Optimized DCAT promises faster SBRT delivery

Monaco 5.1 for Dynamic Conformal Arc Therapy (DCAT) improves plans through Segment Shape Optimization and Variable Dose Rate

• Optimized DCAT is a unique approach to DCAT treatment planning available only in Monaco
• Steep reduction in monitors units significantly reduces treatment time
• Patients with simple lesions not close to OARs are ideal candidates

At UT Health San Antonio, as many as 40 to 60 percent of patients who would normally receive SBRT with VMAT delivery could be candidates for SBRT plans delivered with optimized DCAT, predicts UT Health physicist Sotiri Stathakis, PhD. The benefit for these patients – those who can’t tolerate a long therapy session and/or who present simpler cases that do not require the extreme modulation of VMAT – is a DCAT beam-on time as much as 2.5 times less than that of a VMAT fraction yet with comparable plan quality.

DCAT, optimized with Segment Shape Optimization (SSO) and Variable Dose Rate, is available only with Monaco version 5.1 or newer versions.

In preparation to begin treating lung or liver cancer patients with optimized DCAT later this year, Dr. Stathakis and his team conducted a retrospective study comparing optimized DCAT SBRT plans created by a research version of Monaco 5.1 to VMAT SBRT plans for each of 20 patients previously treated for lung or liver cancer. They recorded target size, PTV, monitor units (MUs), the volume receiving 100 percent of the dose and 50 percent of the dose for each patient’s DCAT and VMAT plan.

“We found that the average number of MUs in optimized DCAT plans was decreased by a factor of 2.5 times – which significantly reduces beam-on time – while plan quality was generally maintained.”

“We found that the average number of MUs in optimized DCAT plans was decreased by a factor of 2.5 times – which significantly reduces beam-on time – while plan quality was generally maintained,” he says. “All of the DCAT and VMAT plans for lung and liver tumors met the recommended dose objectives.”

For lung targets that move due to patient breathing, the optimized DCAT plans were actually superior, he adds. This part of the study was conducted with a moving lung phantom.

MU reduction VMAT v. DCAT

“VMAT spares OARs by blocking the radiation using MLC leaf motion, but if the target is moving the MLC leaves might end up blocking the radiation to the target,” Dr. Stathakis explains. “With DCAT, the target is always in the open field with target motion accounted for in the plan and with the MLC leaves never crossing into the port; the prescribed dose is always delivered. Our results so far show that at the center of the target, the DCAT doses agree within 0.5 percent to plan, whereas the VMAT doses are within three percent. We attribute these differences to the interplay between MLC motion and target motion.”

Overcoming the limitations of “conventional” DCAT

DCAT is similar to VMAT in that the treatment is delivered continuously as the gantry rotates around the patient. Unlike VMAT, however, the beam is not continuously modulated as it rotates; the MLC leaves conform to the 2D target shape as seen from the beam source. The plan quality of traditional DCAT has been inferior to that of VMAT due to issues related to dose rate and dose homogeneity.

“In lung cancer patients, if you use the conventional constant dose rate, the dose will be skewed higher where the distance to the target is shortest,” he says. “You won’t be able to achieve a conformal isodose distribution, even though the MLC shapes are conformed to this arc. This is particularly true for targets that are off center axis.”

Monaco’s variable dose rate (VDR) feature for DCAT optimizes the delivery technique by allowing the delivery of more or less dose at a given gantry angle.

“The segment dose rate changes or the gantry speeds up or slows down to compensate for beam-to-target differences, evening out the dose deposition so that the dose distribution becomes more conformal,” Dr. Stathakis notes.

The Segment Shape Optimization (SSO) capability for DCAT in Monaco is a refinement to IMRT/VMAT plans that is used to improve the match between the ideal fluence-based plan and the segmented, deliverable plan. SSO smooths and clusters DCAT arc control points [segments], then optimizes beam weights and shapes to enhance OAR sparing and dose conformality. It changes the DCAT plan from a forward plan – in which the user provides all the inputs, and calculates, reviews and adjusts until a favorable result is reached – to an inverse plan in which the user enters goals, constraints and optimization parameters and allows Monaco to find the best result.

“Because the planning system can’t perfectly replicate the continuous delivery of a DCAT arc from starting to stopping angles, it has to discretize the arc to create a plan, creating control points every two or one or half-degree of arc, depending on how much dose Monaco decides it needs or can safely deposit per control point. In a full arc, we could end up with 150 to 500 control points depending on the needs of the plan,” he explains. “However, not every control point needs to contribute the same amount of dose to the target. For example if the spinal cord is in beam path that control point will contribute less dose so we don’t over-irradiate the spinal cord. Conversely, another control point might have no OARs in the beam path, so we can deliver much more dose. We also have to shape what the beam sees toward the target. SSO takes those shapes and decides which control points will have higher or lower dose contributions to the target.”

Cases for optimized DCAT

“Forty to 60 percent of the approximately 200 SBRT patients treated at our center per year will be excellent candidates for DCAT, while still maintaining plan quality that is comparable to VMAT.”

Simple, spherical targets with no concavities and which are not close to organs-at-risk are ideal candidates for DCAT, according to Dr. Stathakis.

“We would still use VMAT in certain cases, for example a lung tumor next to a rib or the heart, but for more isolated tumors that don’t have complex features or that are moving excessively, DCAT can be a great option for these patients,” he says. “Forty to 60 percent of the approximately 200 SBRT patients treated at our center per year will be excellent candidates for DCAT, while still maintaining plan quality that is comparable to VMAT.”

The major reduction in MU’s with optimized DCAT translates directly to decreased beam-on time, which is perfect for patients who can’t tolerate a longer treatment without moving, Dr. Stathakis adds.

“The less time the patient is on the table the less movement we’re going to get, the more comfortable the patient is – and at the end of the day, the more patients we can treat,” he observes. “Because DCAT will take less time to deliver, one of the possibilities for the future is deep inspiration breath hold treatments. If the beam-on time is just two minutes with DCAT, versus 10 minutes with VMAT, patients with good lung functionality would be required to hold their breath just three of four times for 20 to 30 seconds each breath-hold. A VMAT treatment would require a lot more breath-holds, so DCAT could be an attractive option for select patients.

“DCAT is a tool that can hopefully be applied to other sites,” Dr. Stathakis adds. “We are looking for application in the pancreas, brain and even prostate treatments. It can simplify and shorten treatments. It won’t be a tool used on every patient, but I think that as we gain more experience we will be able to identify the patients that can benefit from such treatments.”