Short Answer:

Although surface-guided radiation therapy (SGRT) suppliers such as VisionRT may have promoted their system capabilities as including breath-hold gating, clinical practice and AAPM suggest otherwise — breast and intracranial tumors excepted.

Further Discussion:

We do not see SGRT systems as necessarily “competing” with spirometry-based respiratory gating (SDX^®); rather, we see these as complementary.

We believe that camera-based SGRT systems do a great job helping the staff with alignment and positioning of the patient, and monitoring the position of the patient during treatment. SGRT systems help ensure a reproducible patient position for each treatment. They also inform us about alignment discrepancies that can be observed by looking at the surface of the body, as do room lasers.

“Reproducible patient position” (SGRT) cannot be equated with “reproducible target position” (SDX). Cameras inform you about alignment discrepancies that can be seen, as do lasers. SDX ensures target positioning reproducibility for a tumor that you cannot see. SDX helps ensure a reproducible tumor position for each treatment.

Monitoring the position of the body surface using SGRT during breath hold or free breathing does not ensure target position reproducibility for deeper seated tumors, with only two exceptions. For intracranial procedures (Frameless SRS) and for tumors in the breast with DIBH, SGRT monitoring does ensure target position reproducibility.

Therefore, SGRT and SDX are complementary. A clinic needs both in order to position the patient and the tumor reliably and reproducibly.

SDX Respiratory Gating: Bridging the Gap Between Technology and Patient Care in Radiation Oncology

Cancers that occur in the thoracic and abdominal regions – such as breast, lung, liver, esophagus and pancreas – are considered to be “mobile” as they are impacted by the respiratory motion of the lungs. Radiation oncologists and medical physicists recognize the challenges of treating tumors susceptible to movement due to respiration. How can health care providers ensure the highest quality treatment for these patients?

Surface Guided Radiation Therapy (SGRT)

An initial approach could be to monitor the tumor motion by monitoring the body’s surface above the tumor. According to a report of AAPM Task Group 302, “The power of noninvasive real-time surface imaging has been harnessed for motion tracking and management with beam-hold capability (i.e., beam gating). As a result, Surface Guided Radiation Therapy (SGRT) has facilitated two clinical applications: (1) voluntary DIBH treatment for breast cancer and (2) frameless SRS. In both of these applications, the patient’s surface serves as a surrogate for the target without the need for any additional markers. Additionally, SGRT systems can be used to gate the beam either manually or automatically thereby halting treatment when the patient moves out of the specified tolerance.” Notice: not when the tumor moves … when the patient moves out of tolerance.

TG-302 provides guidance on clinical implementation of SGRT for three workflows that have been widely adopted as determined by a survey of physicists in the United States: general patient positioning, DIBH breast cancer treatment, and frameless SRS (brain).

According to the TG-302 report, “Because SGRT relies on tracking the patient’s surface, the accuracy for inferring the location of the internal target is degraded when the surface is not a good surrogate for the target, or when the surface is not visible to the cameras.”

“For SBRT treatment that includes advanced motion management techniques, such as breath hold, SGRT may not track the internal target with sufficient accuracy, and internal imaging may be more appropriate. A synergistic approach is to use SGRT in combination with X-ray imaging to efficiently verify internal target position while ensuring proper patient positioning.”- TG-302

The AAPM Task Group report seems to imply that for other than these two indications – breast and brain – when using SGRT, verification imaging is required for deeper-seated tumors in the thoracic and abdominal regions to make sure the tumor is positioned correctly with respect to isocenter. This verification imaging takes place before the treatment begins.

What happens when respiratory motion is taken into account?

According to Batista V, et. al., “For treatment volumes in the abdomen and pelvis, a reduced accuracy in positioning for both 3-point localization and SGRT is generally observed and SGRT can be used to monitor the correlation between chest and abdominal breathing during lung SBRT to detect baseline drifts, which may alter the tumor position even if the patient position has not changed.”

“Since the surface does not always correlate well with the internal treatment volume in the abdomen and pelvis, it is advisable to combine SGRT with internal verification images.”

Okay, so we use SGRT and X-ray verification images to ensure that the patient and the tumor are positioned correctly with respect to isocenter prior to treatment. What happens once the radiation beam is turned on? Is the patient breathing during treatment? Is the patient holding his/her breath? What about tumor motion once irradiation begins?

