Dosimetric comparison of TomoDirect, helical tomotherapy, and volumetric modulated arc therapy for postmastectomy treatment

Abstract Purpose To compare dose to the targets and organs at risk (OARs) in different situations for postmastectomy patients who require radiation to the chest wall with or without regional nodal irradiation when using three treatment techniques. Methods and materials Thirty postmastectomy radiotherapy (PMRT) patients previously treated by helical tomotherapy (HT) at our institution were identified for the study. The treatment targets were classified in three situations which consisted of, the chest wall (CW) only, the chest wall plus supraclavicular lymph nodes (CW + SPC), and the chest wall plus supraclavicular and whole axillary lymph nodes irradiation (CW + SPC+AXLN). The volumetric modulated arc therapy (VMAT) plans and Tomodirect (TD) plans were created for each patient and compared with HT treatment plans which had been treated. The target coverage, dose homogeneity index (HI), conformity index (CI), and dose to OARs were analyzed. The quality scores were used to evaluate the appropriate technique for each situation from multiparameter results. Results The HT and VMAT plans showed the advantage of target coverage and OARs sparing for the chest wall with regional nodal irradiation with the higher plan quality scores when compared with TD plans. However, TD plans demonstrated superiority to contralateral breast sparing for the chest wall without regional nodal situation reaching the highest of planned quality scores. HT plans showed better HI, CI, and target coverage (P < 0.01) than TD and VMAT plans for all patient situations. Volumetric modulated arc therapy plans generated better contralateral breast and heart sparing at a lower dose than HT. Conclusion The arc‐based techniques, HT and VMAT plans, provided an advantage for complex targets in terms of target coverage and OARs sparing. However, the static beam TD plan was superior for contralateral organ sparing meanwhile achieving good target coverage for the chest wall without regional node situations.


| INTRODUCTION
Postmastectomy radiotherapy (PMRT) presents a complex target volume, generally consisting of the chest wall (CW) and regional lymph nodes. The challenge of treatment planning is that it covers a large, superficial surface which is a thin area and a concave-shaped target. 1 In our clinic, postmastectomy patients were classified in three situations, chest wall only irradiation, chest wall plus supraclavicular nodes irradiation and chest wall including supraclavicular and axillary node irradiation. Each situation presents a variety of target complexity, which affects the selection of treatment techniques for the radiation oncologist.
Previously, PMRT in our clinic was treated with a mixed-beam technique consisting of three-dimensional (3D) technique with medial and lateral tangential field for CW. For increasing the skin dose, 1.0 cm. bolus used for half of the treatment course. Two 3D plans to be generated, the bolus and the nonbolus fractions. The anterior x-ray field was used for supraclavicular lymph nodes (SPC) with prescribed point at 3-4 cm. depth and posterior x-ray field prescribed point at midline depth was used for axillary nodes combined with anterior electrons to treat internal mammary nodes. Subsequently, helical tomotherapy (HT) often becomes the treatment of choice for PMRT due to improved conformality to the target, while sparing the OARs. 2 Tomotherapy can be performed in two modes. First is the HT delivery mode, a technique to treat continuous gantry rotations around the patient, using thousands of narrow beamlets, which are individually optimized to the target. However, TomoDirect (TD) is a nonrotational treatment by coplanar static beams, with the couch moving at a constant speed through a fixed binary multileaf collimator (MLC) that modulates the beam. After the patient has been treated with one gantry angle, the gantry is rotated to a different angle and the patient is again passed through the bore for the delivery of subsequent fields. 3 Heretofore, volumetric modulated arc therapy (VMAT) intensitymodulated delivery technique available with a linear accelerator launched in our center. Volumetric modulated arc therapy is a continuous modulation of the MLC, dose rate, and variable gantry speed to deliver highly conformal dose distributions in a short period of time. 4 Volumetric modulated arc therapy has become another choice of PMRT treatment in our clinic. Therefore, the objectives of this study were to compare the dose to the target and organs at risk (OARs) in different situations of left-sided PMRT patients requiring radiation to the chest wall with or without regional nodal irradiation when using the three treatment techniques, TomoDirect, Helical tomotherapy, and VMAT. Then we evaluated which advantages of each technique were suitable for each situation in our institute.

2.A. | Patients
This study included 30 patients who were treated by helical tomotherapy at our institution for left-sided PMRT between January 2017 and December 2018. The treatment targets were classified in three situations, with ten patients per each situation.

2.B. | Dose prescription and dose constraint
The target volume of the chest wall and regional nodes were localized separately. The prescription dose for all patients was 50 Gy in 25 fractions. The dosimetric constraints were determined from various publications and recommendations. [5][6][7][8][9][10][11][12] The dose to OARs was divided into two dose constraints (Table 1), the patients in the first situation followed the constraint for the CW only irradiation. The second constraint, CW plus regional nodes determined for the patients receiving treatment of the CW and regional nodes which were the second and the third situations. Because only the left-side PMRT patients were enrolled in this study, the dosimetric doses for heart, left anterior descending artery (LAD) were compared in each treatment technique.

