Thyroid V50 Highly Predictive of Hypothyroidism in Head-and-Neck Cancer Patients Treated With Intensity-modulated Radiotherapy (IMRT)

Predicting radiation-induced hypothyroidism in nasopharyngeal carcinoma patients using a deep learning model

Background

Radiation-induced hypothyroidism (RIHT) is a common complication in nasopharyngeal carcinoma patients. Predicting its onset is crucial for effective management and early intervention. This study aims to develop a model based on deep learning survival analysis to predict RIHT in nasopharyngeal carcinoma patients.

Methods

This retrospective study included 535 nasopharyngeal carcinoma patients between January 2015 and October 2020. Cox regression, LASSO-Cox analyses and Spearman correlation test were employed to identify significant predictors. Two deep learning and two machine learning algorithms were trained, tuned, and compared against traditional Cox and NTCP models by C-index, Brier score, and decision curve analysis.

Results

The study observed a 41.7 % incidence of RIHT, with a median time to onset of 15 months. AJCC N stage, thyroid volume and specific dose-volume parameters were identified as potential predictors. DeepSurv model outperformed traditional ones (C-index: DeepSurv 0.75, traditional models ≤ 0.63). While other models were competitive at early post-treatment intervals, deep learning models demonstrated superior performance over time. Calibration and decision curve analysis corroborated the enhanced predictive capability of DeepSurv. Feature importance analysis highlighted thyroid V30 and V50 as the most significant predictors.

Conclusions

DeepSurv demonstrated superior predictive performance for RIHT in nasopharyngeal carcinoma patients compared to traditional models. Deep learning-based predictions offer high accuracy, which may enable personalized patient management and have great potentials in mitigating the risk of RIHT. These findings suggested that incorporating such model into clinical practice could be beneficial for the management of RITH.

Clinical and Dosimetric Predictors of Early Onset Postradiation Hypothyroidism in Patients with Head and Neck Malignancies: A Logistic Regression Analysis

INTRODUCTION

Hypothyroidism commonly occurs as a side effect following radiotherapy for head and neck malignancies, yet limited information exists to predict the risk of postradiation hypothyroidism. This study aims to investigate the clinical and dosimetric factors that may predict early onset postradiation hypothyroidism (EO-PRH).

METHODS

A retrospective study was conducted on patients with head and neck cancer treated between 2018 and 2021, with a minimum follow-up duration of 12 months. The thyroid gland was contoured on computed tomography (CT) scans, and dose-volume histograms were analyzed, incorporating thyroid volume and V5-60 into the analysis. Logistic regression and receiver operating characteristic (ROC) analysis were performed to identify predictors and assess the model's predictive value.

RESULTS

Among the 84 eligible patients, 17 (20.2%) developed hypothyroidism within 1 year. The percentage of thyroid volume receiving 30 Gy (V30) emerged as the sole significant dosimetric predictor of EO-PRH (odds ratio [OR] 1.02, 95% confidence interval [95% CI] 1.005-1.05, p = 0.03). Univariable analysis revealed significant differences in cancer histopathology, primary tumor site, V15,30, and VS15,30 (the volume of the thyroid spared from radiation doses 15 Gy and 30 Gy) between the hypothyroid and euthyroid groups (p < 0.10). Multivariable analysis identified the primary cancer site (OR 9.09, 95% CI 1.59-100) and V30 (OR 1.26, 95% CI 1.007-1.76) as independent significant variables predicting EO-PRH. The predictive model incorporating cancer histopathology, primary tumor site, V15,30, and VS15,30 effectively predicted postradiation thyroid dysfunction (area under the receiver operating characteristic curve [AUC-ROC] 0.84, 95% CI 0.73-0.95, p < 0.001).

CONCLUSIONS

V30 could serve as a dosimetric predictor of hypothyroidism following neck radiotherapy. This study underscores that a predictive model encompassing cancer type and site, along with V15,30 and VS15,30, can effectively predict EO-PRH.

Preliminary study on the correlation between thyroid magnetic resonance parameters and radiation dose after radiotherapy for nasopharyngeal carcinoma

Background

Hypothyroidism is a common sequela after radiotherapy for nasopharyngeal carcinoma (NPC). Magnetic resonance imaging (MRI) has gained prominence in thyroid imaging, leveraging its non-ionizing radiation, high spatial resolution, multiparameter and multidirectional imaging. Few previous studies have investigated the evaluation of radiation-induced thyroid injury by MRI.

Methods

MRI and radiotherapy data of 32 patients who were first diagnosed with nasopharyngeal carcinoma in our hospital from April 2015 to April 2024 and underwent radiotherapy in the radiotherapy department were retrospectively collected. Before, during and after radiotherapy, the thyroid morphology was observed on MR images, and the quantitative parameters of size (width, thickness) were measured on T1-weighted images. The signal intensity (SI) of the thyroid gland was measured on T1-weighted imaging (T1WI), T2-weighted imaging (T2WI) and contrast-enhanced T1-weighted imaging. The differences in thyroid parameters at different time points before and after radiotherapy were compared. The correlation between the MRI quantitative parameters of the thyroid and the radiation dose volume of the thyroid and the radiation dose of the pituitary were analyzed.

Results

The width, thickness and volume of the thyroid decreased gradually before, during and 6 and 12 months after radiotherapy. They were negatively correlated with the mean thyroid dose and V50 (p < 0.05), but were not significantly correlated with the maximum and minimum thyroid doses, V30 and V35 (p > 0.05). The T1WI relative signal intensity (RSI), T2WI RSI, and enhanced T1WI RSI of the thyroid gland gradually decreased from before radiotherapy to during radiotherapy and 6 months and 12 months after radiotherapy. The T1WI RSI, T2WI RSI, and enhanced T1WI RSI during radiotherapy and 6 months and 12 months after radiotherapy were negatively correlated with the mean radiation dose, V40, V45, and V50 of the thyroid gland (p < 0.05), but were not significantly correlated with the maximum radiation dose, minimum radiation dose, V30, and V35 of the thyroid gland or the radiation dose of the pituitary gland (p > 0.05).

Conclusion

Quantitative MRI analysis can non-invasively and effectively show the changes in thyroid shape, size and signal intensity in patients with nasopharyngeal carcinoma before and after radiotherapy, which is crucial for early and accurate assessment of thyroid damage, enabling timely treatment to preserve thyroid function.

