Is the use of a preoperative computed tomography beneficial to reduce the interobserver variability of the CTVboost delineation for breast radiation therapy?

Accelerated Partial Breast Irradiation: Technological Advances and Current Challenges

Accelerated partial breast irradiation is a mature, standard-of-care treatment option for many women with early-stage breast cancer. In this paper, we discuss technological challenges and advances in the delivery of accurate and reproducible accelerated partial breast irradiation.

External partial breast irradiation in prone position: how to improve accuracy?

INTRODUCTION

In view of the limited incremental benefit between whole breast irradiation (WBI), accelerated partial breast irradiation (APBI) and omission of radiotherapy in favorable early-stage breast cancer (ESBC), APBI can only be justified if it combines adequate target coverage with the lowest achievable toxicity. Interobserver exercises demonstrated the difficulty of precise target delineation, especially in prone position; information on accuracy is even scarcer. We tested the impact of inserting an additional indicator clip, marking the depth of the tumor in the breast, and the added value of a preoperative CT in treatment position on precision and accuracy.

MATERIAL AND METHODS

In 12 patients, tumor bed delineation was performed by four radiation oncologists, with CTV_standard (clinical target volume) based on standard delineation guidelines, CTV_clip resulting from a 1-2-cm symmetrical expansion with the indicator clip as center and CTV_{clip_CT} expanding from the midpoint between the indicator clip and preoperative gross tumor volume (GTV) as center. Precision was measured as the mean pairwise Jaccard index (JI_pairs) between observers, accuracy as the mean overlap between GTV and respective CTVs.

RESULTS

JI_pairs was 0.38 for CTV_standard, 0.75 for CTV_clip and 0.59 for CTV_{clip_CT}. Overlap rate of GTV with CTVs was respectively 0.48, 0.67 and improved further to 0.88 for CTV_{clip_CT}. High-dose coverage of GTV (D95 and D90) improved with an indicator clip, but the most optimal result was reached when preoperative CT was added.

CONCLUSIONS

If EB-APBI in prone position is aimed for, an indicator clip intended to mark the depth of the tumor increases the probability of accurate target coverage, but cannot entirely replace the added value of a preoperative CT in treatment position. Avoiding the cost and effort of such CT implies a risk of missing the target, especially when small volumes are aimed for. Increasing target volumes to reduces this risk, questions the concept of APBI.

Boost delineation in breast radiation therapy: Isotropic versus anisotropic margin expansion

PURPOSE

The purpose of this article is to compare isotropic and anisotropic margin expansion with regard to the size of the clinical target volume boost (CTV_boost) and the interobserver variability (IOV).

METHODS AND MATERIALS

Lumpectomy cavities marked with 3 or more surgical clips were delineated by 6 radiation oncologists who specialized in breast radiation therapy. CTV_{boost anisotropic} was 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 CTV_{boost 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 CTV_boost (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 CTV_boost almost doubled by expanding the tumor bed with an isotropic margin compared with anisotropic margins (CTV_{boost isotropic} 94 mL [12.5-331.0] vs CTV_{boost anisotropic} 50 mL [3.2-332.7]; P = .0006). The IOV, assessed by the JI, significantly decreased by using isotropic versus anisotropic margin expansion (JI_{CTV boost isotropic} 0.73 [0.02-0.92] vs JI_{CTV boost anisotropic} 0.51 [0.0-0.8]; P< .0001). Because of the known positive correlation of the IOV and larger volumes, we corrected for CTV_boost volumes. 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 CTV_{boost isotropic} significantly reduced the volume of the delineated CTV_boost with a factor of 1.9 compared with isotropic margin expansion, but it substantially increased the interobserver variability.