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.
Small Lung PTVs
Treating with high energy photons is common in radiation therapy. However, when it comes to small tumors in the lung, the effect of the long recoil electron distances in low-density lung tissue leads to less-than-ideal coverage caused by a lack of lateral electronic equilibrium. This is at least one reason why 90.5% of respondents indicated that they never use 15X or 18X for small lung targets.
Target delineation for lung treatment planning poses challenges given the relatively large amount of motion caused by patient breathing. To create a volume that encompasses all respiratory motion, an internal target volume (ITV) is created. During simulation, 82% of survey takers acquire a 4DCT and use data binned in multiple breathing phases to put together a final image to use as a target. In a separate survey, 64.8% use the 4DCT to create a maximum intensity projection (MIP), while others contour the GTV on each slice and integrate over all phases (22.1%) or other techniques.
Composite and Sequential Plans
Creating composite plans is routine for patients with multiple plans, and when it comes to piecing together a dosimetric picture including prior irradiation, 87.2% of clinics generate a composite isodose distribution if a patient had previous treatment.
In mixed-modality environments, not all departments document their plans the same way. For example, for courses of treatment that involve standard treatment and SRS, only 37.5% generate a composite plan. About the same number of respondents (36.4%) said they create a composite plan if one of the plans involved HDR. A larger proportion—58.7%— create a sum plan for SBRT coupled with other XRT No one reported performing HDR-LDR composite plans.
When a treatment consists of multiple sequential plans as often occurs for breast or prostate cancer, a large majority, 86.7%, print all associated plans along with a composite. One comment indicated this is to provide "proof that each part was looked at individually and totally for coverage, hot/cold spots.”
Small Field Dosimetry Correction Factors
Published in 2017, IAEA’s TRS-483 proposes implementation guidelines for obtaining accurate small static field output factors. A survey from 2019 indicated that 71.4% of users were aware of IAEA TRS-483's new recommendations. Of those who were aware, 93.3% had applied them clinically, and 3.3% were in the process of implementing changes. For those that had implemented correction factors, 94.4% observed no change in how physicians prescribed dose to small targets. Methods for validating final beam models included film (29.6%), comparing to pooled data (25.0%), diodes (18.5%), scintillation detectors, and daisy-chaining between multiple detectors.
Methods for Validating Final Beam Models
AAA vs. Acuros XB
As radiation oncology clinicians, we must balance accuracy with efficiency based on our clinical needs. Eclipse's Analytical Anisotropic Algorithm (AAA) and Acuros XB are both excellent algorithms, but have different clinical strengths and weaknesses. In a 2015 survey on the topic, 78.8% of Eclipse users noted they still use AAA for almost all cases except for SRS/SBRT cases. For users that have both algorithms commissioned, the primary benefit of Acuros was its accuracy in the presence of heterogeneity for IMRT, VMAT, and lung cases.
One user pointed out, for migrating from AAA to Acuros, "You have to explain the difference between Dw to Dm, and have clinicians understand/appreciate that their lung V95% were never really high to begin with...especially the minimum dose to lung PTVs. But other than that, the differences are not as big as you might think. The question remains, however, is it worth the extra calculation time?"
In a 2020 blog post on dose reporting methods for Acuros XB, we polled readers to find out what reporting method they use clinically. Here are the results.
Which cases are most often planned with Acuros?
- Heterogeneous Tissue
- High-Z Implants/Immobilization
- 3D Conformal Arcs
- Head and Neck
TrueBeam linacs are becoming increasingly popular as C-series models are replaced with the all digital platform, which features flattening filter-free (FFF) energies. In a 2019 survey on TrueBeam utilization, all survey respondents indicated they had the 6X energy, but other energies were adopted to fit individual needs. The least common energy reported was 10X-FFF, with an 81.6% adoption rate.
As to how often the different energies were used for patient plans, the 6X beam was most popular, being used on 67% of all patients. The second most used energy was 10X, playing a part in 18.3% of patient plans. One user remarked, “10MV is the bomb, Clinac 18 had it right in 1976…” The 6X-FFF and 10X-FFF were used in 12.1% and 9.3% of plans, respectively, exploiting their high dose rates of 1400 and 2400 MU/min on SBRT and SRS cases. Finally, higher energy beams (15X/18X) were used in approximately 14.2% of cases.
Beam Utilization by Percentage of Patients
Wedges: Physical, Virtual and Dynamic
Physical and virtual wedges are essential tools in shaping dose distributions. Physical wedges appear to be falling out of vogue. A 2015 survey showed only 54.7% of users using physical wedges in any capacity. As field-in-field and enhanced dynamic wedge techniques are both widely available with no need for therapists to manually swap hardware between fields, 45.3% of dosimetrists and physicists admit they no longer use the physical wedge. Physical wedges are still reserved by some clinics for special cases where jaw dimensions do not permit the use of EDWs.
Total Body Irradiation
For those that perform TBI, the most common beam arrangement for treatment is AP/PA at 46.8% or R/L Laterals at 37.1% , with others performing a combination of IMRT, Tomotherapy or 6-field setup. Additionally, 67% of clinicians performing TBI treated patients lying down, while 21.0% chose standing and 14.5% sitting. When performing dose calculation, a majority of the 2016 survey respondents–80.7%–used hand calculations or in-house software, with the remaining using a TPS calculation. A vast majority (83.6%) verify treatment dose using in-vivo dosimetry of some kind. Of the available options, diodes are the most popular with 41.2% of respondents favoring them, followed by OSLDs at 19.6%, TLDs at 15.7%, and MOSFETS at 11.5%. Others used film or ion chambers to confirm the delivered dose.
Contouring accurately is essential when assessing the dose to a given volume or structure. In many clinics, OARS are delegated to the dosimetrists. Physicians typically draw targeted contours that receive prescription dose, though there are exceptions to the rule. For H&N and pelvic lymph node beds, 12.5% and 16.5% of dosimetrists stated they contoured the nodal regions, respectively. Slightly more—29.6%–admitted to contouring the lumpectomy bed. The bowel bag, which qualifies as an OAR, had a much higher rate of dosimetrist contour (65.4%).
What Do Dosimetrists Contour?
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.