Thyroid Dysfunction After Intensity-Modulated Radiotherapy and PD⁃1 Inhibitor Treatment for Locally Advanced Nasopharyngeal Carcinoma

Purpose

Analyze the incidence and risk factors of thyroid dysfunction in patients with advanced nasopharyngeal carcinoma (LA-NPC) after intensity-modulated radiotherapy (IMRT) and PD⁃1 inhibitor treatment and their relationship with treatment efficacy and prognosis.

Methods

Eighty-five LA-NPC patients treated with IMRT and PD-1 inhibitors were retrospectively collected from March 1, 2019, to May 30, 2022. The incidence of thyroid dysfunction after combination therapy was analyzed. The Kaplan-Meier method was used to analyze the relationship between thyroid dysfunction and patient prognosis. Logistic regression analysis was used to screen independent risk factors for thyroid dysfunction.

Results

As of data cutoff (May 31, 2024), the median follow-up time was 27.8 months (range: 25.6 to 32.0 months). The median time of onset of thyroid dysfunction was 8.26 months. The incidence of thyroid dysfunction is 47.06% (40/85), with clinical hypothyroidism being the main cause at an incidence rate of 28.24% (24/85) and clinical hyperthyroidism at an incidence rate of 3.53% (3/85). The incidence of grade 1 thyroid immune-related adverse events (irAEs) was 29.41% (25/85), and the incidence of grade 2 thyroid irAEs was 17.65% (15/85). Patients with thyroid dysfunction had longer overall survival, progression-free survival, and distant metastasis-free survival at both one and two years compared to patients with normal thyroid function, but the difference was not statistically significant (p > 0.05). Multivariate logistic regression analysis showed that pretreatment lactate dehydrogenase (LDH) (p = 0.079) is an independent predictor of thyroid dysfunction after radiotherapy in combination with immunotherapy for LA-NPC.

Conclusion

The study found that the addition of immunotherapy increases the risk and shortens the onset time of thyroid dysfunction in LA-NPC patients treated with chemoradiotherapy. Pretreatment LDH may serve as an independent risk factor for thyroid dysfunction for LA-NPC patients.

Thyroid Dosimetry and Its Association With Radiation-Induced Hypothyroidism in Head and Neck Cancer Patients Treated With Conformal Radiotherapy: An Observational Study

Introduction Hypothyroidism is a common side effect in head and neck cancer (HNC) patients treated with radiotherapy (RT). Conformal RT reduces the dose to normal tissues while delivering higher doses to tumors. However, hypothyroidism remains a significant toxicity in these patients. This study evaluates the incidence of hypothyroidism, thyroid dosimetric parameters, and their correlation with radiation-induced hypothyroidism in HNC patients treated with conformal RT at a regional cancer center in India. Methods Fifty patients with histologically confirmed squamous cell carcinoma of the head and neck, treated with conformal RT, and who underwent pre- and post-treatment thyroid function tests were included in the study. All patients were euthyroid before treatment. The thyroid gland was contoured retrospectively in the approved RT plan. The volume of the contoured thyroid gland and the thyroid dosimetric parameters (Dmean, Dmax, Dmin, and V30-V60) were recorded. Pre- and post-treatment thyroid function test results were compared to look for the incidence of hypothyroidism, and dosimetric data were compared to establish a relation with the incidence of hypothyroidism. Results The median age was 53 years. Most patients had oral cavity primaries (60%), stage III/IVA disease (86%), and received definitive (64%) or adjuvant (36%) RT. Two-thirds underwent concurrent chemotherapy. After a median follow-up of four months, 24% developed hypothyroidism, with a median onset of 16 weeks post-treatment. Pharyngeal primary and concurrent chemotherapy were significant risk factors. Dosimetric analysis revealed Dmean > 57 Gy, V55 > 80%, and V60 > 37% as predictors of hypothyroidism. Conclusion Pharyngeal cancers, concurrent chemoradiotherapy, and higher thyroid doses significantly increase the risk of hypothyroidism. Optimizing RT planning is essential to minimize thyroid toxicity, particularly in high-risk patients.

A multivariable normal tissue complication probability model for predicting radiation-induced hypothyroidism in nasopharyngeal carcinoma patients in the modern radiotherapy era

Radiation-induced hypothyroidism (RHT) is a common long-term complication for nasopharyngeal carcinoma (NPC) survivors. A model using clinical and dosimetric factors for predicting risk of RHT could suggest a proper dose-volume parameters for the treatment planning in an individual level. We aim to develop a multivariable normal tissue complication probability (NTCP) model for RHT in NPC patients after intensity-modulated radiotherapy or volumetric modulated arc therapy. The model was developed using retrospective clinical data and dose-volume data of the thyroid and pituitary gland based on a standard backward stepwise multivariable logistic regression analysis and was then internally validated using 10-fold cross-validation. The final NTCP model consisted of age, pretreatment thyroid-stimulating hormone and mean thyroid dose. The model performance was good with an area under the receiver operating characteristic curve of 0.749 on an internal (200 patients) and 0.812 on an external (25 patients) validation. The mean thyroid dose at ≤45 Gy was suggested for treatment plan, owing to an RHT incidence of 2% versus 61% in the >45 Gy group.

Development and validation of a machine learning model of radiation-induced hypothyroidism with clinical and dose-volume features

BACKGROUND AND PURPOSE

Radiation-induced hypothyroidism (RIHT) is a common but underestimated late effect in head and neck cancers. However, no consensus exists regarding risk prediction or dose constraints in RIHT. We aimed to develop a machine learning model for the accurate risk prediction of RIHT based on clinical and dose-volume features and to evaluate its performance internally and externally.

MATERIALS AND METHODS

We retrospectively searched two institutions for patients aged >20 years treated with definitive radiotherapy for nasopharyngeal or oropharyngeal cancer, and extracted their clinical information and dose-volume features. One was designated the developmental cohort, the other as the external validation cohort. We compared the performances of machine learning models with those of published normal tissue complication probability (NTCP) models.

RESULTS

The developmental and external validation cohorts consisted of 378 and 49 patients, respectively. The estimated cumulative incidence rates of grade ≥1 hypothyroidism were 53.5% and 61.3% in the developmental and external validation cohorts, respectively. Machine learning models outperformed traditional NTCP models by having lower Brier scores at every time point and a lower integrated Brier score, while demonstrating a comparable calibration index and mean area under the curve. Even simplified machine learning models using only thyroid features performed better than did traditional NTCP algorithms. The machine learning models showed consistent performance between folds. The performance in a previously unseen external validation cohort was comparable to that of the cross-validation.

