measured. To assess the IOV, the Jaccard index (JI), defined as the intersection divided by the size of the union of the sample sets, was used (ideal value = 1). The JI was calculated for each case and each observer pair. Linear mixed models were used for all analyses.


RESULTS
A total of 444 delineated tumor beds were evaluated. The mean volume of the CTVboostalmost doubled by expanding the tumor bed with an isotropic margin compared with anisotropic margins (CTVboost isotropic94 mL [12.5-331.0] vs CTVboost anisotropic50 mL [3.2-332.7]; P = .0006). The IOV, assessed by the JI, significantly decreased by using isotropic versus anisotropic margin expansion (JICTV boost isotropic0.73 [0.02-0.92] vs JICTV boost anisotropic0.51 [0.0-0.8]; P< .0001). Because of the known positive correlation of the IOV and larger volumes, we corrected for CTVboostvolumes. With this correction, the difference in IOV remains highly significant (P < .0001) in favor of isotropic margin expansion.


CONCLUSIONS
The use of anisotropic margin expansion from tumorbed to CTVboost isotropicsignificantly reduced the volume of the delineated CTVboostwith a factor of 1.9 compared with isotropic margin expansion, but it substantially increased the interobserver variability.
University Hospitals Leuven/KU Leuven, Department of Radiation-Oncology, Belgium. Electronic address: karolienverhoeven@msn.com.
Karolien
K
Verhoeven
University Hospitals Leuven/KU Leuven, Department of Radiation-Oncology, Belgium.
Stéphanie
S
Peeters
Iridium kankernetwerk Wilrijk, Department of Radiation-Oncology, Belgium.
Katrien
K
Erven
University Hospitals Leuven/KU Leuven, Department of Radiation-Oncology, Belgium.
Hilde
H
Janssen
University Hospitals Leuven/KU Leuven, Department of Radiation-Oncology, Belgium.
Isabelle
I
Kindts
University Hospitals Leuven/KU Leuven, Department of Radiation-Oncology, Belgium.
Erik
E
Van Limbergen
Catholic University Leuven, L-BioStat, Belgium.
Annouschka
A
Laenen
University Hospitals Leuven/KU Leuven, Department of Radiation-Oncology, Belgium.
Saskia
S
Petillion
University Hospitals Leuven/KU Leuven, Department of Radiation-Oncology, Belgium.
Caroline
C
Weltens
D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D018270:Carcinoma, Ductal, Breast / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D000072662:Margins of Excision; D015412:Mastectomy, Segmental; D015588:Observer Variation; D000072378:Radiation Oncologists; D018714:Radiotherapy, Adjuvant / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D013525:Surgical Instruments; D014057:Tomography, X-Ray Computed; D047368:Tumor Burden