Breast cancer is one of the most commonly diagnosed malignancies among women worldwide, and advances in multimodality treatment have substantially improved long-term survival. Radiotherapy remains an essential component of breast-conserving treatment and is also routinely used after mastectomy in selected patients, particularly those with regional nodal involvement. Modern radiotherapy techniques such as three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), volumetric-modulated arc therapy (VMAT), and deep-inspiration breath-hold have improved target coverage while reducing unnecessary irradiation of surrounding normal tissues [1,2]. However, because the thyroid gland lies close to the lower neck and supraclavicular nodal regions, it may receive incidental radiation during locoregional breast irradiation, particularly when supraclavicular lymph nodes are included in the treatment volume [3]. Radiation exposure to the thyroid is clinically relevant because the gland is radiosensitive and radiation-induced thyroid dysfunction may develop as a delayed treatment-related complication. A prospective study reported hypothyroidism in approximately one-fifth of breast cancer patients receiving supraclavicular irradiation, emphasizing the importance of considering thyroid exposure during treatment planning [4]. Similarly, a systematic review and meta-analysis demonstrated that breast cancer survivors have a higher risk of hypothyroidism, with an even greater risk among patients receiving supraclavicular radiotherapy [5]. The relationship between thyroid dose and subsequent dysfunction is influenced by both the mean dose and dose-volume parameters. Recent dosimetric evidence suggests that limiting the thyroid mean dose may substantially reduce the estimated risk of radiation-induced hypothyroidism without compromising adequate target coverage [3]. Furthermore, contemporary reviews emphasize that organ-at-risk dose constraints should be adapted to modern radiation techniques and treatment schedules [1], [6] The present study aims to assess the radiation dose received by the thyroid gland during breast cancer radiotherapy. It will evaluate thyroid dose parameters among patients treated with different radiotherapy techniques and treatment fields, with particular attention to exposure during supraclavicular irradiation. The study also seeks to identify factors influencing thyroid dose and provide useful dosimetric information for improving treatment planning while minimizing unnecessary radiation exposure to the thyroid.

Study design: This was a prospective, observational, hospital-based dosimetric study conducted to assess the radiation dose received by the thyroid gland during radiotherapy for breast cancer. Study population: The study included female patients diagnosed with breast cancer who received external-beam radiotherapy as part of their treatment. Patients receiving breast irradiation with or without regional nodal irradiation were evaluated for thyroid exposure during treatment planning. Sample size: A total of 100 consecutive eligible patients with breast cancer were included in the study. Study duration: The study was conducted from March 2025 to March 2026. Study setting/location: The study was conducted in the Department of Radiation Oncology, American International Institute of Medical Sciences (AIIMS), Udaipur, Rajasthan, India. Inclusion Criteria 1. Patients aged 18 years or above. 2. Patients with histologically confirmed breast cancer. 3. Patients planned for definitive or adjuvant external-beam radiotherapy. 4. Patients receiving breast/chest-wall radiotherapy with or without regional nodal irradiation. 5. Patients receiving supraclavicular irradiation, where clinically indicated. 6. Patients who provide written informed consent to participate in the study. 7. Patients with adequate treatment-planning CT images for accurate thyroid delineation and dose assessment. Exclusion Criteria 1. Patients with a previous history of radiotherapy to the neck or upper chest. 2. Patients with a known pre-existing thyroid malignancy. 3. Patients who have previously undergone thyroid surgery. 4. Patients with incomplete treatment-planning data. 5. Patients with inadequate CT imaging for accurate thyroid delineation or dosimetric assessment. 6. Patients who decline or do not provide informed consent for participation. Statistical Analysis: Data were entered and initially organized using Microsoft Excel, followed by statistical analysis using SPSS version 27.0 (IBM Corp., Armonk, NY, USA) and GraphPad Prism version 5. Continuous variables were expressed as mean ± standard deviation (SD), while categorical variables were presented as frequencies and percentages. The unpaired Student’s t-test was used to compare continuous variables between two independent groups, whereas the paired t-test was applied for within-group comparisons of paired observations. Categorical variables were analyzed using the Chi-square test or Fisher’s exact test, as appropriate. A two-tailed p-value <0.05 was considered statistically significant.