CONCLUSIONS

Our model outperformed traditional NTCP models, with additional capabilities of predicting the RIHT risk at individual time points. A simplified model using only thyroid dose-volume features still outperforms traditional NTCP models and can be incorporated into future treatment planning systems for biological optimization.

Hypothyroidism following Radiotherapy for Head and Neck Cancer: A Systematic Review of the Literature and Opportunities to Improve the Therapeutic Ratio

(1) Background: Radiotherapy (RT) is a central component for the treatment of many head and neck cancers. In this systematic review of the literature, we aimed to characterize and quantify the published evidence on RT-related hypothyroidism, including estimated incidence, clinical risk factors, and dosimetric parameters that may be used to guide clinical decision making. Furthermore, we aimed to identify potential areas of improvement in the prevention and clinical management of RT-induced hypothyroidism, including the role of modern advanced therapeutic techniques. (2) Methods: We conducted a systemic review of the literature in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. PubMed and Google Scholar were searched to identify original research articles describing the incidence, mechanism, dosimetry, treatment, or prevention of radiation-related hypothyroidism for adults receiving RT for the treatment of head and neck cancers. The snowball method was used to identify additional articles. For identified articles, we tabulated several datapoints, including publication date, patient sample size, estimated hypothyroidism incidence, cancer site/type, follow-up period, radiation modality and technique, use of multimodality therapy, method of thyroid function evaluation, and proposed dosimetric predictors of hypothyroidism. (3) Results: One hundred and eleven articles met inclusion criteria, reflecting a range of head and neck cancer subtypes. There was a large variation in the estimated incidence of RT-related hypothyroidism, with a median estimate of 36% (range 3% to 79%). Reported incidence increased in later publication dates, which was likely related to improved screening and longer follow up. There were a wide variety of predictive metrics used to identify patients at high risk of hypothyroidism, the most common of which were volumetric and mean dosimetrics related to the thyroid gland (Vxx%, Dmean). More recently, there has been increasing evidence to suggest that the thyroid gland volume itself and the volume of the thyroid gland spared from high-dose radiation (VSxx) may better predict thyroid function after RT. There were no identified studies investigating the role of advanced radiotherapeutic techniques such as MRI-guided RT or particle therapy to decrease RT-related hypothyroidism. Conclusions: Hypothyroidism is a common toxicity resulting from therapeutic radiation for head and neck cancer with recent estimates suggesting 40-50% of patients may experience hypothyroidism after treatment. Dosimetric predictive models are increasingly able to accurately identify patients at risk of hypothyroidism, especially those utilizing thyroid VS metrics. Further investigation regarding the potential for advanced radiotherapeutic therapies to decrease RT-induced thyroid dysfunction is needed.

Post-radiation primary hypothyroidism in patients with head and neck cancer: External validation of thyroid gland dose-volume constraints with long-term endocrine outcomes

BACKGROUND

Post-radiation primary hypothyroidism is a common late complication in head and neck cancer (HNC) survivors. No radiation dose-volume constraint of the thyroid gland has been externally validated for predicting long-term thyroid function outcomes.

MATERIALS AND METHODS

This external validation study evaluated the diagnostic properties of 22 radiation dose-volume constraints of the thyroid gland proposed in the literature. Radiation dosimetric data from 488 HNC patients who underwent neck irradiation from January 2013 to December 2015 at two tertiary oncology centers were reviewed. The diagnostic metrics of candidate constraints were computed by inverse probability of censoring weighting and compared using time-dependent receiver operating characteristic (ROC) curves with death designated as a competing event. Multivariable regression analyses were performed using the Fine-Gray sub-distribution hazard model.

RESULTS

Over a median follow-up period of 6.8 years, 205 (42.0 %) patients developed post-radiation primary hypothyroidism. The thyroid volume spared from 60 Gy (VS60) had the largest area under ROC curve of 0.698 at 5 years after radiotherapy. Of all evaluated constraints, VS60 at a cutoff value of 10 cc had the highest F-score of 0.53. The 5-year hypothyroidism risks of patients with thyroid VS60 ≥ 10 cc and < 10 cc were 14.7 % and 38.2 %, respectively (p < 0.001). The adjusted sub-hazard ratio for post-radiation primary hypothyroidism for VS60 < 10 cc was 1.87 (95 % confidence interval, 1.22-2.87; p < 0.001).

CONCLUSION

Thyroid VS60 is the best radiation dose-volume parameter to predict the long-term risk of primary hypothyroidism in patients with HNC who underwent neck irradiation. VS60 ≥ 10 cc is a robust constraint that limits the 5-year primary hypothyroidism risk to less than 15 % and should be routinely employed during radiotherapy optimization.

Predictors of radiation-induced hypothyroidism in nasopharyngeal carcinoma survivors after intensity-modulated radiotherapy

Background

The aim of the study is to identify clinical and dosimetric factors that could predict the risk of hypothyroidism in nasopharyngeal carcinoma (NPC) patients following intensity-modulated radiotherapy (IMRT).

Methods

A total of 404 non-metastatic NPC patients were included in our study. All patients were treated with IMRT. The thyroid function were performed for all patients before and after radiation at regular intervals. The time onset for developing hypothyroidism was defined as the time interval between the completion of RT and the first recorded abnormal thyroid hormone test. The cumulative incidence rates of hypothyroidism were estimated using Kaplan–Meier method. Univariate and multivariate Cox regression analyses were performed to detect the most promising factors that were associated with hypothyroidism.

Results

Median follow up was 60.6 months. The 3-, 5- and 7- year cumulative incidence rate of hypothyroidism was 39.4%, 49.1% and 54.7%, respectively. The median time to primary hypothyroidism and central hypothyroidism were 15.4 months (range 2.9–83.8 months) and 29.9 months (range 19.8–93.6 months), respectively. Univariate and multivariate analyses revealed that younger age, female gender and small thyroid volume were the most important factors in predicting the risk of hypothyroidism. Dtmean (mean dose of thyroid), V30-V50 (percentage of thyroid volume receiving a certain dose level) and VS45-VS60 (the absolute volumes of thyroid spared from various dose levels) remained statistically significant in multivariate analyses. Cutoff points of 45 Gy (Dtmean), 80% (Vt40) and 5 cm³ (VS45Gy) were identified to classify patients as high-risk or low-risk group.

Conclusion

Thyroid Vt40 highly predicted the risk of hypothyroidism after IMRT for NPC patients. We recommended plan optimization objectives to reduce thyroid Vt40 to 80%.

Trial registration: Retrospectively registered.

Dose-volume predictors of post-radiation primary hypothyroidism in head and neck cancer: a systematic review

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This systematic review included 29 studies (n = 4,530 patients) on dosimetric predictors of primary hypothyroidism in HNC.

