Occupational Radiation Exposure During SLNB in Endometrial Cancer

INTRODUCTION

Endometrial cancer (EC) is known to be the most common type of gynaecological cancer in developed countries, with a higher risk being presented in direct proportionality with ageing and in patients diagnosed with obesity.^[1] For assessing the lymph node status in these patients, a minimally invasive technique and a better alternative to a radical lymphadenectomy is the sentinel lymph node biopsy (SLNB). The sentinel lymph node (SLN) is defined as the first lymph node that drains lymph from the primary tumour, which is why it is the first place where lymph node metastasis would develop.^[2] Routine lymphadenectomy showed no increased survival rate compared to SLNB, but an increased risk of adverse effects such as lymphedema according to the A Study in the Treatment of Endometrial Cancer (ASTEC) trial.^[3] In addition, during SLNB, more lymph node metastases are detected because nearly 5 per cent of SLNs are present in areas not routinely dissected during pelvic lymphadenectomy, such as lymph nodes in deep internal iliac and presacral regions.^[4] For lymphatic mapping of EC and radio-guided surgery, colloid particles labelled with technetium-99m (Tc-99m) are injected into the cervical stroma on the day of surgery by the gynaecologic surgeon. After the localisation of the SLNs during preoperative lymphoscintigraphic evaluation, the SLNs are found with a gamma probe and removed during surgery.^[5]

Radiation safety is essential during the performance of radio-guided SLNB. Occupational exposure to ionising radiation may occur during both radiotracer application and surgical treatment. The most common method of personal dose monitoring involves the use of thermoluminescent dosimeters (TLDs), which are passive devices used to estimate radiation exposure by measuring the intensity of visible light emitted from a sensitive crystal when heated. The emitted light intensity is proportional to the absorbed radiation dose.

The surgical staff is exposed to varying levels of radiation depending on their proximity to the radiation source. Furthermore, radiation exposure is not uniformly distributed across the body; for example, the dose received by the extremities may differ from that received by the trunk. Personal TLDs are typically worn as finger rings or bracelets to measure extremity exposure, expressed as Hp(0.07), and as badges to measure whole-body dose, expressed as the personal dose equivalent Hp(10). Periodic readings of cumulative radiation doses from TLDs are performed by an authorised dosimetry service.^[6,7]

This study aimed to determine the occupational equivalent and effective radiation doses received by surgical staff involved in radiocolloid administration and SLN biopsy in EC patients.

MATERIAL AND METHODS

This study was designed as a prospective observational evaluation of occupational radiation exposure under routine clinical conditions. Radiation exposure to the surgical staff was evaluated during radio-guided SLNB in endometrial cancer patients between January 2022 and August 2023. Each patient received a cervical injection of 4 mCi of Tc-99m-labelled nanocolloid on the day of surgery. All patients provided standard dual written informed consent for both lymphoscintigraphy and the radio-guided surgical procedure.

RESULTS

Radiation exposure of the surgical staff was recorded during 29 SLNB over 20 months. The mean time interval between tracer administration and surgery was 244.82 ± 26.74 minutes (range: 210-290 minutes), and the mean duration of the surgical procedures was 128.27 ± 20.79 minutes (range: 90-165 minutes). The effective activity at the time of surgery was calculated considering physical decay only, according to the decay equation , assuming a physical half-life of 6.01 hours for 99mTc. Based on the mean delay, the activity at the beginning of surgery was approximately 2.50 mCi (range: 2.29–2.67 mCi). Biological clearance was not considered, representing a conservative estimate of intraoperative activity. Cumulative whole-body and extremity occupational radiation exposures over the 20-month study period are presented in Table 1. Both operators demonstrated low overall exposure, with total whole-body doses of 595 µSv for the assistant and 545 µSv for the primary surgeon. The assistant exhibited slightly higher cumulative values, with proportional discrepancies of 9.17% for whole-body and 8.42% for extremity dose. Across all monitoring periods, extremity doses exceeded whole-body doses.