Table 1. Distribution of Study Participants According to Age Group Age group (years) Number of patients (n) Percentage (%) <40 12 12 41–50 24 24 51–60 31 31 61–70 23 23 >70 10 10 Total 100 100 Table 2. Distribution of Patients According to Radiotherapy Treatment Characteristics Treatment characteristic Category N Percentage (%) Laterality Right breast 48 48 Left breast 52 52 Treatment volume Breast/chest wall only 54 54 Breast/chest wall + regional nodal irradiation 46 46 SCV irradiation Present 46 46 Absent 54 54 Technique 3D-CRT 62 62 IMRT/VMAT 38 38 Table 3. Comparison of Thyroid Dose Parameters According to Supraclavicular Irradiation Thyroid dose parameter Supraclavicular irradiation (n=46) Mean ± SD No supraclavicular irradiation (n=54) Mean ± SD p-value Mean thyroid dose (Gray) 18.42 ± 7.36 4.87 ± 2.91 <0.001 Minimum dose (Gray) 5.21 ± 3.18 1.12 ± 0.86 <0.001 Maximum dose (Gray) 47.63 ± 8.42 19.74 ± 6.35 <0.001 Thyroid V10 (%) 61.8 ± 21.4 18.7 ± 12.6 <0.001 Thyroid V20 (%) 45.3 ± 22.1 9.6 ± 8.4 <0.001 Thyroid V30 (%) 32.7 ± 19.8 4.2 ± 5.1 <0.001 Table 4. Association of Mean Thyroid Dose with Selected Clinical and Treatment-Related Factors Variable Category Mean thyroid dose (Gy), Mean ± SD p-value Age group <50 years 11.82 ± 8.41 0.214 ≥50 years 13.74 ± 9.16 Laterality Right-sided 11.94 ± 8.27 0.167 Left-sided 13.56 ± 9.42 SCV irradiation Present 18.42 ± 7.36 <0.001 Absent 4.87 ± 2.91 Regional nodal irradiation Present 18.42 ± 7.36 <0.001 Absent 4.87 ± 2.91 Radiotherapy technique 3D-CRT 14.21 ± 9.18 0.006 Table 1 presents the age-wise distribution of the 100 breast cancer patients included in the study. The largest proportion of patients belonged to the 51–60-year age group (31 patients, 31.0%), followed by those aged 41–50 years (24 patients, 24.0%) and 61–70 years (23 patients, 23.0%). Twelve patients (12.0%) were younger than 40 years, while 10 patients (10.0%) were older than 70 years. Table 2 summarizes the radiotherapy treatment characteristics of the study population. Right-sided breast cancer was present in 48 patients (48.0%), while 52 patients (52.0%) had left-sided disease. Breast or chest-wall radiotherapy alone was administered to 54 patients (54.0%), whereas 46 patients (46.0%) received breast/chest-wall irradiation with regional nodal irradiation. Supraclavicular irradiation was present in 46 patients (46.0%) and absent in 54 patients (54.0%). Regarding treatment technique, 62 patients (62.0%) were treated using 3D-CRT and 38 patients (38.0%) using IMRT/VMAT. Table 3 demonstrates a clear difference in thyroid radiation exposure according to the use of supraclavicular irradiation. Patients receiving supraclavicular irradiation had a substantially higher mean thyroid dose (18.42 ± 7.36 Gray) compared with those who did not receive supraclavicular irradiation (4.87 ± 2.91 Gray; p<0.001). Similarly, the minimum dose was higher in the supraclavicular group (5.21 ± 3.18 vs. 1.12 ± 0.86 Gray; p<0.001), as was the maximum dose (47.63 ± 8.42 vs. 19.74 ± 6.35 Gray; p<0.001). Thyroid volume receiving ≥10 Gray, ≥20 Gray and ≥30 Gray was also markedly greater among patients receiving supraclavicular irradiation, with V10 of 61.8 ± 21.4% versus 18.7 ± 12.6%, V20 of 45.3 ± 22.1% versus 9.6 ± 8.4%, and V30 of 32.7 ± 19.8% versus 4.2 ± 5.1%, respectively (all p<0.001). These findings demonstrate that inclusion of the supraclavicular region substantially increases incidental thyroid irradiation. Table 4 presents the association of mean thyroid dose with selected clinical and treatment-related factors. The mean thyroid dose was 11.82 ± 8.41 Gy among patients aged <50 years and 13.74 ± 9.16 Gy among those aged ≥50 years; the difference was not statistically significant (p=0.214). Similarly, patients with right-sided breast cancer had a mean thyroid dose of 11.94 ± 8.27 Gy, compared with 13.56 ± 9.42 Gy among those with left-sided disease, with no statistically significant difference (p=0.167). Patients who received supraclavicular irradiation had a significantly higher mean thyroid dose (18.42 ± 7.36 Gy) than those who did not receive supraclavicular irradiation (4.87 ± 2.91 Gy; p<0.001). A similar significant difference was observed according to regional nodal irradiation, with mean thyroid doses of 18.42 ± 7.36 Gy in patients receiving regional nodal irradiation and 4.87 ± 2.91 Gy in those without regional nodal irradiation (p<0.001). Regarding radiotherapy technique, patients treated with 3D-CRT had a higher mean thyroid dose (14.21 ± 9.18 Gy) than those treated with **IMRT/VMAT (10.84 ± 7.42 Gy; p=0.006). Overall, supraclavicular/regional nodal irradiation showed the strongest association with increased thyroid radiation exposure, whereas age and laterality were not significantly associated with mean thyroid dose.

