Patient and physician radiation exposure during minimally invasive lumbar decompression: A prospective assessment of X‐ray exposure risks

Minimally invasive lumbar decompression (mild®) is becoming a popular procedure for treating lumbar spinal stenosis (LSS) secondary to hypertrophic ligamentum flavum (LF). The mild® procedure is commonly performed under live fluoroscopic guidance and carries a risk of radiation exposure to the patient and healthcare.

A 2020 systematic review meta-analysis reported the prevalence of symptomatic LSS to be 11%-39% in the general population worldwide. 2If left untreated, symptoms can lead to significant functional disability.A common cause of degenerative central LSS is ligamentum flavum (LF) hypertrophy. 3Minimally invasive lumbar decompression (mild®) aims to improve symptoms of neurogenic claudication by debulking the thickened LF.
Traditionally, the approach to the treatment of LSS after failed conservative management has been open lumbar decompression with or without fusion, which carries the potential risks of general anesthesia and open surgical complications or postoperative sequelae.Comparatively, the mild® procedure can be performed through a small incision and does not significantly alter the structural integrity of the spine allowing for an improved safety profile. 4Consequently, mild® is associated with decreased risks associated with anesthesia and open surgery.5][6] Importantly, studies have demonstrated significant functional improvement and decreased disability secondary to neurogenic claudication in patients 1 year postprocedure. 6,71][12] The risk comes from the use of fluoroscopy, which is necessary to guide the operator and ensure the safe and effective removal of the posterior fibers of the ligamentum flavum with preservation of the anterior fibers.
To our knowledge, the radiation exposure risk and safety to patients and operating physicians during mild® procedure have not been quantified in the literature.

Subject selection
Our institutional review board approved this prospective study of X-ray exposure risk during the mild® procedure.The study included 41 consecutive mild® procedures performed by a single physician (NM) in the Cleveland Clinic Department of Pain Management between October 2019 and December 2021.The indication for the procedure was symptomatic LSS secondary to hypertrophied LF in patients who failed conservative management and were considered to be too high risk for or were unwilling to undergo open surgical decompression.Patient demographics including sex, age, BMI, and comorbidities including tobacco use, obesity, hypertension, and diabetes mellitus (DM) were documented at the time of enrollment.

Procedure
Each mild® procedure was performed by the same provider, in the same operating room with the same fluoroscopy unit.Specifically, a General Electric OEC 9900 Elite Super C fluoroscopy unit was used for intraoperative imaging guidance.The C-Arm system was compliant with all appropriate medical physics compliance surveys.All imaging was performed with normal automatic brightness in the contralateral oblique view with as-needed confirmation images in the true AP and lateral projections.Pulsed fluoroscopy was used for most of the procedure with occasional continuous fluoroscopy as needed intraoperatively.The physician wore a Mirion Technologies unshielded radiation dosimeter at the collar level.The surgeon wore standard radiation protective gear including a full lead apron, lead glasses, and a thyroid shield.The dosimeter was worn at the collar level external to any lead covering.
An operating room schematic layout of the physician, OR personnel, and patient positions relative to the X-ray source is demonstrated in Figure 1.

Outcome measures
Fluoroscopy time and mean patient skin exposure were measured per procedure and per unilateral level decompressed and recorded in the department's picture F I G U R E 1 1: operating physician, 2: surgical assistant, 3: scrub nurse, 4: circulating nurse, 5: radiation technologist (CO), 6: anesthesiologist, 7: patient, 8: anesthesia machine, 9: surgical instrument table, 10: fluoroscopy screen.All OR personnel were wearing appropriate X-ray protective gear.archiving and communication system (PACS).One exception was subject #26 who was operated on at the main hospital operating room rather than the outpatient surgery center due to his increased morbidity.The main hospital operating room fluoroscopy units did not have the same PACS.As a result, one of the pieces of data was not able to be obtained, namely the total radiation time for radiation exposure for the case.This subject was therefore not included in the calculations involving the duration of radiation exposure.Individual patient procedure data including procedural level(s), laterality, and any procedural complications were included.Surgical specimens obtained during every procedure were sent for histopathology examination to confirm the removal of LF.Postoperative skin radiation exposure complications were assessed for all patients in the recovery area and at the first postoperative follow-up visit.Physician exposure was measured via a personal dosimeter worn during each case.

Statistical analysis
Descriptive analysis was used for patient demographics and calculated using the mean ± standard deviation (SD) if normally distributed and median with interquartile range (IQR) if not.Categorical data were represented as numbers and percentages.The correlation between the observed and predicted values of the response was tested by R-squared analysis.Multivariate logistic regression was used to analyze fluoroscopic time or skin radiation dose during mild®, using the covariates of age, gender, BMI, DM, and smoking (ever/never).Any result with a p-value less than 0.05 was considered statistically significant.All analyses were conducted in JMP.