The annual cumulative doses are summarised in Table 2, considering three possible sequences: January 2022– December 2022; May 2022–April 2023; September 2022-August 2023. The average annual whole-body dose was 284 µSv (range: 90-516 µSv) for the surgeon and 314 µSv (range: 100 – 566 µSv) for the surgeon assistant. The corresponding annual extremity doses were 250 µSv (range: 130-396 µSv) for the surgeon and 270 µSv (range: 140-436 µSv) for the assistant. The highest calculated annual cumulative dose was 566 µSv, observed for the surgeon assistant’s whole-body dose during the period September 2022–August 2023.

Graphical representations [Graphs 1 and 2] illustrate the linear relationship between the number of interventions and the cumulative dose per period. Dosimetric data for the surgeon assistant are shown by the blue line, whereas data for the surgeon are indicated by the red line.

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Graph 1: Display of the cumulated effective dose per the number of radio guided interventions performed. WB 1: Assistant surgeon; WB 2: Surgeon; WB: Whole body; SLNB: Sentinel lymph node biopsy; Hp (10): Personal dose equivalent at a depth of 10 mm

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Graph 2: Display of the cumulated equivalent dose per the number of radio-guided interventions performed. EXT 1: Assistant surgeon; EXT 2: Surgeon; WB: Whole-body; SLNB: Sentinel lymph node biopsy; Hp (0.07): Personal dose equivalent at a depth of 0.07 mm

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Correlation analysis showed a high degree of agreement in all cases, indicating a linear relationship in cumulative dose, with R² values exceeding 0.95. This linearity allows for the extrapolation of values, enabling predictions of radiation exposure for a greater number of radio-guided interventions or over extended periods.

Based on these conditions, the maximum number of radio-guided SLNB procedures that surgeons can safely perform in endometrial cancer patients can be estimated, depending on whether they are considered members of the public (dose limit: 1 mSv) or occupationally exposed individuals (dose limit: 20 mSv).

Graph 3 illustrates that approximately 50 interventions per year would result in reaching the public dose limit. For occupational exposure, the maximum permissible number of SLNB procedures is 800 annually, or 725 if the same individual both administers the radiocolloid and performs the surgical procedure [Graph 4].

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Graph 3: Display of the extrapolated relationship of the effective radiation dose in the population. WB 1: Assistant surgeon; WB 2: Surgeon; WB: Whole-body; SLNB: Sentinel lymph node biopsy; Hp (10): Personal dose equivalent at a depth of 10 mm

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Graph 4: Display of the extrapolated relationship of the effective radiation dose in the professionally exposed individuals. WB 1: Assistant surgeon; WB 2: Surgeon; WB: Whole-body; SLNB: Sentinel lymph node biopsy; Hp (10): Personal dose equivalent at a depth of 10 mm

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Higher occupational doses were observed in periods characterised by increased mean body mass index (BMI) and longer operative duration, as detailed in Table 3. Mean BMI ranged from 28.0 to 36.0 kg/m², while mean procedure time varied between 121.3 and 150.0 minutes. Periods involving a greater number of excised sentinel lymph nodes were associated with higher cumulative exposure, suggesting that procedural complexity and duration may influence occupational dose. SLN laterality (left versus right hemipelvis) was specifically evaluated to assess its potential impact on exposure; however, no observable difference in dose patterns was identified [Table 3].

Graph. 3 Display of the extrapolated relationship of the effective radiation dose in the population WB 1- assistant surgeon, WB 2- surgeon, WB: Whole-body; SLNB: Sentinel lymph node biopsy; Hp

(10): Personal dose equivalent at a depth of 10 mm

Graph. 4 Display of the extrapolated relationship of the effective radiation dose in the professionally exposed individuals WB 1: Assistant surgeon; WB 2: Surgeon; WB: Whole-body; SLNB: Sentinel lymph node biopsy; Hp (10): Personal dose equivalent at a depth of 10 mm

DISCUSSION

The introduction of radio-guided SLNB has improved surgical staging in gynaecological malignancies; however, it raises legitimate concerns regarding occupational radiation exposure due to the handling of unsealed radioactive sources. In this context, this prospective observational study provides longitudinal descriptive data on whole-body and extremity dose equivalents received by the primary surgeon and the surgical assistant over 20 months under routine clinical conditions.