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Average crude incidence of primary hypothyroidism after HNC radiotherapy was 41.4%.

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Thyroid Dmean and V50 were the most widely reported dosimetric predictors for hypothyroidism.

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Thyroid volume is a predictor of hypothyroidism (pooled aOR 0.89 per 1 cc increment) independent of radiation dosimetry.

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Thyroid gland constraints individualized for thyroid volume are crucial in HNC radiotherapy.

Background and Purpose

This systematic review aims to identify radiation dose-volume predictors of primary hypothyroidism after radiotherapy in patients with head and neck cancer (HNC).

Materials and methods

We performed a systematic literature search of Medline, EMBASE and Web of Science from database inception to July 1, 2021 for articles that discuss radiation dose-volume predictors of post-radiation primary hypothyroidism in patients with HNC. Data on the incidence, clinical risk factors and radiation dose-volume parameters were extracted. A meta-analysis was performed using the random-effects model to estimate the pooled odds ratio (OR) of thyroid volume as a predictor of the risk of post-radiation hypothyroidism, adjusted for thyroid radiation dosimetry.

Results

Our search identified 29 observational studies involving 4,530 patients. With median follow-up durations ranging from 1.0 to 5.3 years, the average crude incidence of post-radiation primary hypothyroidism was 41.4 % (range, 10 %–57 %). Multiple radiation dose-volume parameters were associated with post-radiation primary hypothyroidism, including the thyroid mean dose (Dmean), minimum dose, V25, V30, V35, V45, V50, V30–60, VS45 and VS60. Thyroid Dmean and V50 were the most frequently proposed dosimetric predictors. The pooled adjusted OR of thyroid volume on the risk of post-radiation primary hypothyroidism was 0.89 (95 % confidence interval, 0.85–0.93; p < 0.001) per 1 cc increment.

Conclusion

Post-radiation primary hypothyroidism is a common late complication after radiotherapy for HNC. Minimizing inadvertent exposure of the thyroid gland to radiation is crucial to prevent this late complication. Radiation dose-volume constraints individualized for thyroid volume should be considered in HNC radiotherapy planning.

Organs at risk radiation dose constraints

Dose constraints are essential for performing dosimetry, especially for intensity modulation and for radiotherapy under stereotaxic conditions. We present the update of the recommendations of the French society of oncological radiotherapy for the use of these doses in classical current practice but also for reirradiation.

Modern Rotational Radiation Techniques with Volumetric Modulated Arc Therapy or Helical Tomotherapy for Optimal Sparing of the Lung and Heart in Left-Breast Cancer Radiotherapy Plus Regional Nodal Irradiation: A Comparative Dosimetric Analysis

Simple Summary

For advanced left-breast cancer patients, adjuvant radiotherapy (RT) with regional nodal irradiation (RNI) has been indicated to reduce cancer recurrence and mortality. Modern arc RT techniques, volumetric-modulated arc therapy (VMAT), or helical tomotherapy (HT), can minimize normal organ exposure without compromising disease control. The aim of this study is to identify which arc technique is optimal for patients receiving left-breast RT with RNI, and to explore distinct RNI volumes with or without IMN. A total of 108 eligible patients were enrolled (70 VMAT, 38 HT). VMAT reduced the mean dose and low-dose exposure to the heart, ipsilateral lung, whole lung, contralateral breast, and esophagus compared with HT. The advantage of VMAT for normal organ sparing was distinct when performing RNI with IMN irradiation. To limit normal organ exposure and reduce potential toxicities, VMAT is the optimal technique for patients with left-breast cancer who are undergoing RT with RNI.

Abstract

Background: For advanced breast cancer with lymph node involvement, adjuvant radiotherapy (RT) with regional nodal irradiation (RNI) has been indicated to reduce cancer recurrence and mortality. However, an extensive RT volume is associated with normal organ exposure, which increases the toxicity and affects patient outcomes. Modern arc RT techniques can improve normal organ sparing compared with conventional techniques. The aim of this study was to explore the optimal technique for left-breast RT with RNI. Methods: We retrospectively reviewed patients receiving RT with RNI for left-breast cancer. We used modern arc RT techniques with either volumetric-modulated arc therapy (VMAT) or helical tomotherapy (HT) with a novel block technique, and compared differences in dosimetry parameters between the two groups. Subgroup analysis of RNI with or without internal mammary node (IMN) volume was also performed. Results: A total of 108 eligible patients were enrolled between 2017 and 2020, of whom 70 received VMAT and 38 received HT. The median RT dose was 55 Gy. No significant differences were found regarding the surgery, RT dose, number of fractions, target volume, and RNI volume between the VMAT and HT groups. VMAT reduced the heart mean dose more than HT (3.82 vs. 5.13 Gy, p < 0.001), as well as the cardiac parameters of V5–V20, whole-lung mean dose, lung parameters of V5–V20, and contralateral-breast and esophagus mean dose. In the subgroup analysis of RNI with IMNs, the advantage of VMAT persisted in protecting the heart, lung, contralateral breast, and esophagus. HT was beneficial for lowering the thyroid mean dose. For RNI without IMN, VMAT improved the low-dose exposure of the heart and lung, but HT was similar to VMAT in terms of heart, whole-lung, and contralateral-breast mean dose. Conclusions: For patients with left-breast cancer receiving adjuvant RT with RNI, VMAT reduced the exposure dose to the heart, lung, contralateral breast, and esophagus compared with HT. VMAT was superior to HT in terms of normal organ sparing in the patients who underwent RNI with IMN irradiation. Considering the reduction in normal organ exposure and potential toxicity, VMAT is the optimal technique for patients receiving RNI when deep inspiration breath-hold is not available.

Multivariate NTCP Model of Hypothyroidism After Intensity-Modulated Radiotherapy for Nasopharyngeal Carcinoma

Objective

To evaluate the incidence of hypothyroidism in patients with nasopharyngeal carcinoma after intensity-modulated radiotherapy (IMRT), analyze its correlation with multiple influencing factors such as thyroid exposure dose, thyroid volume, and gender, and construct a multivariate-based normal tissue complication probability (NTCP) model for the occurrence of hypothyroidism after IMRT.

Materials and Methods

The thyroid hormone levels of patients at different points in time before and after radiotherapy were tested, and statistics on the incidence of hypothyroidism after treatment were obtained. The dose-volume data of patients’ thyroids were converted into EQD2 equivalent dose values. The correlation between hypothyroidism after radiotherapy and thyroid exposure dose, thyroid volume, gender, and other factors was analyzed, and an NTCP model was constructed.

