Commissioning and initial stereotactic ablative radiotherapy experience with Vero

Abstract

The purpose of this study is to describe the comprehensive commissioning process and initial clinical performance of the Vero linear accelerator, a new radiotherapy device recently installed at UT Southwestern Medical Center specifically developed for delivery of image-guided stereotactic ablative radiotherapy (SABR). The Vero system utilizes a ring gantry to integrate a beam delivery platform with image guidance systems. The ring is capable of rotating ± 60° about the vertical axis to facilitate noncoplanar beam arrangements ideal for SABR delivery. The beam delivery platform consists of a 6 MV C-band linac with a 60 leaf MLC projecting a maximum field size of 15 × 15 cm² at isocenter. The Vero planning and delivery systems support a range of treatment techniques, including fixed beam conformal, dynamic conformal arcs, fixed gantry IMRT in either SMLC (step-and-shoot) or DMLC (dynamic) delivery, and hybrid arcs, which combines dynamic conformal arcs and fixed beam IMRT delivery. The accelerator and treatment head are mounted on a gimbal mechanism that allows the linac and MLC to pivot in two dimensions for tumor tracking. Two orthogonal kV imaging subsystems built into the ring facilitate both stereoscopic and volumetric (CBCT) image guidance. The system is also equipped with an always-active electronic portal imaging device (EPID). We present our commissioning process and initial clinical experience focusing on SABR applications with the Vero, including: (1) beam data acquisition; (2) dosimetric commissioning of the treatment planning system, including evaluation of a Monte Carlo algorithm in a specially-designed anthropomorphic thorax phantom; (3) validation using the Radiological Physics Center thorax, head and neck (IMRT), and spine credentialing phantoms; (4) end-to-end evaluation of IGRT localization accuracy; (5) ongoing system performance, including isocenter stability; and (6) clinical SABR applications.

Figure 1

Head and neck end‐to‐end localization phantom (a), with various inserts for hidden targets, film and ion chamber dosimetry (top‐left), radiograph showing hidden targets (bottom‐left), and CT scan with pinpoint ionization chamber in place (bottom‐right). Thorax end‐to‐end localization phantom (b), showing the small and large unit density lung tumors into which dosimeters can be placed (bottom‐left) and hidden targets on axial and sagittal CT reconstructions (right). Holes to place ion chambers within the vertebral body and spinal cord are also visible in the sagittal reconstruction (bottom‐right).

Figure 2

Vero percent depth dose for $1\times 1$ to $15\times 15\hspace{0.17em}{\mathrm{m}}^2$ fields, compared with similar data from a Novalis Tx unit.

Figure 3

Inline profiles (a) and crossline profiles (b) at three depths in water, normalized to ${\mathrm{D}}_{\text{max}}$.

Figure 4

Diagonal profiles for a $15\times 15\hspace{0.17em}{\text{cm}}^2$ field at three depths in water, measured using a stereotactic diode and pinpoint chamber.

Figure 5

Inline profiles in air (a) for $2\times 2$ to $15\times 15\hspace{0.17em}{\text{cm}}^2$ fields, normalized to the central axis $(2\times 2)$, 80% of the central axis $(10\times 10)$, and 60% of the central axis $(15\times 15)$. Crossline profiles in air (b) for the same conditions.

Figure 6

Leakage profile through the center of the closed MLC, as indicated.

Figure 7

Planned dose distributions for RPC head and neck IMRT, spine and thorax phantoms (left to right).

Figure 8

SBRT plan for a patient with small lung metastases, showing ITV, PTV, $\text{PTV}+2\hspace{0.17em}\text{cm}$, and central zone contours (top left). Dose calculation performed using Monte Carlo (bottom left) and pencil beam (bottom right) algorithms, with the corresponding DVH for each (top right).

Figure 9

SBRT plan for a patient with spinal metastases, showing GTV, CTV and cord contours (left) and dose distribution showing 20, 14, and 10 Gy levels (middle). The calculated dose distribution superimposed on film dosimetry is shown at top right, with the resulting gamma distribution $(3\mathrm{\%}/3\hspace{0.17em}\text{mm})$ at bottom right.