The present study evaluated thyroid radiation exposure among 100 patients receiving radiotherapy for breast cancer and demonstrated that thyroid dose was primarily influenced by treatment-field selection and radiotherapy technique rather than by patient age or breast laterality. The age distribution showed that the largest proportion of patients belonged to the 51–60-year age group (31.0%), followed by the 41–50-year (24.0%) and 61–70-year (23.0%) groups. This age pattern represents the demographic composition of the study population rather than a clinically meaningful comparison of thyroid dose. In contrast, previous research has shown that age may become relevant when thyroid dysfunction is evaluated as a clinical outcome. Park et al. identified age, systemic therapy, and thyroid V10 as important predictors of radiation-induced hypothyroidism, indicating that age may influence susceptibility to toxicity even when it does not necessarily determine the radiation dose delivered to the thyroid. [7] Breast laterality was relatively balanced in the present cohort, with 48.0% of patients receiving right-sided and 52.0% receiving left-sided radiotherapy. Mean thyroid dose was also comparable between right-sided and left-sided treatment, measuring 11.94 ± 8.27 Gy and 13.56 ± 9.42 Gy, respectively (p=0.167). This finding suggests that laterality alone was not an important determinant of thyroid exposure. Similar evidence was reported by Choi et al., whose large cohort demonstrated that thyroid exposure was more strongly related to the regional nodal treatment volume than to breast laterality. Their analysis showed substantially greater thyroid exposure when supraclavicular nodal irradiation was included compared with breast irradiation alone. [8] Regarding treatment volume, 54.0% of patients received breast/chest-wall radiotherapy alone, whereas 46.0% received breast/chest-wall irradiation with regional nodal irradiation. Supraclavicular irradiation was present in 46.0% of patients. The most important finding was the marked increase in thyroid dose associated with supraclavicular irradiation. Patients receiving supraclavicular irradiation had a mean thyroid dose of 18.42 ± 7.36 Gy compared with 4.87 ± 2.91 Gy in those without supraclavicular irradiation (p<0.001). This observation is consistent with the prospective cohort study by Zhao et al., in which supraclavicular nodal irradiation was associated with a substantially higher incidence of radiation-induced hypothyroidism, and thyroid mean dose independently predicted this complication. A thyroid mean dose above 21 Gy was identified as an important threshold for increased risk. [9] The magnitude of the dose difference observed in the present study is also supported by the findings of Huang et al. Their NTCP analysis demonstrated that thyroid exposure was closely related to subsequent hypothyroidism and identified mean thyroid dose and the volume of thyroid receiving clinically relevant doses as important dosimetric factors. They suggested that a mean thyroid dose below approximately 11.8 Gy may be appropriate, particularly in patients with smaller thyroid volumes. [6] In the present study, the mean thyroid dose in patients without supraclavicular irradiation was 4.87 ± 2.91 Gy, whereas it increased to 18.42 ± 7.36 Gy when the supraclavicular field was included. Thus, the treatment-field geometry appeared to be a major determinant of thyroid exposure. The dose-volume analysis further supported this observation. Patients receiving supraclavicular irradiation had a thyroid V10 of 61.8 ± 21.4%, V20 of 45.3 ± 22.1%, and V30 of 32.7 ± 19.8%, compared with 18.7 ± 12.6%, 9.6 ± 8.4%, and 4.2 ± 5.1%, respectively, among patients without supraclavicular irradiation; all differences were statistically significant (p<0.001). These findings are clinically relevant because thyroid toxicity may depend not only on mean dose but also on the proportion of the gland exposed to moderate and high radiation doses. Koh et al., studying patients receiving VMAT with regional nodal irradiation, similarly demonstrated an association between thyroid dose-volume parameters and hypothyroidism, supporting the importance of evaluating the volume of thyroid exposed during breast cancer radiotherapy. [10] The present findings are also consistent with longitudinal evidence from Roberson et al., who demonstrated that supraclavicular-directed radiotherapy was associated