D. Minkema

Multiinstitutional study on target volume delineation variation in breast radiotherapy in the presence of guidelines

PURPOSE This study aims to determine magnitude, causes and consequences of post-operative breast tumour target volume delineation variation among radiation oncologists in the presence of guidelines. MATERIALS AND METHODS Excision cavities, CTVs and PTVs of eight breast cancer patients were delineated on CT scans by 13 Dutch radiation oncologists (observers) from 12 Dutch institutes participating in the international Young Boost Trial. Delineated volumes and conformity indices were determined. CTV delineation variation (SD) was determined for anatomically relevant regions. Non-parametric statistics were performed to establish effects of observers, patient characteristics and regions on delineation variation. RESULTS Even in the presence of delineation guidelines considerable delineation variation is present (0.24<SD<1.22 cm). Presence of clips or seroma reduced interobserver variation (0.24<SD<0.62 cm). Region-specific analysis showed distinct regions of higher variability per patient. This could not always be ascribed to anatomical features, suggesting interobserver variation is not solely due to lack of image quality. CONCLUSIONS In this study, interobserver delineation variation in breast tumour target volume delineation is larger than, e.g. setup inaccuracies and results from limited reliable visual guidance as well as interpretation differences between observers, despite guidelines. Reduction of delineation variation is essential in view of current developments in planning techniques, particularly for External Partial Breast Irradiation.

Breast Patient Setup Error Assessment: Comparison of Electronic Portal Image Devices and Cone-Beam Computed Tomography Matching Results

PURPOSE To quantify the differences in setup errors measured with the cone-beam computed tomography (CBCT) and electronic portal image devices (EPID) in breast cancer patients. METHODS AND MATERIALS Repeat CBCT scan were acquired for routine offline setup verification in 20 breast cancer patients. During the CBCT imaging fractions, EPID images of the treatment beams were recorded. Registrations of the bony anatomy for CBCT to planning CT and EPID to digitally reconstructed-radiographs (DRRs) were compared. In addition, similar measurements of an anthropomorphic thorax phantom were acquired. Bland-Altman and linear regression analysis were performed for clinical and phantom registrations. Systematic and random setup errors were quantified for CBCT and EPID-driven correction protocols in the EPID coordinate system (U, V), with V parallel to the cranial-caudal axis and U perpendicular to V and the central beam axis. RESULTS Bland-Altman analysis of clinical EPID and CBCT registrations yielded 4 to 6-mm limits of agreement, indicating that both methods were not compatible. The EPID-based setup errors were smaller than the CBCT-based setup errors. Phantom measurements showed that CBCT accurately measures setup error whereas EPID underestimates setup errors in the cranial-caudal direction. In the clinical measurements, the residual bony anatomy setup errors after offline CBCT-based corrections were Σ(U) = 1.4 mm, Σ(V) = 1.7 mm, and σ(U) = 2.6 mm, σ(V) = 3.1 mm. Residual setup errors of EPID driven corrections corrected for underestimation were estimated at Σ(U) = 2.2mm, Σ(V) = 3.3 mm, and σ(U) = 2.9 mm, σ(V) = 2.9 mm. CONCLUSION EPID registration underestimated the actual bony anatomy setup error in breast cancer patients by 20% to 50%. Using CBCT decreased setup uncertainties significantly.

Breast-Conserving Therapy: Radiotherapy Margins for Breast Tumor Bed Boost

PURPOSE To quantify the interfraction position variability of the excision cavity (EC) and to compare the rib and breast surface as surrogates for the cavity. Additionally, we sought to determine the required margin for on-line, off-line and no correction protocols in external beam radiotherapy. METHODS AND MATERIALS A total of 20 patients were studied who had been treated in the supine position for 28 daily fractions. Cone-beam computed tomography scans were regularly acquired according to a shrinking action level setup correction protocol based on bony anatomy registration of the ribs and sternum. The position of the excision area was retrospectively analyzed by gray value cone-beam computed tomography-to-computed tomography registration. Subsequently, three setup correction strategies (on-line, off-line, and no corrections) were applied, according to the rib and breast surface registrations, to estimate the residual setup errors (systematic [Sigma] and random [sigma]) of the excision area. The required margins were calculated using a margin recipe. RESULTS The image quality of the cone-beam computed tomography scans was sufficient for localization of the EC. The margins required for the investigated setup correction protocols and the setup errors for the left-right, craniocaudal and anteroposterior directions were 8.3 mm (Sigma = 3.0, sigma = 2.6), 10.6 mm (Sigma = 3.8, sigma = 3.2), and 7.7 mm (Sigma = 2.7, sigma = 2.9) for the no correction strategy; 5.6 mm (Sigma = 2.0, Sigma = 1.8), 6.5 mm (Sigma = 2.3, sigma = 2.3), and 4.5 mm (Sigma = 1.5, sigma = 1.9) for the on-line rib strategy; and 5.1 mm (Sigma = 1.8, sigma = 1.7), 4.8 mm (Sigma = 1.7, sigma = 1.6), and 3.3 mm (Sigma = 1.1, sigma = 1.6) for the on-line surface strategy, respectively. CONCLUSION Considerable geometric uncertainties in the position of the EC relative to the bony anatomy and breast surface have been observed. By using registration of the breast surface, instead of the rib, the uncertainties in the position of the EC area were reduced.

Clinical Factors Associated With Seroma Volume Reduction in Breast-Conserving Therapy for Early-Stage Breast Cancer: A Multi-Institutional Analysis

PURPOSE To correlate clinical factors with seroma volume and reduction; and to determine whether cone-beam CT (CBCT) could be used clinically to monitor seroma reduction. PATIENTS AND METHODS This investigation included 102 women from five institutions with stage T1-2 breast cancer treated with breast-conserving therapy. Two CT scans were acquired: the planning CT (CT1) and a second CT (CT2) during radiotherapy (RT). Seroma was contoured on all scans, and correlations between seroma characteristics and clinical factors were investigated by univariate and multivariate analyses. In a substudy, 10 of the 102 patients received multiple CBCT scans during RT. Seroma were contoured by two observers in the substudy. Fifteen time points at which CT and CBCT were performed within 2 days were identified. The levels of correlation in seroma contours between CBCT and CT and between the two observers were examined. RESULTS The mean relative seroma reduction from CT1 to CT2 was 54% (p < 0.001). A significant inverse relationship was found between relative seroma reduction per week and number of RT fractions given by univariate and multivariate analyses (p = 0.01, 0.03). The mean difference in contoured seroma volumes between CT and CBCT was 12% (3.3 cm(3)). When assessing the relative difference in seroma contours between Observer 1 and Observer 2, an interobserver difference of 12% was demonstrated. Neither discrepancy was clinically significant. CONCLUSIONS Radiotherapy seems to hinder seroma reduction. Volume discrepancies between CBCT and CT were minor, with low interobserver variation, indicating that CBCT might be useful in monitoring seroma reduction.