Overcoming the Challenges of Tumor Motion: How SDX Makes a Difference

The SDX Respiratory Gating System is an aid for ensuring internal target position reproducibility based on spirometry measurement. SGRT systems and room lasers are essential aids for aligning and positioning the patient with respect to the radiation treatment machine isocenter. SGRT is also used to continuously monitor the patient position during treatment, secondary to X-ray imaging techniques such as CBCT (used during initial setup), as well as monitor intra-fraction patient surface motion. However, beneath the surface, motion matters.

Challenging treatment sites requiring respiratory motion management, which stand to benefit the most from high-precision DIBH, in order from greatest to least benefit:

• Hepatocellular carcinoma (HCC)
• Non-HCC upper abdomen
• Pancreas
• Esophagus
• Thoracic malignancies (NSCLC) – lung
• Breast
• Lymphoma – mediastinum
• Bile duct
• Lymphoma (other than mediastinum)
• Kidney
• Retro-peritoneum

These sites are commonly treated with SBRT/SABR: higher dose, 2-5 fractions. Managing respiratory motion with breath-hold is advisable.

Breath hold benefits can:

• Reduce the size of the PTV: high dose restricted to only volume that needs it
• Separation between tumor and critical OARs avoids unnecessary complications
• Reduce normal tissue exposure/dose, and minimize/eliminate treatment side effects
• Target position reproducibility; escalate dose to tumor (SBRT)

Key Insights

SDX takes these benefits and adds the precision of ensuring that the same volume of air is in the lungs during training, imaging (CT-Simulation)/treatment planning, and each treatment session. When the patient holds his/her breath at the same air volume, the position of the tumor is located in the same place each time.

Why does this matter?

When patient-breath hold is administered by verbal instruction, the volume of air being held is not reproducible. There is no quantitative measurement of air volume. It’s guesswork. Yes, the target is motionless in space, but is the target center-of-mass located at isocenter? Nobody knows, unless some form of imaging is used to reveal internal anatomy. Imaging was already used during pre-treatment setup/positioning. We must be judicious about the use of CBCT/X-ray imaging repeatedly during treatment; too much imaging dose can add up to the equivalent of another treatment fraction. Even SGRT combined with verbally-instructed breath-hold does not improve target position reproducibility.

Recall the Key Insights: SDX ensures that during breath-holds, the same volume of air is in the lungs during each treatment session as it was during imaging (CT-Simulation)/treatment planning. And when the breath is held at the same air volume, measured precisely by SDX, the position of the tumor is reproducible during each treatment.

Breath-hold benefits without SDX vs. with SDX

• Reduce the size of the PTV: Without SDX, make an ITV large enough to encompass the tumor excursion and treat the entire volume; with SDX, tumor is stationary and its position is fixed – in the same location within the body for each treatment.
• Separation between tumor and critical OARs: Without SDX, the degree of separation is not measurable with the desired precision; with SDX, the degree of separation is the same for each treatment (e.g., getting a lung tumor away from the heart).
• Reduce normal tissue exposure/dose: Without SDX, we don’t know the exact position of the tumor since the volume of air may not be the same – tumor position uncertainty opens the possibility of greater normal tissue dose; with SDX, the exact position of the tumor is fixed – therefore the volume in space being irradiated will contain the tumor (PTV) and nothing else.
• Target position reproducibility: Without SDX, we don’t know the exact position of the tumor since the volume of air may not be the same; with SDX, the exact position of the tumor is fixed.

In an ideal world, the treatment room will have room lasers (backup to SGRT), SGRT for positioning and position monitoring, and SDX for precise and reproducible breath-holds.

In Conclusion

SGRT cameras/software inform you about patient body alignment discrepancies that can be seen, as do room lasers. SDX has no contribution to make when assessing patient alignment or monitoring patient position. SDX does ensure target positioning reproducibility for a tumor that you cannot see.

SGRT/camera VS. SDX/spirometry

Patient Position ≠ Tumor Position

Body Surface Position ≠ Tumor Position

Visible Displacement ≠ Invisible Displacement

Contact SDX

Get in touch today to learn how the SDX System can transform patient care through advanced respiratory gating technology. Contact us at (833) 739-3967 in North Miami Beach, FL, or fill out our online form for more information.