Volumetric modulated arc therapy

Introduction

Volumetric modulated arc therapy (VMAT) was first introduced in 2007 and described as a novel radiation technology designed to deliver treatment quickly and accurately. If appropriate for the treatment of cancer, it can reduce the number of patient visits and greatly improving recovery time. Volumetric Modulated Arc Therapy (VMAT) can be used for treating various types of cancer as prostate, head and neck tumors, as well as a range of other cancerous tumors (1).

All of them used data of Varian accelerators with its implementation of VMAT called RapidArc® (Varian Medical Systems, Inc., USA) and either prototypes of planning or optimizing systems or Varian’s Eclipse™. The ability to generate complex dose distributions is highly dependent on the optimization algorithm and capabilities of the delivery system used for treatment (2) [16, 17].

Image courtesy of Varian Medical Systems (3)

   

The main advantages of Volumetric Modulated Arc Therapy (VMAT) are precision and speed. Volumetric Modulated Arc Therapy (VMAT) focuses the radiation on the tumor while protecting healthy tissues (1). Each Volumetric Modulated Arc Therapy (VMAT) may reduce treatment times to as little as two minutes (3). Faster treatments improve the accuracy of radiation delivery, in addition to improving patient convenience and quality of life.

Volumetric Modulated Arc Therapy (VMAT) uses photons (W-rays) generated by a medical linear accelerator. The VMAT technique allows beam-on during a full gantry rotation of 360° with simultaneous modulation of the multileaf collimator (MLC) and a variation of gantry rotation speed as well as dose rate (4). Very small beams with varying intensities are aimed at a tumor and then rotated 360 degrees around the patient (4). This results in attacking the target in a complete 3D (three-dimensional) manner,  radiation therapist will be able to see the tumour during patient´s treatment and target it directly.

Volumetric Modulated Arc Therapy (VMAT) Treatment involves three basic steps: diagnosis, treatment planning, and treatment delivery. As part of the diagnosis, the medical team generates 3D diagnostic images (usually CT and/or PET) of the patient’s anatomy and then uses these images to specify the dose of radiation needed to treat the tumor (3). This article aims to discuss the current use of VMAT techniques in clinical process of radiation therapy that can be divided into as simulation and patient data acquisition, treatment planning and treatment delivery.

Simulation and patient data acquisition

The process of virtual simulation and data acquisition can be summarized in the following steps:

• Determination of the patient’s treatment position.

• Setting references on the patient’s skin.

• Acquisition of CT data and transfer to the virtual simulation workstation.

• Volume definition, which consists of delineation of target volumes (tumors) and of healthy surrounding structures (normal tissues and organs at risk).

• Determination of the treatment isocenter with respect to the references.

• Choice of plan parameters such as number of fields, energy, field size and orientation.

Modern treatment planning systems (TPSs) incorporate packages for virtual simulation. In consequence, it is possible to transfer the CT data to the TPS and carry out the volume definition, virtual simulation and treatment planning in a single integrated system. CT simulators offer important advantages over conventional simulators, allowing for complex and accurate treatment planning based on 3D data sets.

References

1. M Teoh, CH Clark, K Wood, S Whitak, et.al. Volumetric modulated arc therapy: a review of current literature and clinical use in practice. Br J Radiol. 2011 Nov; 84(1007): 967–996.
2. Palma DA, Verbakel WF, Otto K, Senan S: New developments in arc radiation therapy: a review. Cancer Treat Rev 2010, 36: 393-399
3. Cleveland Clinic. Volumetric modulated arc therapy. Mars, 2017. http://my.clevelandclinic.org/health/articles/volumetric-modulated-arc-therapy
4. Wolff D, Stieler F, Welzel G, Lorenz F, Abo-Madyan Y, et al. Volumetric modulated arc therapy (VMAT) vs. serial tomotherapy, step-and-shoot IMRT and 3D-conformal RT for treatment of prostate cancer. Radiother Oncol 2009, 93: 226-33.