A survey of surface imaging use in radiation oncology in the United States

Abstract Surface imaging (SI) has been rapidly integrated into radiotherapy clinics across the country without specific guidelines and recommendations on its commissioning and use aside from vendor‐provided information. A survey was created under the auspices of AAPM TG‐302 to assess the current status of SI to identify if there is need for formal guidance. The survey was designed to determine the institutional setting of responders, availability and length of its use, commissioning procedures, and clinical applications. This survey was created in REDCap, and approved as IRB exempt to collect anonymized data. Questions were reviewed by multiple physicists to ensure concept validity and piloted by a small group of independent physicists to ensure process validity. All full members of AAPM self‐identified as “therapy” or “other” were sent the survey link by email. The survey was active from February to March 2018. Of 3677 members successfully contacted, 439 completed responses; the summary of these responses provides insight on current surface imaging clinical practices, though they should not be assumed to be representative of radiation oncology as a whole. Results showed that 53.3% of respondents have SI in their clinics, mostly in treatment rooms, rarely in simulation rooms. Half of those without SI plan on purchasing it within 3 years. Over 10% have SI but do not use it clinically, 36.8% classify themselves as “expert” users, and 85.5% agreed/strongly agreed that SI guidelines are needed. Initial positioning with SI is most common for breast/chestwall and SRS/SBRT treatments, least common for pediatrics. Use of SI for intra‐fraction monitoring follows a similar distribution. Gating with SI is most prevalent for breast/chestwall (66.0%) but also used in SBRT (33.0%), and non‐SBRT lung/abdomen (<30%) treatments. SI is a rapidly growing technology in the field with widespread use for several anatomic sites. Guidelines and recommendations on commissioning and clinical use are warranted.

ability to perform these tasks without the use of ionizing radiation.
The technical characteristics and a description of how current commercially available surface imaging systems work have been described elsewhere. 2 In brief, these systems can monitor the patient's position in real time using optical light and compare it to a given reference from either the external contour of the planning CT or an SI system-acquired capture. Typical applications of these systems, based on current literature, mainly include open-mask stereotactic radiosurgery (SRS) procedures and breast radiotherapy, particularly deep inspiration breath-hold (DIBH) treatments for leftsided breast patients. 1,3 Literature describing SI use for other sites is more limited. While it is evident that this technology is being increasingly used in radiation oncology, its prevalence, implementation workflows, or scope of use in the field have not been described to date. An electronic survey was conducted in an effort to compile this information.

| ME TH ODS
A questionnaire was designed to assess the extent of use of SI for radiotherapy in the United States and gain more insight on its implementation in the field. Questions were crafted to inquire about the availability of this technology in clinics, existing commissioning procedures, and its role in current clinical practice regarding both its applications and common treatment sites of use (see Table S1). This survey was deemed IRB exempt after institutional board review at The University of Chicago as all the responses were anonymized and aggregated and could not be related back to the participants. The survey, along with text outlining its purpose, length, participation consent, and anonymity of results, was sent out via email to all full members of the American Association of Physicists in Medicine who self-identified as specializing in "therapy" or "other" and had a mailing address in the U.S. Both the survey questions and the text used in the survey invitation are listed in Table S1. The survey was active from February to March of 2018.
Study data were collected and managed using REDCap (Research Electronic Data Capture) electronic data capture tools hosted at The University of Chicago. 4 REDCap is a secure, web-based application designed to support data capture for research studies, providing (a) an intuitive interface for validated data entry; (b) audit trails for tracking data manipulation and export procedures; (c) automated export procedures for seamless data downloads to common statistical packages; and (d) procedures for importing data from external sources.
Survey questions were organized into two sections. The first one was to determine the institutional setting of the responder, the availability and duration of use of the technology, and the commissioning process performed upon initial acquisition of the system(s). The second section focused on the clinical uses of surface imaging, including applications (e.g., initial positioning, intra-fraction monitoring, gating) and types of treatment (e.g., anatomical site and type of procedure conventional, stereotactic, pediatric). All questions were reviewed by more than ten physicists for concept validity. The survey was tested by a small cohort of physicists independent from the survey creators to ensure response process validity prior to deployment for data collection. The survey length was intended to be brief: 10 min for participants who had surface imaging and 2 min for those who did not.

| RESULTS
There were 205 undeliverable emails of the 3882 emails originally sent. We received 509 responses, 439 were complete. Only complete responses were used. The overall response rate was 13.8% (from self-identified "therapy" and "other" AAPM members). The response rate from "therapy" only members was 14.7%. reported that their clinic plans to purchase an SI system within the next 1-3 years. Out of the respondents with SI in their clinics, most (59.4%) report their SI equipment was installed on or after 2015, and 10.7% indicated that although they have SI at their facilities, it is not being used clinically. Only 36.8% of reported users classify their level of expertise as "expert," and 85.5% of all respondents with SI agree to strongly agree that guidelines for the clinical use of surface imaging are necessary.

