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 TruebeamTM 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.


| INTRODUCTION
Radiation therapy plays an important role for patients who undergo breast conservation therapy (BCT) which includes both breast conserving surgery (BCS) and radiotherapy. Breast conservation therapy preserves the breast normal tissue as much as possible without compromising survival. Breast conservation surgery as known as lumpectomy, quadrantectomy, partial mastectomy, or segmental mastectomy depending on how the tissue has been removed is important which includes resection of the primary tumor with or without axillary nodes followed by radiotherapy to eradicate the residual microscopic disease of the breast tissue. 1,2 According to cancer statistics 3 in 2020, there were about 276 480 (30% of estimated new cases for all sites in female) new cases of breast cancer. In women, breast cancer has high incidence rate as compared to other types of cancer.
Depending on the patient staging, 4,5 for early-stage breast cancer with stage I and II, the conservative surgery and radiation therapy are standard alternatives to mastectomy. Radiation therapy after lumpectomy has been known for long-term local control equivalent to mastectomy 5,6 on the order of 85-95% with similar survival outcomes. Furthermore, postlumpectomy radiation therapy is associated with reduction in local recurrence and improved overall survival rate as compared to surgery alone. 7 Therefore, radiation therapy is considered a better approach for postlumpectomy treatment as compared to lumpectomy alone.
Patients who have early-stage breast cancer and received a lumpectomy can get sequential boost (10)(11)(12)(13)(14)(15)(16)(17)(18) to postlumpectomy preceded by whole breast radiation therapy (46-50.4 Gy with 1.8 to 2 Gy daily fractions) as per the Radiation Therapy Oncology Group (RTOG) 1005. 5 Current clinical practice is to treat the whole breast followed by a coned down boost to the lumpectomy cavity using electrons for superficial cavities and photons for deep-seated cavities. While both photons and electrons aim for conformal irradiation to the target while minimizing the dose to organs-at-risk (OARs), selection of one versus the other should be carefully considered to avoid toxicity. 8 In a previous publication, 9 we have developed the clinical feasibility of gantry static couch motion (GsCM) technique for treating deep-seated brain tumors. This technique was found beneficial for brainstem tumors or targets in the middle of optic chiasm and brainstem. The main advantageous of this approach in brain tumors was a sharp dose fall-off anteriorly and posteriorly to the target which spared normal tissues such as optical track and brainstem. In the current study, two partial arcs were simulated by adding multiple conformal static fields for each side. The GsCM technique is conceptually similar to volumetric-modulated arc therapy (VMAT) because it includes dose modulation. However, the modulation is achieved without inverse optimization. For GsCM the couch is dynamic with a static gantry (ie, GsCM utilizes a fixed gantry and rotating couch).
In this study we investigated the potential dosimetric advantages of the GsCM technique for sequential/concurrent boost of postlumpectomy cavities as compared to existing three-dimensional conformal radiation therapy (3D-CRT), conventional wedge pair in supine position (WPS), and wedge pair in decubitus position (WPD) techniques. The dose volume histogram (DVH) for lungs, normal breast tissue, and heart was calculated and compared with that of 3D-CRT and conventional wedge pair (WPS, WPD) techniques. The dosimetric accuracy of the plan delivery was evaluated by ion chamber array measurements in phantom.

| MATERIALS AND METHODS
In this study, the GsCM technique was implemented for the boost treatments of (n = 20) breast patients. This retrospective study has received an institutional review board (IRB#19-1025) approval to conduct the comparison of different treatment planning techniques for breast boost treatments. This study is focused on women who have large breast size resulting in a postlumpectomy cavity that is >5 cm deep. Use of electrons in this case is inferior due to greater skin dose and inadequate coverage distally to the lumpectomy cavity. Therefore, comparison with electrons was excluded from this study.

2.A | The gantry-static couch-motion (GsCM) technique
In this study the GsCM technique was implemented by first selecting the medial and lateral beams arranged with 2°delta couch angles.
The GsCM concept begins somewhat similar to dynamic conformal arc, in the sense that each beam initially conforms to the planning target volume (PTV) plus margin at every segment, which is actually a fixed static beam. However, the MLCs were conformed with a static gantry and a variable couch. After, the dose calculation, the beam weighting was adjusted to achieve good conformity around the target. Next, a fluence editing option was used to make the dose distribution homogenous as possible within the PTV. The MLCs were then generated with a limit of 1 segment per field. The groups of static beams then became like VMAT with both dose rate and aperture modulation. The main differences between standard VMAT and our GsCM technique are the moving couch instead of gantry, use of nontraditional inverse optimization, and the dose delivery at 2°i ncrements rather than continuous.  was generated by adding 0.5 cm margin to CTV (a total of 1.5cm margin around the GTV) to account for set up uncertainty. Figure 1 shows the location of lumpectomy cavity (contoured in pink color as the gross tumor volume) for a right breast (a) and a left breast (b) patient included in this study. Bolus was not required due to the deep-seated nature of the lumpectomy cavities, as shown in Fig. 1.
The contralateral breast, ipsilateral lung, and heart structures were used as meaningful OARs for plan comparison.
The dose constraints to the OARs were chosen based on the NASBP B-39/RTOG 0413 10 in addition to those listed in RTOG 1005 study to compare standard treatments to the GsCM technique.
These dose constraints were supported by the study conducted by Popescu et al. 11 on simultaneous couch and gantry dynamic arc rotation (CG-Darc) for APBI and Baglan et al. 12 on accelerated partial breast irradiation using 3D-CRT approach. Both studies found these constraints meaningful for assessing normal tissue toxicity, tumor control, and better cosmetic results.    format was generated, combining the fields into deliverable arcs. The isodose distribution for this technique is shown in Fig. 3(a). Couch angles (20°-40°) were selected to spread out the fields and avoid collision of the gantry head and treatment couch. Each field included a 60°wedge angle and the heel of wedge was kept anteriorly for all the fields. A 5 mm margin which defines the MLC aperture around the PTV was used to account for beam penumbra. The field arrangement for this technique is shown in Fig. 3(b).
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.
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.

