ESTRO 36 Abstract Book

S881 ESTRO 36 _______________________________________________________________________________________________

throughout the respiratory cycle on the basis of an imaged subspace. Material and Methods 4DCT pre-treatment scans of ten NSCLC patients were used to assess the motion of the bronchial tree in order to determine the minimum set of transformations necessary to describe the motion across the respiratory cycle. The bifurcation points (BPs) of the main airways were initially used to characterise the respiratory motion and determine the minimum number of BPs required to be monitored to accurately infer the position of all BPs. The accuracy of using the transformations resulting from the BP investigation to account for the respiratory motion of the airway sub-structures between the BPs was assessed using the Dice similarity index, maximum Hausdorff distance and the percentage of data points within the structure that are within 0.2/0.5 cm from the baseline structure. Further investigation into the optimal transformation required to minimise the mis-registration of the airway sub-structures throughout the respiratory cycle was performed including aligning the centre of mass of subsections of airways between the BPs. Results It was found that when the BPs were regionally paired and a single transformation applied to both BPs, the mis- registration errors were reduced from 2.19 to 0.18 cm, suggesting small differential motion between regionally paired BPs. This may also indicate that the transform may reduce the mis-registration of the airway sub-structures between the BPs. The BP transformations did not always improve the registration of the airway sub-structure in terms of maximum Hausdorff distance. The optimal method found to minimise the maximum Hausdorff distance was by aligning the centre of mass of the sub-structures. Table 1 Range of results from registration techniques. Conclusion Affine transformations have successfully been used to align paired bifurcation points within the bronchus to within 0.18 cm. This demonstrates that it is possible to describe the motion of all the BPs on the basis of a spatial subset of data. Similarly, improvements in the registration of the airway structure has also been achieved, reducing the maximum Hausdorff distance from 1.14 cm to 0.56 cm by using a small subset of information - the centre of mass of sub-structures - at different phases of the respiratory cycle. EP-1627 Anatomical advantages of deep inspiration breath hold for breast radiotherapy: a geometric analysis L. Conroy 1 , E. Watt 1 , S. Quirk 2 , J.L. Conway 2 , I.A. Olivotto 2 , T. Phan 3 , W.L. Smith 2 1 University of Calgary, Department of Physics & Astronomy, Calgary- Alberta, Canada 2 University of Calgary, Department of Oncology, Calgary- Alberta, Canada 3 Tom Baker Cancer Centre, Department of Radiation Oncology, Calgary- Alberta, Canada Purpose or Objective Numerous studies have proven the dosimetric benefits of deep inspiration breath hold (DIBH) for cardiac sparing in left breast radiotherapy; however, the anatomical advantages of this maneuver remain largely uncharacterized on a population basis. We examine the motion of the heart with respect to the target breast and

chest wall (CW) between end-exhalation and DIBH from a tangent beam’s-eye-view (BEV) perspective. Material and Methods Two computed tomography scans were acquired for 10 consecutive left-sided breast cancer patients: a DIBH scan and an end-exhalation breath hold (EEBH) scan. Breast and CW were contoured on the DIBH scan according to RTOG consensus guidelines. The heart was contoured on the DIBH scan using a validated heart atlas. Contours were propagated to the EEBH scan using the MIM Maestro TM Adaptive Recontour Workflow and edited where necessary. For all DIBH and EEBH scans, an in-house MATLAB program was used to measure the separation between the heart and CW at an angle approximately perpendicular to the BEV of the medial tangent beam on each axial slice. This is the estimated location of the heart that is at greatest risk of entering the tangent beam. Breath hold amplitudes were measured as the AP distance between DIBH and EEBH body contours at the mid- line/nipple-line intersection. Results DIBH causes the CW to move superiorly/anteriorly and the heart to move inferiorly/posteriorly. This relative motion facilitates coverage of the whole breast with tangents while sparing the heart (Figure 1). The median (range) breath hold amplitudes (difference between EEBH and DIBH) was 1.3(0.7–2.1) cm in the anterior direction, and the superior border (base) of the heart shifted 1.0(0.2–2.2) cm inferiorly. The relative motion of the CW with respect to the heart in DIBH reduced the amount of heart that would be at risk of entering the tangent field by 1.5(0.6– 3.8) cm (Figure 1b).The median maximum increase in heart-CW separation was 1.9(1.2–2.4) cm, at a location 4.8(1.6–6.2) cm inferior to the end-exhale heart base position (Figure 2, colored circles). Conclusion In contrast to dosimetric evaluations, results from population-based geometric analyses can be applied to a range of breast treatment strategies (e.g., partial breast irradiation, modified wide-tangents). This study provides evidence towards predicting the potential benefit of using DIBH from a free-breathing scan, and can be used to inform treatment planning strategies robust to inter- and intra-fraction motion of DIBH treatments.