Tomotherapies, Helical

Patients with breast cancer involving internal mammary chain and clavicular nodes are routinely treated with TomoTherapy in our center. Positioning is started with aligning the patient in the TomoTherapy virtual isocenter using lasers or Catalyst. The couch is displaced 70 cm towards the real isocenter in the cranial direction (+Y) and a daily MVCT image is taken. We use coarse slices and fine reconstruction leading to 3 mm slice thickness. After this MVCT image, the couch is displaced back to the virtual isocenter, after which the MVCT image fusion with the planning kVCT is performed. Finally, the position is corrected for in X, Y, Z and roll (left–right, craniocaudal, ant–post, roll) and displaced back towards the gantry for treatment.
All treatments were performed in 25 fractions. Boost volumes, when present, were treated by delivering 2.4 Gy/fraction as integrated boost. Treatment planning is performed in the helical mode with TomoEdge⁽²⁰⁾ (Accuray) using a 5 cm field width and using details described in the previous work by Crop et al.⁽²¹⁾ This results in a treatment beam‐on time of approximately 5–7 min. Typical volumes of the different PTVs were in the order of 40 cm³ for internal mammary chain PTV, 1000 cm³ for the breast, 100 cm³ for infraclavicular node, and 100 cm³ for the supraclavicular node.
The patient is positioned with both arms up on a breast board with an inclination of 7°. The TomoTherapy couch is positioned as low as possible: 21–22 cm below the virtual isocenter. This allows for the highest amount of liberty, low thread effect,⁽²²⁾ and better Catalyst camera view. The difference between the initial position and positioning result after MVCT fusion was evaluated in order to compare laser‐based and Catalyst‐based positioning. Forty patients with Catalyst‐based setup and 55 patients with laser‐based setup where included, resulting in, respectively, 810 Catalyst‐only sessions and 666 laser positioning‐only sessions. Thirty‐one of these patients had both laser‐based and Catalyst‐based setup on different days. Couch sag for TomoTherapy is different in comparison with the initial kVCT image. This leads to a different height position of the patient in the real isocenter, 70 cm further located, compared to the position in the virtual isocenter. This couch sag thus depends on the patient weight and the treatment location. Couch sag was evaluated with two methods: by adding weight to a MVCT‐scanned phantom and as the uncorrected mean Catalyst‐image bias between patient position outside the bore and inside the bore.

To reduce breathing motion and set-up error, patients were laid in the supine position on appropriate immobilization devices depending on the location of the radiotherapy target [13 (link)]. In planning computed tomography (CT), 2- to 3-mm slice thickness axial images were acquired using a 64-row multi-detector CT (Aquilion CX, Toshiba Medical, Otahara, Japan). Contrast-enhanced CT images were acquired and fused to the planning CT images to delineate the target, but unenhanced CT images were used for dose calculation to keep calculation accuracy [14 (link)]. Contouring of target volumes and normal structures was performed on the Pinnacle (3) version 9 treatment planning system (Philips Medical System, Eindhoven, Netherlands). The contours created in the treatment planning system were exported to the Tomotherapy Hi-Art treatment planning system v4.0, where TomoHelical and TomoDirect plans were generated.

Both RapidArc and IMRT plans were generated with 6 MV photon beam in a linear accelerator equipped with Millennium 120 MLC using the Eclipse treatment planning system. Dose calculation was performed with an anisotropic analytic algorithm. RapidArc plans were devised by the use of two coplanar full arcs. Collimators were rotated from 101 to 201 to minimize the effect of tongue and groove. IMRT plans were devised by using five coplanar beams. TOMO plans were generated with 6 MV photon beam using Tomotherapy Planning Workstation (TomoHD version 1.0.0, Accuray Inc., Sunnyvale, CA). Dose calculation was performed with a superposition or convolution algorithm. The planning parameters were as follows: field width ¼ 2.5 cm, pitch ¼ 0.43, and modulation factor ¼ 3–3.5. To ensure consistency of planning techniques, all treatment plans were devised by physicists with over 3 year clinical experience in IMRT, RapidArc, and Tomotherapy planning. Planning requirements and techniques for planners were also aligned by training, standard protocol, and procedures of the department. Dose-volume histograms (DVHs) were generated and evaluated with the medical physicist until the desired plan was reached. The angles of the rays, the angles of the filters, and the weight ratio were used to optimize the coverage of the planned target volume and to minimize the dose to the OARs. After creating the DVHs and using the required parameters, the radiation Conformity index (CI) was calculated. It is defined as a ratio between the volume covered by the reference isodose, which according to ICRU, is 95% isodose, and the target volume (TV) designated as PTV (equation (1)). A better PTV conformity corresponds to lower values of the CI. The Dose homogeneity index (DHI) is defined as a ratio between the dose reached in 95% of the PTV volume (D ≥ 95%) and the dose reached in 5% (D ≥ 5%) of the PTV volume (equation (2)). A higher DHI signified poor homogeneous irradiation of the TV. $$\text{Conformity}\text{index}{(\text{CI})}_{\text{RTOG}}=V_{\text{RI}}/\text{TV},$$ where V_RI = Reference isodose volume and PTV = Plan target volume. $$\text{Dose}\text{homogeneity}\text{index}(\text{DHI})=D_{\ge 95\%(\text{within PTV})}/D_{\ge 5\%(\text{within}\text{PTV})}.$$ [14 (link)]
The D_98% (doses received by 98% of the PTV volume) and D_2% (doses received by 2% of the PTV volume) were defined as the minimum and maximum doses. The data from the DVHs obtained from all the plans were analyzed. The dose distributions in the grade II gliomas of a typical case, by using VMAT, IMRT, and TOMO are shown in Figure 1.

Seven patients diagnosed with histological grade II glioma in our hospital were included between September 2020 and February 2021 in our study. All patients with surgically and oncologically treated glioma previously had a clearly defined indication for glioma treatment under recommendations by the Radiation Therapy and Oncology Group (RTOG) to administer selective radiotherapy. All patients participating in the study had a complete surgical resection. All patients did not have cerebral radiation history. The TOMO plans were designed on a tomotherapy treatment system with 6 MV photon beams and optimized using the least-squares optimization method. VMAT and IMRT plans were designed on the Varian Eclipse treatment planning system with the 6 MV photon beams generated by the Varian IX linear accelerator. The summary of patients, tumor, and treatment details are shown in Table 1. Written evidence of informed consent was obtained from the participant or the participant’s legally acceptable representative.