Results

A total of 69 patients with nasopharyngeal carcinoma were enrolled in this study. Twelve months after radiotherapy, a total of 24 patients (34.8%) developed hypothyroidism. Univariate analysis and multivariate analysis revealed that the average thyroid dose and thyroid volume are the most important factors affecting hypothyroidism after radiotherapy. The NTCP model constructed based on the average dose and thyroid volume has a good degree of fit.

Conclusion

The volume and average dose of the thyroid gland are the key factors affecting the occurrence of hypothyroidism in patients with nasopharyngeal carcinoma after radiotherapy. The NTCP model constructed based on multivariate construction suggests that reducing the average dose of the thyroid to the greatest extent is an effective way to protect thyroid functions.

Radiation-induced hypothyroidism in patients with nasopharyngeal carcinoma treated with intensity-modulated radiation therapy with or without chemotherapy: Development of a nomogram based on the equivalent dose

OBJECTIVE

The aim of this study was to establish a nomogram for predicting radiation-induced hypothyroidism (RHT) based on an equivalent dose at 2 Gy per fraction (EQD₂) in patients with nasopharyngeal carcinoma (NPC) treated with intensity-modulated radiation therapy (IMRT) with or without chemotherapy.

METHODS

Two hundred forty-four eligible patients with NPC were recruited for this study. Patients' clinical factors and dose-volume parameters of the thyroid gland were retrieved from medical records and the IMRT treatment planning system, respectively. The irradiation doses were converted into EQD₂ for analysis. Least absolute shrinkage and selection operator (LASSO) regression analysis and multivariate logistic regression analysis were performed to identify optimal predictors of RHT for constructing the nomogram.

RESULTS

With a median follow-up of 63.0 months, the cumulative incidence rates of RHT at 3 months and 1-, 2-, 3-, 4- and 5- year after IMRT were 10.2%, 36.2%, 47.6%, 54.2%, 58.8% and 69.4%, respectively. Four independent factors for predicting RHT, including gender, age, pretreatment volume of the thyroid gland and V_35Gy(3Gy) of the thyroid gland, were identified and incorporated into the nomogram. The area under the ROC curve of the nomogram was 0.747 (95% confidence interval 0.685 - 0.809). Calibration curves and DCA curves showed that the nomogram was in good agreement with the actual observations and clinical usefulness.

CONCLUSIONS

The nomogram proposed in this study provides a reliable estimate of RHT risk in patients with NPC after IMRT and appears to have the potential to be a useful tool for widespread clinical applications.

Research progress of radiation-induced hypothyroidism in head and neck cancer

This paper reviews the factors related to hypothyroidism after radiotherapy in patients with head and neck cancer to facilitate the prevention of radiation-induced hypothyroidism and reduce its incidence. Hypothyroidism is a common complication after radiotherapy in patients with head and neck cancer, wherein the higher the radiation dose to the thyroid and pituitary gland, the higher the incidence of hypothyroidism. With prolonged follow-up time, the incidence of hypothyroidism gradually increases. Intensity modulated radiotherapy should limit the dose to the thyroid, which would reduce the incidence of hypothyroidism. In addition, the risk factors for hypothyroidism include small thyroid volume size, female sex, and previous neck surgery. The incidence of radiation-induced hypothyroidism in head and neck cancer is related to the radiation dose, radiotherapy technique, thyroid volume, sex, and age. A prospective, large sample and long-term follow-up study should be carried out to establish a model of normal tissue complications that are likely to be related to radiation-induced hypothyroidism.

Thyroid V50 is a risk factor for hypothyroidism in patients with nasopharyngeal carcinoma treated with intensity-modulated radiation therapy: a retrospective study

BACKGROUND

We investigated the risk factors of radiation-induced thyroid dysfunction, then combined the clinical factors and optimum thyroid dosimetric parameters to predict the incidence rate of hypothyroidism (HT) and to guide individualized treatment.

METHODS

A total of 206 patients with histologically proven nasopharyngeal carcinoma (NPC) treated at the Cancer Hospital of the University of Chinese Academy of Sciences between January 2015 and August 2018 were included. Dose-volume histogram (DVH) data, including mean dose, absolute volume, V₂₀, V₂₅, V₃₀, V₃₅, V₄₀, V₄₅, V_50, V₅₅, and V₆₀ were extracted and used as dosimetric parameters. A logistic regression analysis model was built to identify predictors related to HT occurring within 2 years.

RESULTS

Sex, N stage, thyroid volume, mean thyroid dose, and thyroid V₂₀ and V₅₀ were significantly different between patients with and without HT. Logistic regression analysis showed that N stage, thyroid volume, and thyroid V₅₀ were independent predictors of HT. The radiosensitivity of the thyroid decreased as the thyroid volume increased. Patients with N stage > 1 had significantly higher HT incidence (37.38%) than patients with N stage ≤1 (13.11%). The incidence of HT was 54.55% in patients with thyroid V₅₀ > 24% and was 34.15% in patients with thyroid V₅₀ ≤ 24%.

CONCLUSIONS

The incidence of HT is significantly associated with N stage, thyroid volume, and thyroid V₅₀. More attention should be paid to patients with NPC with thyroid volume ≤ 12.82 cm³ and advanced N stage disease.

Dose-volume derived nomogram as a reliable predictor of radiotherapy-induced hypothyroidism in head and neck cancer patients

Background

The aim of this study was to determine the possible predictive value of various dosimetric parameters on the development of hypothyroidism (HT) in patients with head and neck squamous cell carcinoma (HNSCC) treated with (chemo)radiotherapy.

Patients and methods

This study included 156 patients with HNSCC who were treated with (chemo)radiotherapy in a primary or postoperative setting between August 2012 and September 2017. Dose-volume parameters as well as V10 toV70, D02 to D98, and the VS10 to VS70 were evaluated. The patients’ hormone status was regularly assessed during follow-up. A nomogram (score) was constructed, and the Kaplan-Maier curves and Log-Rank test were used to demonstrate the difference in incidence of HT between cut-off values of specific variables.

Results

After a median follow-up of 23.0 (12.0–38.5) months, 70 (44.9%) patients developed HT. In univariate analysis, VS65, Dmin, V50, and total thyroid volume (TTV) had the highest accuracy in predicting HT. In a multivariate model, HT was associated with lower TTV (OR 0.31, 95% CI 0.11–0.87, P = 0.026) and Dmin (OR 9.83, 95% CI 1.89–108.08, P = 0.042). Hypothyroidism risk score (HRS) was constructed as a regression equation and comprised TTV and Dmin. HRS had an AUC of 0.709 (95% CI 0.627–0.791). HT occurred in 13 (20.0%) patients with a score < 7.1 and in 57 (62.6%) patients with a score > 7.1.