R E SU LT S
A total of 41 patients with LSS were treated using the mild® procedure.There were 23 male and 18 female patients included with a mean age of 75.2 years and a mean BMI of 29.The study population demographics are summarized in Table 1.All patients had 1-or 2-level disease requiring bilateral or unilateral decompression.The most common spinal levels treated were L3-L4 and L4-L5.Most cases (38) required bilateral decompression while the remaining cases underwent either unilateral decompression or both unilateral at one level and bilateral decompression at a second level (Tables 2 and 3).Postoperative pathology reports confirmed that all intraoperative patient specimens were LF tissue.

Patient exposure
Mean fluoroscopy time per procedure was 4.2 ± 1.8 min with a range of 2.3-11.3min.Mean fluoroscopy time per unilateral level was 1.3 ± 1.8 min with a range of 1.1-5.6 min.
Mean patient skin radiation exposure was 1.1 ± 0.9 mGym 2 per procedure with an average of 0.6 ± 0.5 mGym 2 of skin exposure per unilateral level decompressed (Table 5).
No patients reported radiation-specific adverse outcomes on initial or subsequent follow postoperative visits.

Physician exposure
Cumulative physician radiation exposure from all 41 mild procedures performed over the 2-year period was 337 mRem with respect to deep tissue, 238 mRem to eye tissue, and 1219 mRem to shallow tissue.Average  4).Of note, the ocular exposure was calculated by the radiation safety team based on the dosimeter reading the surgeon was wearing during the procedures.The surgeon did not utilize lead glasses during the procedure.The average total fluoroscopy duration for L3-L4 mild® procedure was 3.89 ± 0.6 min with an average radiation exposure of 115.1 ± 50.2 mGy (Table 3).The average total fluoroscopy duration for L4-L5 mild® procedure was 4.5 ± 2.4 min with an average radiation exposure of 162.8 ± 136.0 mGy (Table 3).
The three multilevel cases were not included in the calculations for the average total fluoroscopy duration and average radiation exposure per level because the data were only collected as a summation for the entire case rather than per level within the multilevel case.

Statistical analysis
A multivariate regression analysis was performed to compare baseline patient demographics to increased total skin exposure (Table 8).This demonstrated that increased BMI was a statistically significant risk factor for increased skin exposure to radiation during the procedure.A cutoff point of 142 mGy was used for the analysis because this corresponds to the mean entrance skin dose.

DI SC US SION
Minimally invasive lumbar decompression (mild®) is a percutaneous, fluoroscopy-guided procedure that involves debulking the hypertrophied LF in patients with LSS to relieve symptoms of neurogenic claudication.It has become an increasingly popular treatment option because it carries less risk for tissue injury, blood loss, and postoperative pain and allows for shorter recovery time compared with other more invasive procedures.
Additionally, it provides significant pain relief and improved functionality while being less invasive and more cost-effective compared with open intervention.As with all procedures using fluoroscopy, there is an element of radiation exposure risk to the healthcare providers and patients.The specific radiation risk associated with the mild® procedure was evaluated in this study.
The adverse effects of radiation exposure are categorized as stochastic or non-stochastic (deterministic).Stochastic effects refer to the development of cancer or genetic mutations.The probability of these effects occurring increases proportionally to radiation dose, but the severity of such manifestations is independent of dose.Conversely, non-stochastic adverse effects such as skin erythema, epilation, cataract formation, and even death are dose dependent.Since the development of these nonstochastic effects is predictable, it is important to monitor procedural radiation exposure to ensure that dose thresholds are not exceeded.

