Flattening filter free VMAT for a stereotactic, single‐dose of 30 Gy to lung lesion in a 15‐min treatment slot

Abstract Cone‐beam CT‐guided single dose of lung stereotactic body radiotherapy (SBRT) treatment with a flattening filter free (FFF) beam and volumetric modulated arc therapy (VMAT) is a safe and highly effective treatment modality for selective small lung lesions. Four‐dimensional (4D) CT‐based treatment plans were generated using advanced AcurosXB algorithm for heterogeneity corrections. 6X‐FFF beam produced highly conformal radiosurgical dose distribution to the target and reduced lung SBRT fraction duration to less than 10 min for a single dose of 30 Gy, significantly improving patient comfort and clinic workflow. Early follow‐up CT imaging results (mean, 8 months) show high local control rates (100%) with no acute lung or rib toxicity. Longer clinical follow‐up in a larger patient cohort managed in this fashion is underway to further validate this treatment approach.


| INTRODUCTION
Due to recent advances in technology, stereotactic body radiotherapy (SBRT) has become standard of care for medically inoperable early stage non-small-cell lung cancer (NSCLC) patients. [1][2][3][4] SBRT protocol RTOG-0915 (Arm 1) allowed a single dose of 34 Gy treatment for early stage I peripheral NSCLC patients when dosimetric criteria were achieved. 5  Both treatment schedules provided equivalent tumor local control and overall survival rates with minimal toxicity. Therefore, a stereotactic, single dose of 30 Gy is an equally effective treatment for the selected NSCLC patients and is gaining popularity.
Treatment delivery developments, including volumetric modulated arc therapy (VMAT) and flattening filter free (FFF) beams have reduced SBRT treatment time significantly and improved patient compliance. [8][9][10][11] Removal of the flattening filter from the gantry reduces head scatter, out-of-field dose, residual electron contamination, and delivers treatments with higher dose rates up to factors of 2.33 for 6X-FFF and 4 for 10X-FFF beams compared to the traditional flattened beams. [12][13][14][15][16] Because of the reduced treatment times, VMAT with FFF beams is particularly appealing for delivering a sin- All patients were treated with cone beam CT-guided VMAT with 6X-FFF (1400 MU/min) beam on Truebeam Linac. VMAT plans were individually designed using multiple noncoplanar partial arcs (3-4 arcs with ± 10-15°couch kicks) chosen to achieve the planning objective for each patient. Patient-specific collimator rotations and jaw tracking options were used. All dose distributions were computed using the advanced AcurosXB algorithm for heterogeneity corrections with photon optimizer MLC algorithm [18][19][20] (AcurosXB, version 13.6) implemented in the Eclipse treatment planning system. The calculation grid size was 1.25 mm and dose to medium reporting mode was used.
Prescription dose was 30 Gy in 1 fraction; at least 95% of the PTV received the prescription dose and the maximum dose to the PTV was limited to 130% (fall within the ITV) of the prescription dose.
Before delivering each SBRT treatment, a daily quality assurance (QA) check was performed on kilovoltage to megavoltage imaging isocenter coincidence, including IsoCal measurement for precise and accurate target localization. The IsoCal localization accuracy for Truebeam was <0.5 mm. All the quality assurance procedures including patient-specific QA were in compliance for SBRT treatment delivery. 17 Patient-specific VMAT-SBRT QA was performed using an Octavius 4D (PTW, Freiburg, Germany) phantom with an Octavius 1500 detector array insert and the average pass rate was 97.6 ± 2.7% for 3%/2 mm criteria.
Patients were initially positioned using external marks and inroom lasers, followed by pretreatment free-breathing cone beam CT scan. Every patient setup prior to single-dose lung SBRT was performed using an in-house SBRT/IGRT protocol by co-registering pretreatment cone beam CT with the planning CT scans at the Truebeam Linac (see Fig. 1). Image registration was performed automatically based on region of interest and bony landmarks, followed by manual refining performed by the treating physician to ensure that the tumor was registered with the ITV contoured on the planning CT. The patient position was then corrected for 6 degrees of freedom (DOF) according to the results of soft tissue registration and the treatment was delivered. Those 6-DOF couch corrections were within the limits of our departmental SBRT protocol guidelines for each patient (translational shifts within ± 2.0 mm and rotational shifts within ± 2.0°in each direction). The patient setup, tumor matching on cone beam CT scan, and treatment delivery were monitored and verified by the treating physician and physicist. Figure 1 shows the planned isodose color wash superimposed with daily CBCT images after the couch corrections were applied.
For comparison of dosimetry and treatment delivery efficiency, all cases were replanned using identical VMAT geometry with traditional flattened 6X-FF beam with maximum available dose rate of 600 MU/min. The same prescribed dose, planning objectives during plan optimization, and requirement for plan evaluation were used as F I G . 1. Axial, coronal, and sagittal views of CBCT images (see inset) co-registered with planning CT images (see back at coronal and sagittal views) used for image-guided SBRT treatment. In addition to anatomical landmarks, the planned dose cloud was superimposed. CBCT images were acquired in free breathing with abdominal compression, and soft-tissue three-dimensional (3D) matching was performed manually. SBRT, stereotactic body radiotherapy.
for 6X-FFF plans. The major RTOG parameters evaluated for target coverage include: 1. Conformity index (CI): ratio of prescription isodose volume to the PTV. CI less than 1.2 is highly desirable; CI = 1.2-1.5, acceptable with minor deviations.
2. Gradient index (GI): ratio of 50% prescription isodose volume to the PTV. GI has to be smaller than 3-6, depending on the PTV.
3. Maximum dose at any point 2 cm away from the PTV margin in any direction (D 2cm ): D 2cm has to be smaller than 50-70%, depending on the PTV size.

