Published by Radformation Survey Team on 1/7/2021
Items of Consensus
How often does a group of physicists agree on something? Let’s put it this way: when over 80% of respondents agree on a survey topic, it’s worth pointing out. Here are the items of consensus for the surveys that fall into this particular category.
Brachytherapy for Gynecologic Cancer
Brachytherapy is a common alternative or adjunct to external beam radiation therapy for the treatment of gynecologic cancer. For the treatment of cervical cancer specifically, the radionuclide of choice is Ir-192, with 96.5% of clinics using it according to a 2015 survey. Historically, Cs-137 was a popular option for gynecologic indications. According to the survey, only 3.5% use Cs-137 for cervical cancer. However, a 2016 survey indicated that 23.3% of respondents still use Cs-137 for GYN indications. One survey taker admitted, “We have Cs-137 ‘just in case,’ but refer most patients out for HDR. We haven’t used the Cs sources in 3+ years.”
IMRT/VMAT Beam Selection
Beam energy and filtration varies between treatment sites to achieve the desired dose rate and distribution. The most preferred energy for VMAT is 6X for most sites, including mediastinum (68.9%), lung (63.0%), and the brain (81.3%). Among survey takers, 10X is most preferred for pelvis treatments, with 56.3% of respondents choosing 10X over other energies. Most users reported using FFF beams solely for SRS and SBRT cases.
Custom cutouts are used in electron treatments to create the boundary of the treatment field. The shape will influence the monitor units needed for a given fraction. To account for the cutout factor, a 2013 survey showed 32.5% of respondents use an existing library of premeasured cutouts, 31.2% measure each cut out individually, 20.8% rely on software and 14.6% choose to ignore the cutout factor unless the field is below a certain size. For those that choose to measure cutout factors individually, 81.3% use an ion chamber in solid water, 17.2% use a diode or MOSFET, and 1.6% use film in solid water.
The choice of SSD convention for electron fields appears to be split fairly evenly within the community. In a 2020 survey, 54.2% indicated using a source-to-bolus distance of 99cm and adding 1cm bolus (100SSD) while the remaining 46.8% used an SSD of 101 cm. One user noted their preference for 100SD when performing hand calculations, “Anything other than 100 source to surface distance would require an inverse square correction.”
IMRT Boost Techniques
Boosts involve the delivery of additional dose to a reduced treatment area, and are important in treating and controlling high-risk sites. Depending on the site, additional dose is achieved using a simultaneous integrated boost (SIB)—in which elevated dose is delivered along with lower dose levels within the same plan—or delivering plans in a sequence. For prostate, brain, lung, and breast treatment, respondents in a 2017 survey heavily favored sequential boosts at 78.3%, 71.9%, 73.9%, and 83.9% respectively. When it came to head and neck (H&N) cases, users chose to perform a SIB boost 74.0% of the time.
For those that opted for sequential boosts, 58.1% chose to start treatment with the boost plan while 41.9% chose to treat the boost plan later in the course after other plans.
Use of Sequential Boost Planning Per Site
IMRT/VMAT QA Methods
Patterns in IMRT/VMAT QA methods appear to be relatively static over the past several years, with only minor changes over time. In 2013, 3D detector-based patient-specific QA was the predominant method at 67.0% followed by 2D detectors at 15.4%, EPID at 8.8%, and film at 3.3%.
In 2017, 3D detector QA remained the predominant method with 51.6% indicating it was the primary tool for IMRT/VMAT QA. EPID use increased over the four year period, becoming the second most used with a 20.1% adoption rate. 2D detectors dropped to 12.6% and film went to 1.3%.The most noticeable change was the development of log file-based QA, which increased in popularity from 0% in 2013 (it was not a response option) to 11.9% in 2017. One user hailed the use of of log file QA for its ability to catch equipment issues, stating, “Any minor equipment issues like MLC position, gantry speed, dose rate, etc won't be disclosed by examining 2D or 3D detector or EPID/Film based measurements. However, the logfile shows up all treatment parameters used and they should be compared to the treatment parameters from TPS to verify dose difference caused by equipment. The point dose measurement is to ensure the delivered dose is consistent with the planned dose.”
IMRT/VMAT QA Methods: 2013 -vs- 2017
Electron MUs and Isodose Plans
The way we perform electron monitor unit (MU) calculations is changing. From hand calculations in homogeneous media to 3D volumetric images and Monte Carlo calculations, our field has seen rapid changes in dose calculation methods in just the last decade. In a 2013 survey, 86.5% of respondents used independent hand calculations to determine MUs in electron treatments. At the same time, only 13.5% admitted to using results from a treatment planning system (TPS).
Just one year later, the number of users performing independent hand calculations dropped to 75.6%, and a follow-up 2016 survey saw the number decrease further to 45.7% of respondents. For those using TPS calculations, more sites are incorporating inhomogeneities than in previous years. In 2013, 76.9% created TPS plans to account for heterogeneities, but by 2016 that number shot up to 92.7% of respondents.
Trends in Electron Plan Calculations
IMRT/VMAT Planning and Air Cavities
When it comes to planning, managing air cavities is important due to changes in dose build-up around the tissue-air boundaries. The disparity in tissue densities can pose a challenge for TPS algorithms. Some clinics choose to modify air cavities to compensate for these shortcomings. In a 2017 survey, 73.7% of those planning H&N sites said did not perform any modifications. Among those that did perform modifications (26.3%), 50% relied on Eclipse's Air Cavity Correction, 30% performed either an air subtraction or TPS inhomogeneity calculation, and 20% contoured and manually assigned tissue density.
Regarding air cavities due to flatulence in pelvis IMRT/VMAT planning the results were similar: 65.2% did not perform any modification. Of those that did modify (34.8%), 40.0% contoured and assigned a tissue density, 32.5% relied on Eclipse's Air Cavity Correction, and 27.5% chose to re-simulate the patient or override the HU.
Air Cavity Modifications - Head-Neck
Air Cavity Modifications - Pelvis
A Special Thanks
Thanks to Scott Dube for providing access to over 275 medical physics community surveys for public use. For further reference, a JACMP article by Kisling, et al. provides a complementary analysis of survey results.