Dosimetric benefits of gantry-static couch-motion (GsCM) technique for breast boost radiation therapy: Reduced dose to organs-at-risk and improved dosimetric indices

Abstract

To evaluate the clinical feasibility and dosimetric benefits of a novel gantry-static couch-motion (GsCM) technique for external beam photon boost treatment of lumpectomy cavity in patients with early-stage breast cancer in comparison to three-dimensional conformal radiotherapy (3D-CRT), wedge pair in supine position (WPS), and wedge pair in decubitus position (WPD) techniques. A retrospective review was conducted on breast patients (right breast, n = 10 and left breast, n = 10) who received 10 Gy boost after 50 Gy to whole breast. The treatment plans were generated using an isocentric-based GsCM technique (a VMAT type planning approach) integrating couch rotational motion at static gantry positions. Static fields for each tangential side were merged using a Matlab^® script and delivered automatically within the Varian Truebeam^TM STx in Developer Mode application as a VMAT arc (wide-angular medial and short-angular lateral arcs). The dosimetric accuracy of the plan delivery was evaluated by ion chamber array measurements in phantom. For both right and left breast boost GsCM, 3D-CRT, WPS, and WPD all provided an adequate coverage to PTV. GsCM significantly reduced the ipsilateral lung V30% for right side (mean, 80%) and left side (mean, 70%). Heart V5% reduced by 90% (mean) for right and 80% (mean) for left side. Ipsilateral breast V50% and mean dose were comparable for all techniques but for GsCM, V100% reduced by 50% (mean) for right and left side. The automated delivery of both arcs was under 2 min as compared to delivering individual fields (30 ± 5 min). The gamma analysis using 2 mm distance to agreement (DTA) and 2% dose difference (DD) was 98 ± 1.5% for all 20 plans. The GsCM technique facilitates coronal plane dose delivery appropriate for deep-seated breast boost cavities, with sufficient dose conformity of target volume paired with sparing of the OARs.

Keywords: 3D conformal external beam planning; breast boost; couch motion-based dose calculation; trajectory-based delivery; wedge pair.

Fig. 1

The location of lumpectomy cavity is shown for a right breast (a) and a left breast (b) patient included in this study. In both pictures, the lumpectomy cavities were contoured in pink color as the gross tumor volume.

Fig. 2

The field arrangement of gantry static couch motion (GsCM) technique for the case in Fig. 1(a). The GsCM field arrangement consists of two oblique arcs aiming at the isocenter as shown in an axial view (a), coronal view (b), sagittal view (c), and a three‐dimensional view (d). Each arc utilizes multiple static fields (e) conformed around the target volume (f). For each beam, MLCs were conformed around the planning target volume as shown in the beams‐eye‐view (f) for right‐sided target with single isocenter approach.

Fig. 3

Comparison of the field arrangement and their respective axial, coronal, sagittal, and three‐dimensional (3D) view of isodoses overlay for the right breast lumpectomy boost treatment plan of the case in Fig. 1(a): (a) gantry static couch motion (GsCM), (b) 3D‐conformal radiation therapy (3D‐CRT), (c) wedge pair in supine position (WPS), and (d) wedge pair in decubitus position (WPD) techniques.

Fig. 4

Delivery and validation of gantry static couch motion technique was conducted utilizing an ion chamber array (a) scanned in a vertical position to avoid any side beam entrance. Measured (b) and planned (c) dose were compared using a criteria (d) of 2 mm distance to agreement (DTA), 2% dose difference (%DD), and two‐dimensional gamma (e) passing rate of more and equal to 95% and dose profile comparison from left to right (transverse) direction (f).

Fig. 5

Dose volume histogram (DVH) comparison for a left breast patient (a,b) and a right breast patient (c,d). The graph shows the dose reduction to 10%, 30%, and 50% of the volume getting prescription dose to heart and ipsilateral lung using gantry static couch motion technique while maintaining same PTV coverage as compared to three‐dimensional‐conformal radiation therapy, wedge pair in supine position, and wedge pair in decubitus position.