4.
Percentage of normal lung receiving dose equal to 20 Gy or more (V20Gy): V20Gy should be less than 10% per protocol, V20Gy less than 15% is acceptable with minor deviations. V20Gy is for total lungs minus the ITV.
Furthermore, the gradient distance (GD) was defined as the average distance from 100% prescribed dose to 50% prescribed dose  Table 1) compared to traditional 6X-FF plan. Statistically significant p-values are shown in bold (see Table 1).
All other doses to OAR including rib and skin were much lower than RTOG requirement and dosimetrically superior with 6X-FFF beam compared to traditional 6X-FF beam (not shown here). For the patient shown in Fig. 2, the PTV was 10.7 cc (2.71 cm diameter) and located in the middle right lung. VMAT plan consisted of four noncoplanar partial arcs (total of 10,090 MU was delivered for a single dose of 30 Gy). Beam on time was 7.21 min with 6X-FFF beam for the Truebeam Linac. In this case, the VMAT plan gave CI, D 2cm , GI, GD, and V20 were 1.20, 53.8%, 7.23, 1.37 cm, and 0.6% with 6X-FFF beam, all parameters within RTOG compliance.  Table 2).    Much tighter radiosurgical dose distributions with 6X-FFF beam were due to the unique beam profile, softer energy spectrum,

3.C | Clinical follow-up outcomes
2. This is a radiosurgical dose distribution in three views (axial-, coronal-and sagittal) and the corresponding DVH for ITV (red), PTV (orange) and OAR for patient #6 treated with noncoplanar VMAT plan. This plan was normalized to deliver PTV D95 full dose, and the blue isodose colorwash (50% isodose spillage) constricted within D 2cm around the target volume. PTV was 10.7 cc and located in the right-middle lobe. The cross-hair shows the isocenter location and OAR contours of ribs, esophagus, bronchial tree, spinal cord, normal lung, and skin are shown. OAR, organs at risk; PTV, planning target volume; VMAT, volumetric modulated arc therapy. smaller out-of-field scatter and leakage characteristics compared to traditional 6X-FF beam as discussed above. The main advantages of the 6X-FFF VMAT plan was significant reduction of BOT. Compared to traditional flattened 6X-FF beam, the total number of MU did not change significantly while using 6X-FFF beams, suggesting that both plans had similar plan complexity and hence provide similar beam modulation. However, due to the faster dose rate, the average BOT for 6X-FFF VMAT plan was 6.5 ± 1.5 min that was much shorter than 6X-FF VMAT plan (15.1 ± 3.6 min) and hence significantly affecting the overall treatment time.

CONF LICT OF I NTEREST
None.