Conclusions

The dose volume parameters VS65, Dmin, V50, and TTV had the highest accuracy in predicting HT. The HRS may be a useful tool in detecting patients with high risk for radiation-induced hypothyroidism.

International Guideline on Dose Prioritization and Acceptance Criteria in Radiation Therapy Planning for Nasopharyngeal Carcinoma

PURPOSE

The treatment of nasopharyngeal carcinoma requires high radiation doses. The balance of the risks of local recurrence owing to inadequate tumor coverage versus the potential damage to the adjacent organs at risk (OARs) is of critical importance. With advancements in technology, high target conformality is possible. Nonetheless, to achieve the best possible dose distribution, optimal setting of dose targets and dose prioritization for tumor volumes and various OARs is fundamental. Radiation doses should always be guided by the As Low As Reasonably Practicable principle. There are marked variations in practice. This study aimed to develop a guideline to serve as a global practical reference.

METHODS AND MATERIALS

A literature search on dose tolerances and normal-tissue complications after treatment for nasopharyngeal carcinoma was conducted. In addition, published guidelines and protocols on dose prioritization and constraints were reviewed. A text document and preliminary set of variants was circulated to a panel of international experts with publications or extensive experience in the field. An anonymized voting process was conducted to rank the proposed variants. A summary of the initial voting and different opinions expressed by members were then recirculated to the whole panel for review and reconsideration. Based on the comments of the panel, a refined second proposal was recirculated to the same panel. The current guideline was based on majority voting after repeated iteration for final agreement.

RESULTS

Variation in opinion among international experts was repeatedly iterated to develop a guideline describing appropriate dose prioritization and constraints. The percentage of final agreement on the recommended parameters and alternative views is shown. The rationale for the recommendations and the limitations of current evidence are discussed.

CONCLUSIONS

Through this comprehensive review of available evidence and interactive exchange of vast experience by international experts, a guideline was developed to provide a practical reference for setting dose prioritization and acceptance criteria for tumor volumes and OARs. The final decision on the treatment prescription should be based on the individual clinical situation and the patient's acceptance of optimal balance of risk.

[Post-treatment follow-up of head and neck cancer patients]

Post-therapeutic follow-up of patients with head and neck cancer involves numerous professionals. The radiation oncologist should play an active role in this process. His oncological knowledge and technical expertise position him as a cornerstone for the detection of recurrences from the treated tumor, the research of second primary cancers and the screening of potential side-effects induced by the different treatments administered. To improve the benefits/costs ratio and allow good patient-compliance, follow-up programs should be built through close collaboration between the different contributors and planned according to a feasible schedule. Paraclinical exams must be arranged to respond to accurate objectives. Patient-education is essential to ensure the patient's full understanding and active participation. Finally, the transfer of the long-term follow-up of cancer survivors from specialists to primary care physicians is relevant but would require a prospective evaluation of its efficiency for this specific population.

A review on the dosimetrical and radiobiological prediction of radiation-induced hypothyroidism in radiation therapy of head-and-neck cancer, breast cancer, and Hodgkin’s lymphoma survivors

A review on the radiobiological modeling of radiation-induced hypothyroidism after radiation therapy of head-and-neck cancers, breast cancer, and Hodgkin’s lymphoma is presented. The current review is based on data relating to dose-volume constrains and normal tissue complication probability (NTCP) as a function of either radiobiological or (pre)treatment-clinical parameters. Also, these data were explored in order to provide more helpful criteria for radiobiological optimization of treatment plans involving thyroid gland as a critical normal organ.

Revisiting the dose constraints for head and neck OARs in the current era of IMRT

Head and neck cancer poses a particular challenge in radiation therapy, whilst being an effective treatment modality it requires very high doses of radiation to provide effective therapy. This is further complicated by the fact that the head and neck region contains a large number of radiosensitive tissues, often resulting in patients experiencing debilitating normal tissue complications. In the era of intensity-modulated radiation therapy (IMRT) treatments can be delivered using non-uniform dose distributions selectively aimed at reducing the dose to critical organs-at-risk while still adequately covering the tumor target. Dose-volume constraints for the different risk organs play a vital role in one's ability to devise the best IMRT treatment plan for a head and neck cancer patient. To this end, it is pivotal to have access to the latest and most relevant dose constraints available and as such the goal of this review is to provide a summary of suggested dose-volume constraints for head and neck cancer RT that have been published after the QUANTEC reports were made available in early 2010.

A dosimetric study on radiation-induced hypothyroidism following intensity-modulated radiotherapy in patients with nasopharyngeal carcinoma

The objective of the present study was to investigate the association between thyroid gland-dosimetric parameters and hypothyroidism induced by intensity-modulated radiotherapy in patients with nasopharyngeal carcinoma (NPC). A total of 52 patients with NPC treated in the Department of Radiation Oncology of The Affiliated Hospital of Xuzhou Medical University, from May 2008 to December 2016 were retrospectively enrolled in the present study and divided into two groups based on thyroid function: The euthyroid and hypothyroid groups. The association between hypothyroidism and clinical or dosimetric parameters were analyzed. Females had a significantly increased probability of suffering from radiation-induced hypothyroidism (RIHT), compared with males (P=0.010). The occurrence of RIHT was significantly negatively associated with thyroid volume prior to radiotherapy (P=0.048). Furthermore, the mean dose (Dmean) and V50 in the hypothyroidism group were significantly increased, compared with the euthyroidism group (P=0.017 and P=0.023, respectively). During the treatment optimization period, dose constraints associated with the thyroid gland demonstrated a significantly protective effect on thyroid function compared with the unconstrained group (P=0.034). According to the receiver operating characteristic curves, the threshold value was 5,160 cGy for Dmean and 54.5% for V50. The 3-year cumulative incidence of RIHT was 67.8% when the Dmean value was >5,160 cGy and 44.6% when the Dmean was <5,160 cGy (log rank test, P=0.036). Furthermore, the 3-year cumulative incidence was 66.1% when the V50 was >54.5%, and 29.9% when the V50 was <54.5% (log rank test, P=0.025). In conclusion, RIHT is associated with radiation dose, particularly with Dmean and V50 of the thyroid gland. Dose constraints associated with the thyroid gland significantly reduced the incidence of hypothyroidism compared with the unconstrained group.

