SRS / SBRT

Couch Kicks and Vertex Angles

In cranial SRS, vertex beams are used to create dose homogeneity and improve target conformality. A 2020 survey on the topic revealed that 62.1% of respondents use vertex angles in 50% or more of their cranial SRS cases to help shape their dose distributions. But not all clinicians utilize couch kicks in the same way. 13.3% said they use two angles, 16.7% used three angles, 40% used four angles, and 16.7% used five or more angles. Two replied in the free response section that they use just one single couch angle for SRS plans.

When it came to performing QA, specifically using a MapCHECK, there was no consensus on setup regarding vertex angles. 44.8% set all couch angles to zero for patient-specific QA, with some citing the potential for high dose delivery to electronics. Meanwhile, 44.8% used the actual couch angles, perhaps to work in couch walk uncertainty into the analysis.

SRS Volume and Quality Assurance

SRS volumes have increased in step with the number of clinics offering the special procedure, and for a number of reasons: fewer fractions overall, increased cost-effectiveness, and similar toxicity in healthy tissue when treatment is delivered accurately. In 2016, 38.5% of users clinics performed 10 SRS treatments or fewer over a year. Three years later, that number has reduced to 10.3%. For the remaining categories, 11-30, 31-50, 51-100, 101-200 all exhibited an increase over the three years with the exception of the 201+ group.

How each center chooses to perform SRS cases with multiple lesions is a point of contention. In a 2019 survey, 36.6% of survey takers indicated they treat multiple targets using a single isocenter, 24.4% employ multiple isocenters, and 39.0% admit to using both depending on the case. When it comes to quality assurance for multi-met cases, a survey shows that 79.1% and 80.0% of clinicians thought that current methods for patient-specific QA and machine QA, respectively, were sufficient in verifying single iso, multi-lesion SRS cases. Those that did not agree noted a need for additional QA steps, such as an off-axis Winston-Lutz to test for rotational inaccuracies.

SRS Case Volume Per Department

SBRT Lung

For VMAT Lung SBRT, the use of flattening filter free (FFF) beams for treatment is relatively ubiquitous, with 74.4% using FFF for treatment planning according to a 2016 survey. Lung SBRT plans face a variety of challenges in treatment planning and delivery. One very important clinical decision is how to handle respiratory motion during treatment (immobilization or gating). The community appeared divided over abdominal compression, with 46.8% of clinicians using abdomen compression and 43.5% choosing to not limit motion in this way. The same survey showed that 68.8% that perform SBRT using VMAT and FFF did not make attempts to reduce the modulation during planning, and 88.3% did not limit the dose rate during treatment.

For patient simulation, capturing tumor motion is critical for accurate targeting. The most common imaging technique for SBRT treatment planning is the use of time average scans at 50%. Unadulterated free-breathing scans (without any density corrections) were acquired 29.1% of the time, while others that used free-breathing scans added correction densities to the ITV and/or PTV to augment their dose calculation accuracy (16.2%).

Lung cancer is the third most common form of cancer in the United States, according to the Centers for Disease Control and Prevention (CDC). As SBRT has been shown to have excellent outcomes, it has seen widespread adoption in the last decade. The predominant technique for treating lung with SBRT is VMAT/Rapid Arc, with 54.8% employing rotational delivery according to a 2015 survey. 3DCRT follows at 15.1%, IMRT at 8.7%, and Tomotherapy at 1.6%. Others did not have a preferred method, stating the delivery technique was patient dependent. Users suggest reviewing the literature on minimum field size for correct dose delivery, target doses, fractionation, normal tissue dose constraints, and QA to those looking to start an SBRT program.

Winston Lutz Test Practices

The Winston-Lutz (WL) test is the gold standard for isocenter congruence verification and an integral part of mechanical QA, especially for departments with SRS and SBRT programs. In advance of an SRS, 52.0% said they perform the test the morning of the treatment, 40.8% perform immediately before treatment, and 7.2% run the QA test the night before. For SRT and SBRT treatments, a larger proportion, 69.7% , performed WL testing the morning of, 19.7% immediately before, with 10.6% testing the night before.

Unsurprisingly, the majority of respondents indicated that the physicist performed the WL testing—77.3%—while other departments delegated the data collection to the therapist (20.0%) or dosimetrist (2.7%). A portal imager acquisition was the most popular option for data collection (74.8%) while others used film (25.2%).