October
As per the National Council on Radiation Protection (NCRP) guidelines, the maximum allowed annual exposure for radiation workers is 5 Rem to the body and 50 Rem to an extremity. 16Based on our results, a physician would exceed the body exposure safety limit after performing approximately 610 mild® procedures annually and would exceed the extremity exposure threshold after approximately 1684 mild procedures annually.Assuming 225-250 working days in a year, a physician would need to perform two to three mild® procedures every day for a full year to reach the upper limit of this safety threshold.Considering a provider in a typical interventional pain practice operates one two times per week, the risk of reaching annual radiation safety thresholds by performing mild® is low, although additional X-ray exposure from all other interventional cases should be accounted for as well.
Patient radiation exposure in this study was reasonably low.The maximum skin dose observed was 511.8 mGy.The threshold for adverse cutaneous reaction is 2000 mGy in a single session resulting in early transient skin erythema (Table 7).None of the patients included in the study reported any adverse skin effects after the procedure or on subsequent follow-up.
While no skin changes were directly observed, our data indicated a positive correlation between increasing BMI with increased skin exposure during mild® (Figure 2).Other anatomical factors that might increase the X-ray exposure include advanced degenerative changes leading to shingled laminae and densely mineralized bone that might increase the time to get adequate exposure to the LF.In such instances, increasing the use of pulsed fluoroscopy might be warranted to minimize radiation exposure.Fluoroscopic times and patient radiation exposures compared similarly albeit slightly longer, when compared to other interventional procedures that require fluoroscopic guidance (Table 6).For example, Bindal et al. reported a mean fluoroscopic time of 1.7 min (0.8-3.7 min) and skin exposure of 59.5 mGy (8.3-252 mGy) in 24 cases of minimally invasive transforaminal lumbar interbody fusion (TLIF). 17Additionally, Hwang et al. demonstrated an average patient X-ray exposure time during transforaminal epidural steroid injection (TFESI) of 0.7 min (0.23-1.9 min) with a mean skin radiation exposure of 136.8 mGy (37.4-529.6 mGy). 18owever, the frequency of each procedure should be taken in account to adequately evaluate the radiation exposure risk.For example, a patient with LSS typically undergoes an average of three TFESIs annually, which could result in a cumulative fluoroscopic time of up to 5.7 min.Given the additive effect of radiation exposure over several years, earlier intervention with mild® may carry less risk compared with repeated TFESIs. 2,19,20verall, our study demonstrated that real life, physician, and patient radiation exposure from the mild® procedure is within safety limits, which further suggests that the benefits of the mild® procedure are more likely to outweigh the proposed risks.However, the long-term and stochastic effects from scatter radiation are not well understood and could bely an underappreciated potential risk to providers and OR personnel. 17,21,22Additionally, the data in this study were collected during cases performed by a single physician in a single center.The average length of radiation exposure per case is provider-specific and dependent on physician training, skill level, experience, radiology technician experience, and other operating room support staff experience.It is important to consider these variations when estimating the radiation exposure risk and therefore assessing the overall safety of the procedure.

CONC LUSION
As minimally invasive surgical techniques continue to be adopted and utilized, it is important to appreciate the possible safety considerations for such treatment modalities.This study is an attempt to quantify the radiation safety exposure to the physician and patient during the mild® procedure.Patient radiation exposures are relatively low with no reported adverse skin reactions identified postoperatively.Physician exposures were observed to be within acceptable safety limits.However, operator safety thresholds could be reached if a physician performs over 600 mild® cases annually.Overall, the radiation exposure is relatively similar to some of the other interventional fluoroscopically guided procedures performed by pain management physicians.
While it is difficult to definitively quantify the risk of radiation exposure during fluoroscopic procedures, it can be estimated with the methods used in this study.Providers should be aware of the risk and attempt to minimize radiation exposure to themselves and to patients.Since the mild® procedure is relatively new, further evaluation of the variation in fluoroscopy times and skin exposure based on operator technique could be useful in identifying actionable methods for reducing radiation exposure to both physicians and patients.

L I M I TAT ION S
The primary limitation of this study is that the data collected are from a single center and a single operator.This may limit in some ways generalizability.However, it also provides standardization to minimize interoperator skill set or experience discrepancies.As previously stated, exposure time and factors like the use of pulsed vs. continuous X-ray vary based on operator skill level, experience, and X-ray technician experience with the procedure.Another limitation to consider is cataract formation was not included as an outcome measure.While it is a possible side effect of radiation exposure, it was not feasible to assess during the relatively short time frame of our study.Lastly, the use of one personal dosimeter may have limited the accuracy of radiation exposure measurements.The addition of a ring dosimeter may have provided more comprehensive results.

F I G U R E 2
Multivariate analysis BMI and age versus total dose.
Descriptive statistics.Summary of radiation exposure levels, laterality, and demographics for patients undergoing mild®.per procedure was 8.2 mRem in the deep tissue, 5.8 mRem eye tissue, and 29.7 mRem in shallow tissue.The average exposure per unilateral level decompressed was 4.1 mRem in the deep tissue, 2.9 mRem in the eye tissue, and 14.7 mRem in the shallow tissue (Table T A B L E 1a Due to increased morbidity, the MILD procedure for subject 26 was performed in a different facility with different C arm software.It was not used in the calculation for total fluoro time as these data were not collected by the system.exposure Radiation exposure during selected interventional procedures.Skin adverse effects of single radiation exposure.
a Numbers in parentheses are ranges.TA B L E 7