2.C. | Treatment planning
This was a retrospective study with images from CT simulation with structure delineation for 30 PMRT patients imported into two treat- T A B L E 1 The dose constraints for postmastectomy radiotherapy (PMRT) patients with irradiated planning target volume (PTV) chest wall (CW) and chest wall plus regional nodes (CW + RN).

Chest wall only irradiation
Chest wall + regional nodes irradiation All plans used a jaw width of 2.5 cm, a pitch of 0.25, and a modulation factor between 3.0 and 3.2. The beam placement for chest walls used seven beams in IMRT mode, three beams for medial tangential, and three beams for the lateral tangential direction. We added another beam in anterior oblique direction for improved target conformity. TomoDirect mode skin flash was applied to compensate for the intrafraction movement by retracting three leaves (1.8 cm). In the case of the second and the third situations of patient who were treated for SPC and full axillary lymph nodes, the beams were placed to the planning target volume of regional nodes (PTV-RN) with three directions in anterior and two oblique beams.

2.C.2. | Helical tomotherapy plan setting
HT treatment plans were created using a jaw width of 2.5 cm, a pitch of 0.43, and a modulation factor of 3.0. We created a directional block to limit the entrance dose to OARs for both lung, contralateral breast, heart, and spinal cord. The optimization iterations were completed when the planning goals were met or until the plan could no longer be improved.

2.C.3. | Volumetric modulated arc therapy plan
setting VMAT treatment plans were created using two partial VMAT arcs of 210°-240°with start and stop angles of the first arc set to 295°and 145°, respectively, for chest wall only irradiation and 275°and 155°, respectively, for chest wall with regional nodes irradiation. The skin flash function was applied to compensate for the intrafraction movement by retracting four leaves (2.0 cm).

2.D. | Dosimetric comparison metrics
The targets of each situation were compared in three treatment techniques from the following quantities: target coverage (V 95% ), homogeneity index (HI), and conformity index (CI), calculation: 13,14 where D 2% , D 98%, and D 50% denote the near-minimum, near-maximum and median dose, respectively. An HI of zero indicates that the dose distribution is almost homogeneous. The conformity index, calculated as 1 where TV is the target volume, PIV is the volume of the 95% of prescribed isodose value and TV PIV is the volume of the target that is covered by the 95% of the prescribed isodose value. The larger value of CI representing better dose conformity. To better analyze the most superior technique for each situation from the multiparameter results, we summarize from the quality score table of the plans. 13 In the quality score   for all three situations, indicating better homogeneity with HT for both chest wall and chest wall plus regional nodes irradiation as shown in Table 2.
The HT plans showed to be superior in target coverage, HI and CI for all situations of PTV chest wall and PTV regional node as shown in Table 2  Abbreviations: AXLN, axillary nodes; CI, conformity index; CW, chest wall; HI, homogeneity index; HT, TomoHelical; LAD, left anterior descending artery; SPC, supraclavicular lymph nodes; TD, TomoDirect; VMAT, volumetric modulated arc therapy.
chest wall without regional nodal situation and reached the highest score of plan quality as shown in Table 4.

3.B.2. | Chest wall with nodal irradiation
HT plans demonstrated to be significantly lower than the other two plans for ipsilateral lung sparing. However, VMAT showed the lowest dose to the heart and TD plans still showed a significantly lower dose for contralateral lung, breast, and spinal cord for all situations of patient treatments. The score of plan quality shows the advantage of the arc-based IMRT over the TD plans for the complex target situation as shown in Table 4. with IMRT field for the optimal dose mixture were explored. 15,16 So, from the overall results and plan quality score we suggest choosing TD with the highest score which was suitable for chest wall only irradiation of PMRT. However, the VMAT and HT were the most suitable for chest wall plus regional node irradiation with the highest plan quality score.

| CONCLUSION
The arc-based techniques as HT and VMAT plans provided the advantage for complex targets in terms of target coverage and OARs sparing. However, static beam as TD plans showed to be superior for contralateral organ sparing meanwhile achieved the good target coverage for chest wall without regional node situation.

ACKNOWLEDG MENTS
The authors thank the staff of the Division of Therapeutic Radiology and Oncology, Faculty of Medicine, Chiang Mai University, for supporting this study.

CONFLI CT OF INTEREST
No conflict of interest.
(a) (b) (c) F I G . 3. The transverse view of the computed tomography images of planning target volume chest wall structure (blue color) for one patient for each situation (a) the first situation (chest wall only), (b) the second situation (chest wall included supraclavicular lymph nodes), and (c) the third situation (chest wall included supraclavicular and axillary lymph nodes irradiation).