Validation of previously reported predictors for radiation-induced hypothyroidism in nasopharyngeal cancer patients treated with intensity-modulated radiation therapy, a post hoc analysis from a Phase III randomized trial

This study aimed to validate previously reported dosimetric parameters, including thyroid volume, mean dose, and percentage thyroid volume, receiving at least 40, 45 and 50 Gy (V40, V45 and V50), absolute thyroid volume spared (VS) from 45, 50 and 60 Gy (VS45, VS50 and VS60), and clinical factors affecting the development of radiation-induced hypothyroidism (RHT). A post hoc analysis was performed in 178 euthyroid nasopharyngeal cancer (NPC) patients from a Phase III study comparing sequential versus simultaneous-integrated boost intensity-modulated radiation therapy. RHT was determined by increased thyroid-stimulating hormone (TSH) with or without reduced free thyroxin, regardless of symptoms. The median follow-up time was 42.5 months. The 1-, 2- and 3-year freedom from RHT rates were 78.4%, 56.4% and 43.4%, respectively. The median latency period was 21 months. The thyroid gland received a median mean dose of 53.5 Gy. Female gender, smaller thyroid volume, higher pretreatment TSH level (≥1.55 μU/ml) and VS60 < 10 cm3 were significantly associated with RHT in univariate analyses. Only pretreatment TSH ≥ 1.55 μU/ml and VS60 < 10 cm3 were significant predictors in multivariate analysis. Our results suggested that patients with pretreatment TSH ≥ 1.55 μU/ml should be cautious about the risk of RHT. The VS60 ≥ 10 cm3 is recommended for treatment planning.

Development of a normal tissue complication probability (NTCP) model for radiation-induced hypothyroidism in nasopharyngeal carcinoma patients

BACKGROUND

The objectives of this study were to build a normal tissue complication probability (NTCP) model of radiation-induced hypothyroidism (RHT) for nasopharyngeal carcinoma (NPC) patients and to compare it with other four published NTCP models to evaluate its efficacy.

METHODS

Medical notes of 174 NPC patients after radiotherapy were reviewed. Biochemical hypothyroidism was defined as an elevated level of serum thyroid-stimulating hormone (TSH) value with a normal or decreased level of serum free thyroxine (fT4) after radiotherapy. Logistic regression with leave-one-out cross-validation was performed to establish the NTCP model. Model performance was evaluated and compared by the area under the receiver operating characteristic curve (AUC) in our NPC cohort.

RESULTS

With a median follow-up of 24 months, 39 (22.4%) patients developed biochemical hypothyroidism. Gender, chemotherapy, the percentage thyroid volume receiving more than 50 Gy (V50), and the maximum dose of the pituitary (Pmax) were identified as the most predictive factors for RHT. A NTCP model based on these four parameters were developed. The model comparison was made in our NPC cohort and our NTCP model performed better in RHT prediction than the other four models.

CONCLUSIONS

This study developed a four-variable NTCP model for biochemical hypothyroidism in NPC patients post-radiotherapy. Our NTCP model for RHT presents a high prediction capability.

TRIAL REGISTRATION

This is a retrospective study without registration.

Postradiation hypothyroidism in head and neck cancers: A Department of Veterans Affairs single-institution case-control dosimetry study

We performed a case-control study to characterize the dose-volume relationship and other variables leading to hypothyroidism after head and neck (H&N) cancer radiation therapy (RT) in a homogenous Veterans Affairs (VA) population. All records of patients receiving RT for various H&N cancers at a single VA medical center between 2007 and 2013 (n = 143) were screened for post-RT thyroid stimulating hormone (TSH) levels (n = 77). The thyroid gland was contoured on each slice of the planning computed tomography scan when available (hypothyroid: n = 18; euthyroid > 2 years: n = 16), and dose-volume histograms based on physical dose and biologically equivalent dose (BED) were compared systematically to find the significant dose-volume thresholds that distinguish the patients who developed clinical hypothyroidism. Dosimetric and clinical variables were considered in univariate and multivariate analysis. Preirradiation prevalence of hypothyroidism was 8 of 143 (5.6%). After RT, 36 of 77 (47%) screened patients had abnormally high TSH, of which 22 of 36 (61%) had clinical hypothyroidism after 1.29 ± 0.99 years. The median follow-up durations were 3.3 years and 4.7 years for euthyroid and hypothyroid patients, respectively. Compared with the euthyroid cohort (n = 41), these hypothyroid patients displayed no significant difference in age, gender, primary tumor site, thyroid volume, hypertension, diabetes, or use of chemotherapy, surgery, or intensity-modulated radiation therapy (IMRT). They were more likely to have had stage 3 or 4 cancer than euthyroid patients (86.5% vs 73.2%, p = 0.01). The odds ratios of hypothyroidism for stage 3 + 4 cancers and V50Gy < 75% were 5.0 and 0.2, respectively (p < 0.05). Equivalent BED threshold of V75Gy₃ < 75% gave an odds ratio of 0.156 for developing hypothyroidism (p = 0.02). The prevalence of post-RT clinical hypothyroidism was relatively high for patients with H&N cancers and warrants routine surveillance, especially in those with higher stage malignancy. V50Gy < 75% may be a useful guideline to avoid hypothyroidism. We also show BED data which could be used for unconventionally fractionated schemes, and V75Gy₃ < 75% may be a useful guideline.

Predictors of Hypothyroidism in Hodgkin Lymphoma Survivors After Intensity Modulated Versus 3-Dimensional Radiation Therapy

PURPOSE

To identify predictors of hypothyroidism after chemoradiation therapy for Hodgkin lymphoma (HL) and to compare outcomes after intensity modulated radiation therapy (IMRT) with those after 3-dimensional (3D) conformal radiation therapy (CRT).

METHODS AND MATERIALS

Ninety patients who underwent involved-site IMRT in 2009 through 2014 were evaluated for treatment-induced hypothyroidism, defined as elevated thyroid-stimulating hormone or decreased free thyroxine levels (or both). Receiver operating characteristic curve analysis identified individuals at low versus high risk based on dosimetric variables. Dosimetric cutoff points were verified with an external data set of 50 patients who underwent 3D-CRT.