Radiation exposure in medical practice may result in deterministic or stochastic effects. While deterministic effects occur only above threshold doses, stochastic effects are probabilistic and theoretically possible even at low levels of exposure.^[8] According to ICRP recommendations, the annual effective dose limit for occupationally exposed workers is 20 mSv, averaged over five years, with no single year exceeding 50 mSv, while the corresponding limit for members of the public is 1 mSv per year. In addition, the equivalent dose limits for the skin (averaged over 1 cm²) and the extremities are 500 mSv per year for occupationally exposed workers and 50 mSv per year for the public.^[9,10] In the present study, cumulative whole-body and extremity doses for both operators remain substantially below these regulatory thresholds, confirming the radiological safety of radio-guided SLNB when standard radiation protection principles are applied.

All patients received 4 mCi of 99mTc-labelled radiotracer. Considering a mean delay of approximately 4 hours between injection and surgery, the effective activity at the time of operation was estimated at approximately 2.50 mCi after correction for physical decay only. Biological clearance was not included in the decay model, representing a conservative assumption that likely overestimates intraoperative activity and, consequently, occupational exposure.

A proportional relationship between assistant and surgeon dose was observed throughout the study period. Although the assistant performs the radiotracer injection and is therefore subject to an additional short exposure interval (approximately 5–10 minutes), the difference in cumulative exposure between operators was modest. This indicates that the injection phase contributes only slightly to overall occupational dose, whereas the dominant determinant of exposure is shared intraoperative manipulation and proximity to the radioactive field.

Period-level analysis across the five independent monitoring intervals further demonstrated consistent proportional discrepancies of approximately 9% for whole-body and 8% for extremity exposure between operators. Exploratory paired testing yielded moderate-to-large effect sizes; however, statistical significance was not achieved, reflecting the limited number of independent observation periods. These findings reinforce that occupational exposure patterns were stable over time and primarily influenced by cumulative intraoperative workload rather than the brief injection phase alone.

Extremity doses consistently exceeded whole-body doses, which is expected given the closer positioning of the hands to the operative site during gamma probe localisation and node excision. Extremity-to-whole-body dose ratios remained stable across monitoring intervals, ranging approximately between 0.70 and 0.90 for both operators. This proportional consistency further supports a predominantly geometric exposure pattern determined by operative proximity rather than by isolated procedural components. Increased occupational doses were observed during monitoring periods characterised by higher mean body mass index (BMI), longer operative duration, and a greater number of excised sentinel lymph nodes. Elevated BMI may necessitate deeper dissection and prolonged gamma probe manipulation, thereby increasing the time spent in close proximity to the radioactive source. However, given the aggregated and period-based nature of the dataset, these associations should be interpreted descriptively rather than inferentially.

Importantly, SLN laterality (left versus right hemipelvis) did not influence exposure patterns. This likely reflects the comparable positioning and proximity of both operators relative to the operative field, resulting in similar exposure geometry irrespective of nodal location.

A comparison of occupational radiation exposure reported in previous studies and the present analysis is summarised in Table 4. Previous occupational risk assessment studies have evaluated radiation exposure to surgical teams involved in radio-guided SLNB, including surgeons, assistants, nurses, anaesthesiologists, and anaesthetists.^[11–16] These studies consistently reported low doses, with the highest exposure typically observed in the primary surgeon, while exposure to scrub nurses, anaesthesiologists, and anaesthetists was comparable to background levels. Radiation measurements were obtained using TLDs worn as rings on the non-dominant index finger,^[13] chest-level badges, or electronic personal dosimeters.^[12]Across these studies, a wide range of tracer doses was employed, resulting in a broader spectrum of exposure values: cumulative annual doses ranged from 0.1 to 0.8 mSv, corresponding to 1–24 μSv per procedure for Hp(10) and 6.69–17.5 μSv per procedure for Hp(0.07).^{[12,13,14,17]}