3.A | Surface imaging for initial positioning
Participants who indicated that SI has been clinically implemented in their department (n = 209) were asked to elaborate on their use of SI for initial positioning. This included specifying both the type of reference surface being used for this purpose (DICOM surface from the planning CT acquired during simulation, camera-acquired surface at simulation, or camera-acquired in the treatment room) and the treatments and sites for which SI was being employed for initial setup. Survey responses showed that the majority of users, 63.2%, perform initial positioning based on a single type of reference surface for every fraction of a patient's treatment (DICOM surface).  site/type are compiled in [ Fig. 1(a)]. The frequency of use of SI for initial positioning is highest for breast (routinely: 64.9%), SRS (routinely: 50.5%), and SBRT (routinely: 42.3%). It is rarely used for pediatric patients (never: 64.8%), GU/Prostate (never: 62.8%), and other pelvic or abdominal treatments (never: 65.8%). Note that respondents were given the option of selecting "Not Applicable" for treatment sites/types that are not treated in their clinic (see Table S1).

3.B | Surface imaging for intra-fraction monitoring
Participants were also asked about their use of SI for intra-fraction

Intra-FracƟon Monitoring
RouƟnely Occasionally Rarely Never Don't know

(b)
F I G . 1. Use of surface imaging for initial patient positioning (a) and intra-fraction monitoring (b) by site/treatment type. "Other" includes abdominal treatments (liver, pancreas, etc.), non-GU/prostate pelvis treatments, primary brain, and electron treatments. Note the "n" for each site/treatment type is listed in the x-axis. This number differs from 209 (total number of respondents using SI clinically) because some of them indicated these categories as "NA -Not Applicable." NA responses have been excluded from these results.
select the reference surface type for intra-fraction monitoring depending on the treatment site or patient. The breakdown of the use of SI for intra-fraction monitoring per treatment site/type is shown [ Fig. 1(b)]. Similar to initial positioning, the frequency of SI for each site/treatment type is listed in the x-axis. This number differs from the n in Fig. 1 because only respondents using SI for initial positioning of the indicated site/ treatment type were given these questions. This number is further decreased in graph b of this figure because some respondents indicated that the use of bolus for these treatments is "NA -Not Applicable" in their clinic.
Respondents were also asked what internal imaging verification tools, if any, they used to confirm the respiratory gating position given by SI. Figure 3 summarizes those results. Respondents were allowed to select more than one modality per site, if applicable.
Except for breast/chest wall treatments, which are verified with planar MV imaging slightly more frequently than with planar kV imaging (63.8% and 61.6%, respectively), the most common modalities for verification are planar kV imaging and CBCT/CT on rails. Volumetric imaging (CBCT/CT on rails) is more widely used than planar kV imaging for SBRT (78.3% vs 53.6%), non-SBRT lung (74.5% vs 61.8%), and non-SBRT abdomen (73.8% vs 54.8%).

| DISCUSSION
The use of SI in radiotherapy is increasing rapidly and it is important to understand how and for what purpose it is being used. This can help characterize the current status of the technology and identify areas of need for official guidelines and recommendations for safe application.
To the authors' knowledge, this is the first survey ever published on this topic. Although this survey has a low response rate, this limitation is not uncommon in such studies in medical physics. 5,6 Due to the anonymous nature of the survey, no specific information was collected on the respondents' employers, and over 75% of This assessment is reinforced by the fact that the majority of the participants with surface imaging (85.5%) agree/strongly agree with the need for national recommendations on the use of these systems.
A large proportion of respondents with SI report having the same vendor (see Table 1), which was the first vendor to offer this technology in the U.S. market. Responses also indicate that surface imaging is most commonly found in treatment vaults (98.7%) rather than simulation rooms (33.6%). Since surface imaging systems have the capability of gating the treatment beam based on the patient's position being in or out of tolerance, participants were asked if this feature was available and clinically used. A total of 57.7% of respondents with surface imaging in their clinic, including photon and proton treatment machines, reported using the beam gating capability.
The results collected in this study show that surface imaging is most commonly used for breast (with and without breath-hold) and SRS treatments, which is reflective of the current body of literature published on this technology 1,3 . In addition, these two sites are expected to have a robust surface-to-target positional correlation which makes them ideal candidates for SI use. Respondents who indicate the use of SI for initial positioning, typically also use it for intra-fraction monitoring. As seen in Fig. 1

| CONCLUSION S
Surface imaging is an attractive imaging technology due to its ability to aid in initial positioning, intra-fraction monitoring, and beam gating without the use of ionizing radiation. Although our results cannot be generalized due to the limited response rate of the survey, they present the medical physics community with an overview of current uses and practices in the field. Currently, our results indicate that the majority of clinical applications are for breast (with and without DIBH), SRS, and SBRT treatments. Lower rates of use were reported for other treatments such as for pediatric and lung cancer. Onequarter of respondents with SI capabilities reported no or slow clinical implementation. As the rates of adoption are expected to increase, and different techniques for commissioning and implementation may introduce systematic errors into patient setup and monitoring, national guidelines on the clinical implementation of surface imaging are needed to expedite and standardize its use.

ACKNOWLEDG MENTS
We thank the members of AAPM TG-302 for their feedback in com-

SUPPORTING INFORMATION
Additional supporting information may be found online in the Supporting Information section.