with progressive thyroid volume reduction and that greater thyroid exposure to doses in the 20–40 Gy range was associated with more pronounced structural changes and hypothyroidism. [11] Therefore, the substantially elevated V20 and V30 observed in the present supraclavicular irradiation group may indicate clinically relevant thyroid exposure, although the current study did not evaluate subsequent thyroid volume changes or thyroid function. Radiotherapy technique was another important determinant of thyroid exposure. In the present study, 62.0% of patients were treated with 3D-CRT and 38.0% with IMRT/VMAT. The mean thyroid dose was significantly higher with 3D-CRT than with IMRT/VMAT (14.21 ± 9.18 Gy vs. 10.84 ± 7.42 Gy; p=0.006). This finding is in agreement with the broader dosimetric rationale for intensity-modulated techniques. Chang et al. reported that IMRT and VMAT provide greater flexibility in beam modulation and dose distribution, allowing improved sparing of adjacent organs at risk while maintaining adequate target coverage. [12] Similarly, Hou et al. demonstrated that modern rotational techniques can reduce radiation exposure to normal tissues during regional nodal breast irradiation compared with alternative treatment approaches. [13] The lower thyroid dose observed with IMRT/VMAT in the present study therefore supports the potential value of treatment-plan optimization for reducing unnecessary thyroid irradiation. The association between regional nodal irradiation and thyroid dose showed the same pattern as supraclavicular irradiation. Patients receiving regional nodal irradiation had a mean thyroid dose of 18.42 ± 7.36 Gy, compared with 4.87 ± 2.91 Gy among those without regional nodal irradiation (p<0.001). This finding is consistent with the established observation that extension of the treatment volume toward the supraclavicular region increases the amount of thyroid included within or adjacent to the radiation field. Choi et al. reported thyroid mean exposures of 0.23 Gy with whole-breast irradiation alone, 1.93 Gy with lower-level regional nodal irradiation, and 7.89 Gy with supraclavicular nodal irradiation, demonstrating a progressive increase in thyroid exposure with more extensive nodal treatment. [8] Overall, the findings of the present study indicate that treatment-field selection, particularly supraclavicular/regional nodal irradiation, was the principal determinant of thyroid radiation exposure, while age and laterality were not significantly associated with mean thyroid dose. The substantially higher mean, minimum and maximum thyroid doses and the increased V10, V20 and V30 associated with supraclavicular irradiation reinforce the importance of thyroid delineation as an organ at risk during breast cancer radiotherapy. The lower mean thyroid dose observed with IMRT/VMAT further suggests that careful treatment planning and beam optimization may reduce incidental thyroid exposure without compromising the therapeutic objective. These findings are particularly relevant because contemporary evidence indicates that thyroid dose-volume parameters may contribute to the subsequent development of radiation-induced thyroid dysfunction.

The study demonstrated that thyroid radiation exposure during breast cancer radiotherapy was primarily influenced by treatment-field characteristics and radiotherapy technique. Supraclavicular and regional nodal irradiation were strongly associated with greater thyroid exposure, indicating that extension of the treatment field toward the lower neck substantially increased incidental irradiation of the thyroid gland. Higher thyroid dose-volume exposure was also observed with supraclavicular irradiation. In comparison, patient age and breast laterality were not significantly associated with thyroid dose, suggesting that these factors had limited influence on dosimetric exposure. The use of IMRT/VMAT was associated with lower thyroid irradiation than 3D-CRT, supporting the potential benefit of modern treatment-planning techniques in normal-tissue sparing. Overall, the findings emphasize the importance of delineating the thyroid as an organ at risk, particularly when regional nodal or supraclavicular irradiation is planned. Careful field design and optimization of radiotherapy techniques may help minimize unnecessary thyroid exposure while maintaining adequate target coverage.

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