In contrast to most reports, Burrah et al. observed higher radiation exposure in the surgical assistant than in the primary surgeon, likely due to closer proximity to the injection.^[17] A similar pattern was observed in our study, with the surgical assistant receiving a slightly higher dose. Accordingly, this dose was higher than the corresponding value reported for the assistant in our previous breast cancer SLNB study in which radiotracer administration was performed by nuclear medicine.^[14]

Radiation exposure to medical personnel during SLNB procedures is influenced by several factors. These include the activity of the radiotracer, the operator’s proximity to the patient, the distance from both the injection site and the SLNs, and the duration of exposure, which is directly related to the length of the surgical procedure and the timing of tracer administration.^[18] To mitigate radiation exposure, several strategies can be employed: reducing the administered tracer dose, increasing the interval between radiotracer injection and SLNB, maximising distance from the patient whenever feasible, and minimising exposure time. Studies using reduced tracer activity in same-day protocols have demonstrated particularly low occupational exposure. Burrah et al. demonstrated that lowering the administered activity from 40 MBq (day-before injection) to 20 MBq (same-day injection) resulted in reduced radiation doses, indicating that dose reduction in oneday protocols can decrease occupational exposure without compromising procedural efficacy.^[17] The importance of operating room personnel positioning relative to the patient has been highlighted in a study by Coventry and colleagues, which demonstrated that proximity to the injection site and the SLN significantly increases radiation exposure. The authors further recommended early resection of the primary tumour—containing more than 95% of the injected activity—and placement of the excised radioactive tissue at a distance greater than 2 meters from staff.^[19] These findings were confirmed by Kimura et al., who measured exposure dose rates using dosimeters placed at 28 cm and 36 cm from the injection site, yielding 3 μSv/h at 28 cm and 1 μSv/h at 36 cm. While shielding devices and lead aprons are not routinely required for surgical staff, their use may be considered for pregnant surgeons or nurses, as they can reduce exposure by up to 66%. In the same study, Kimura et al. measured radiation exposure in surgeons and operating room personnel using personal dosimeters placed both inside and outside 0.25 mm lead protective clothing at the abdominal level. Recorded doses were 1 μSv inside the protective clothing and 4 μSv outside.^[15]

In our study, the average annual whole-body effective dose was 0.31 mSv and 0.28 mSv for the surgeon and surgeon assistant, respectively, while the average extremity equivalent dose was 0.27 mSv and 0.25 mSv. These values remain well below the recommended annual dose limits for both radiation workers and the general public/non-radiation medical staff. Based on the average dose per procedure, surgeons would need to perform fewer than 50 interventions per year to reach the public dose limit of 1 mSv. A gynaecologic surgeon classified as an occupationally exposed worker could safely perform up to 800 SLNB procedures annually; if the same surgeon also performs tracer administration, this number is reduced to approximately 725 procedures per year. While these findings indicate a high margin of safety under current practice, further optimisation strategies may contribute to additional dose reduction. Future studies could further explore the use of lower Tc-99m activities, real-time electronic dosimetry, and evaluation of shielding strategies to optimise occupational radiation protection during radio-guided SLNB.

LIMITATIONS

The present study has several limitations. The relatively small sample size (29 SLNB procedures) and the use of aggregated observation periods rather than individual case-level dosimetry preclude detailed statistical modelling and limit the generalisability of the findings. In addition, the absence of real-time dosimetry prevented step-specific exposure analysis. Consequently, cumulative occupational dose measured over the monitoring period could only be approximated according to procedural roles, and precise attribution of exposure to individual procedural steps was not possible. Nevertheless, the longitudinal design provides meaningful real-world insight into occupational exposure trends during routine implementation of radio-guided SLNB. Future multicentre studies incorporating larger cohorts and real-time monitoring may enable more detailed evaluation and targeted optimisation of radiation protection strategies.

CONCLUSION

Our study demonstrates that the equivalent and effective radiation dose received by the gynaecologic surgeons performing radiocolloid administration and radio-guided SLNB in EC patients is low and well below established annual exposure limits. Accordingly, routine radiation monitoring or protective shielding for surgical staff is not required under standard operating conditions. The findings should be interpreted within the context of a single-centre observational design.