2.C.4 | Plan comparison and dosimetric indices
The dose distribution for GsCM, 3D-CRT, WPS, and WPD was compared by overlaying the isodoses on axial, coronal, and sagittal slices as shown in Fig. 3. A two-sided paired t-test statistical analysis was performed with P ≤ 0.05 considered significant. All plans were normalized equally (D95 = 100%) as recommended by ICRU report 83 13 to compare the mean doses to PTVs. Dosimetric distributions were evaluated using the homogeneity index (H.I.), 14 conformity index (C.I.), 15 and gradient measure (G.M.). Briefly: where D98%, D2%, and D50% are dose received by 98%, 2%, and 50% of the volume. Homogeneity Index values approaching zero are considered as an ideal value for plan comparison.
where V 95% is the volume enclosed by isodose surface of 95% prescription dose and V PTV is the target volume. Conformity index approaching 1 is considered an adequate plan for comparison.
where R p and R 50 are the equivalent sphere radius of the prescription and half prescription isodoses. Gradient measure describes dose fall off from the PTV for the central slice.
T A B L E 2 Right breast dosimetric parameters show improvement in planning target volume (PTV) coverage, ipsilateral lung, and heart. GsCM is superior when compared to 3D-CRT, WPS, and WPD. All results are shown as mean AE standard deviation (ρ ≤0:05 to consider statistically significant). For heart, the volume getting 5% of the dose (V5%) was reduced by 97%, 99%, and 98% (mean) for GsCM compared to 3D-CRT, WPS, and WPD, respectively. Same trend of reduction was noted for dose getting to 30% and 50% of the heart volume (D30%, D50%) and the maximum dose. mization. This is due to the inability to limit the fields to a single segment. We limited to a single segment to best mimic VMAT.

Regardless
In this study, comparison of dose distribution (Conformity Index), dose fall off (Gradient Index), and dose volume histograms (V95%) of each technique revealed similarities between GsCM and 3D-CRT and superiority of GsCM over WPS and WPD. For ipsilateral breast F I G . 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.
Left breast dosimteric parameters show improvement in PTV coverage, ipsilateral lung, and heart. GsCM is superior when compared to 3D-CRT, WPS, and WPD. All results are shown as mean AE standard deviation (ρ ≤0:05 to consider statistically significant). to delivering all the fields individually. We found that using these features, GsCM can be easily implemented in clinic, once dose delivery and couch motion are allowed in clinical mode.

Parameter (objective) GsCM
Implementation of the GsCM technique is limited by characterization of patient motion and mitigation of potential collision during dose delivery as well as couch and gantry motion. In order to address this risk, 3D modeling of the planning system can be utilized for gantry and couch angles selection to avoid potential collision.
Furthermore, utilization of laser guard interlocks during treatment delivery can be considered. With regards to patient motion, a wireless bra/custom mesh type overlay on the patient can be used to minimize motion and ensure setup repeatability between fractions.
To study the couch motion, patient motion, and couch speed integration during beam delivery are not part of this study and will be discussed in a future study.
One of the limitations uncovered in this study was the short couch angular span on the lateral side of the beam for both right and left breast patients who have lumpectomy cavity located more medially. This can be mitigated, using two separate isocenters, one for each arc so that wider angular span can be achieved for both arcs. For the medial arc, the isocenter is usually positioned at the geometric center of the boost volume since there is enough clearance for large angular spans no matter where the boost volume is located. For the lateral arc, collision can be a major concern if the boost volume is located close to the medial side of patient. For cases like this, in order to encompass the whole PTV within the treatment field of view, the couch needs to be moved anteriorly and laterally from the isocenter of the medial arc, both shifts can be performed automatically within our GsCM technique framework. The data presented in this study are for single isocenter technique only.
The reduction in high dose to normal breast tissue, spread of low dose to ipsilateral lung and heart, and significant dose confor-

ACKNOWLEDG MENTS
The author would like to thank the Northwell Health and Stony Brook University for this collaborative work.

CONFLI CT OF INTEREST
There is no conflict of interest declared in this article.