Apr 13, 2021
VMAT TBI provides homogenous dose distributions and effective OAR protection with the patient in a comfortable supine position
Dr. C.P. Bhatt
In two Elekta case studies, Dr. C.P. Bhatt from the Sarvodaya Cancer Institute in India and Dr. I.F. Durmuş from Yeni Yuzyil University Gaziosmanpasa Hospital in Istanbul, Turkey share their experiences of implementing VMAT for the delivery of Total Body Irradiation (TBI). They describe how VMAT TBI is easily and safely implemented in centers where VMAT is already a standard treatment delivery technique, without the need for bunker modifications, customized treatment accessories, or special training.
TBI is used in the treatment of several diseases, including multiple myeloma, leukemias, lymphomas, and some solid tumors, and has a role to play in conditioning patients with hematological malignancies prior to receiving a bone-marrow transplant (BMT)1,2. Traditional TBI delivery requires an extended source to surface distance (≥ 4 m) and the patient is positioned upright in a TBI stand with customized shields to protect organs at risk (OAR). Challenges associated with this method include non-uniform dose distributions and lack of OAR protection, long patient setup and treatment times, high delivery uncertainties, and time-consuming customization of stands and shields.
By comparison, linac-based VMAT TBI offers excellent dose homogeneity, high precision in patient positioning and dose delivery, less toxicity for specific OARs, and improved patient comfort3. In addition, the open design of Elekta linacs permits inter/intrafraction kV CBCT imaging and optical surface image guidance, which help to assure patient positioning for accurate dose delivery3.
Improved TBI planning
Dr. I.F. Durmuş
“Accurate and homogenous dose distributions for VMAT TBI are achieved using the gold standard Monte Carlo dose calculation algorithm in Monaco® and the Agility™ multileaf collimator,” comments Dr. Durmuş.
“VMAT offers a number of advantages for TBI,” Dr. Bhatt explains. “The patient lies on the couch in a comfortable supine position, which is easier for simulation, improves overall treatment setup, and increases patient comfort. Furthermore, VMAT planning for TBI provides a more conformal and homogenous dose distribution to the target and reduces dose to critical organs, such as the lungs, kidneys.”
“With Agility’s 5 mm leaf width, 40 x 40 cm field size, and effective leaf speeds of 6.5 cm/s (combined with the jaw speed of 9 cm/s), Monaco effectively optimizes the VMAT plan to achieve a uniform and homogenous dose to the large PTV, while restricting OAR dose effectively with the system’s virtual 1 mm leaf width capabilities,” he continues. “In addition, since VMAT TBI requires a high number of MUs, Agility’s very low transmission (less than 0.5% for 6 MV) helps to reduce the contribution of transmission dose to the VMAT TBI plan.”
Figure 1. A VMAT TBI plan, showing dose color wash and low dose islands for lungs, kidneys and liver.
Both sites deliver 12 Gy in six fractions over three days for TBI, using multiple isocenters and VMAT arcs. The patient is simulated in the feet-first position as well as the head-first position, depending on the length of the patient, and a composite plan is created, using Monaco’s bias dose function to manage high dose regions at the junction. Multi Criteria Optimization (MCO) can also be used to decrease OAR dose as much as possible, without compromising the target coverage.
Increased departmental efficiency
In addition to improving TBI plan quality, Monaco allows fast and efficient planning, while VMAT delivery reduces labor-intensive, time-consuming tasks that are associated with conventional TBI.
“By optimizing multiple isocenters in a single optimization, utilizing MCO to reduce OAR dose as much as possible, Monaco reduces planning time and improves overall plan quality,” says Dr. Bhatt.
“By optimizing multiple isocenters in a single optimization, utilizing MCO to reduce OAR dose as much as possible, Monaco reduces planning time and improves overall plan quality.”
“In addition, TBI using VMAT improves departmental productivity because it is less labor-intensive than conventional TBI,” he adds. “The VMAT method allows the patient to lie in a comfortable supine position, without the need for customized stands or placement of shields, saving significant setup and verification time.”
“Compared to alternative TBI techniques, this method provides greater accuracy, reliability, and OAR protection, with reduced uncertainties,” concludes Dr. Durmuş. “Homogenous dose delivery can be achieved with minimal side effects, resulting in high BMT success and long-term disease-free survival.”
How Elekta solutions benefit VMAT TBI
Assoc. Prof. Bora Taş, Head of the MR-Linac and Linac Business Line for Elekta RTIMEA (Region Turkey, India, Middle East and Africa) and one of the pioneers of linac-based VMAT for TBI, implemented this technique to his clinical practice in 20153.
“Linac-based VMAT for TBI offers higher dose conformality due to its 360° of beam application (compared to the standard fixed beam approach); Agility’s 160 interdigitating 5 mm leaves across the full 40 cm by 40 cm field width; and the unique capabilities of Monaco and Agility to move the Y jaws from segment to segment,” he says.
“The jaws can be parked at different positions for each segment and even within the width of an MLC leaf,” he explains. “The leaves and jaws can be placed in 1 mm increments, creating a virtual leaf width of between 5 mm and 1 mm. This provides steeper dose gradients, allowing high dose to be delivered to the target while ensuring low dose to surrounding healthy tissue or critical structures. Lower MLC leakage, fast leaf and dynamic jaw speeds and IntelliBeam combine to ensure faster arcs and high definition conformality.”
The dual arc capability of Monaco is useful for treating large fields because Monaco optimizes and segments one side during the first rotation (figure 2A) and the other side during the second rotation (figure 2B). Since the MLC leaves do not travel from edge to edge, the TPS can create more effective segmentation with less MUs for large treatment fields. In addition, high or low dose areas in the junction region between head-first and feet-first plans can be eliminated using the bias dose feature in Monaco 5.11. Monaco ensures maximum dose to the target while providing better critical structure sparing (reducing the risk of acute side effects) and better delivery efficiency.
“Linac-based VMAT TBI has clear advantages compared to conventional methods, making its use very attractive,” Professor Taş concludes. “With partial low dose kV CBCT to ensure accurate patient positioning, this method offers excellent dose homogeneity, high precision dose delivery, less toxicity for specific OARs, and improved patient comfort.”
“Linac-based VMAT TBI has clear advantages compared to conventional methods, making its use very attractive.”
To access the case studies, click here.