RESULTS

In the IMRT group, most patients (75 [83%]) had stage II HL, and the median prescribed dose was 30.6 Gy; in the 3D-CRT group, 32 patients (64%) had stage II HL, and the median prescribed dose was 32.0 Gy. No differences were found in the proportions of patients with bilateral (P = .982) or unilateral (P = .074) neck involvement between the 2 groups. Hypothyroidism rates were marginally higher in the IMRT group, with estimated 3-year rates of freedom from hypothyroidism of 56.1% in the 3D-CRT group and 40% in the IMRT group (P = .057). Univariate analysis showed that smaller thyroid volume and higher thyroid dose were associated with hypothyroidism in both groups (P < .05). In the IMRT group, the percentage of the thyroid gland volume receiving ≥25 Gy (V25) and the absolute volume of the thyroid gland spared from 25 Gy (VS25Gy) were the strongest predictors of hypothyroidism (P = .001 and P < .001, respectively). Cutoff points of 63.5% (V25) and 2.2 mL (VS25Gy) classified patients as high risk (80%-82%) or low risk (37%-44%) (P < .001). Use of a thyroid avoidance structure reduced the incidence of hypothyroidism (P < .05) in the IMRT group.

CONCLUSIONS

The percentage of the thyroid receiving 25 Gy and the volume of the thyroid spared from 25 Gy predicted the risk of hypothyroidism after either IMRT or 3D-CRT for HL. IMRT may confer a higher risk than 3D-CRT unless a treatment avoidance structure is used during planning.

Intensity modulated radiotherapy with fixed collimator jaws for locoregional left-sided breast cancer irradiation

The purpose of this study is to evaluate the intensity modulated radiotherapy (IMRT) with the fixed collimator jaws technique (FJT) for the left breast and regional lymph node. The targeted breast tissue and the lymph nodes, and the normal tissues were contoured for 16 left-sided breast cancer patients previously treated with radiotherapy after lumpectomy. For each patient, treatment plans using different planning techniques, i.e., volumetric modulated arc therapy (VMAT), tangential IMRT (tangential-IMRT), and IMRT with FJT (FJT-IMRT) were developed for dosimetric comparisons. A dose of 50Gy was prescribed to the planning target volume. The dose-volume histograms were generated, and the paired t-test was used to analyze the dose differences. FJT-IMRT had similar mean heart volume receiving 30Gy (V30 Gy) with tangential-IMRT (1.5% and 1.6%, p = 0.41), but inferior to the VMAT (0.8%, p < 0.001). In the average heart mean dose comparison, FJT-IMRT had the lowest value, and it was 0.6Gy lower than that for the VMAT plans (p < 0.01). A significant dose increase in the contralateral breast and lung was observed in VMAT plans. Compared with tangential-IMRT and VMAT plans, FJT-IMRT reduced the mean dose of thyroid, humeral head and cervical esophageal by 47.6% (p < 0.01) and 45.7% (p < 0.01), 74.3% (p = < 0.01) and 73% (p = < 0.01), and 26.7% (p = < 0.01) and 29.2% (p = < 0.01). In conclusion, compared with tangential-IMRT and VMAT, FJT-IMRT plan has the lowest thyroid, humeral head and cervical esophageal mean dose and it can be a reasonable treatment option for a certain subgroup of patients, such as young left-breast cancer patients and/or patients with previous thyroid disease.

Nomogram for radiation-induced hypothyroidism prediction in nasopharyngeal carcinoma after treatment

OBJECTIVE

The aim of this study was to develop a nomogram for radiation-induced hypothyroidism (RHT) prediction.

METHODS

We collected data from 164 patients with nasopharyngeal carcinoma (NPC) in our previous prospective study. Biochemical hypothyroidism was defined as a serum thyroid-stimulating hormone level greater than the normal value. We collected both clinical and dose-volume factors. A univariate Cox regression analysis was performed to identify RHT risk factors. Optimal predictors were selected according to the least absolute shrinkage and selection operator (LASSO). We then selected the Cox regression models that best balanced the prediction performance and practicability to build a nomogram for RHT prediction.

RESULTS

There were 38 (23.2%) patients who developed RHT, and the median follow-up was 24 months. The univariate Cox regression analysis indicated that gender, minimum dose, mean dose (D_mean) and V₂₅-V₆₀ [V_x (%), the percentage of thyroid volume receiving >x Gy] of the thyroid were significantly associated with RHT. The variables of gender, receiving chemotherapy or not (chemo), D_mean and V₅₀ were selected using the LASSO analysis. A nomogram based on a three-variable (gender, chemo and V₅₀) Cox regression model was constructed, and its concordance index was 0.72. Good accordance between prediction and observation was showed by calibration curves in the probability of RHT at 18, 24 and 30 months.

CONCLUSION

This study built a nomogram for RHT in NPC survivors by analyzing both clinical and dose-volume parameters using LASSO. Thus, the individual dose constraint could be achieved in a visual format. Advances in knowledge: This study used LASSO to more accurately address the multicollinear problem between variables. The resulting nomogram will help physicians predict RHT.

Dosimetric Predictors of Hypothyroidism After Radical Intensity-modulated Radiation Therapy for Non-metastatic Nasopharyngeal Carcinoma

AIMS

To investigate dosimetric predictors of hypothyroidism after radical intensity-modulated radiation therapy (IMRT) for non-metastatic nasopharyngeal carcinoma (NPC).

MATERIALS AND METHODS

Patients with non-metastatic NPC treated with radical IMRT from 2008 to 2013 were reviewed. Serum thyroid function tests before and after IMRT were regularly monitored. Univariable and multivariable analyses were carried out for predictors of biochemical and clinical hypothyroidism.

RESULTS

In total, 149 patients were recruited. After a median follow-up duration of 3.1 years, 33 (22.1%) and 21 (14.1%) patients developed biochemical and clinical hypothyroidism, respectively. Eight (24.2%) patients who had biochemical hypothyroidism developed clinical hypothyroidism later. Univariable and multivariable analyses revealed that the volume of the thyroid (P=0.002, multivariable), VS60 (the absolute thyroid volume spared from 60 Gy or less) (P<0.001, multivariable) and VS45 (P<0.001, multivariable) of the thyroid were significant predictors of biochemical hypothyroidism. The freedom from biochemical hypothyroidism was longer for those whose VS60 ≥ 10 cm(3) (mean 90.9 versus 62.6 months; P<0.001) and VS45 ≥ 5 cm(3) (mean 91.9 versus 65.2 months; P=0.001). Similarly multivariable analyses revealed that VS60 (P=0.001) and VS45 (P=0.003) were significant predictors of clinical hypothyroidism. The freedom from clinical hypothyroidism was longer for those whose VS60 ≥ 10 cm(3) (91.5 versus 73.3 months; P=0.002) and VS45 ≥ 5 cm(3) (91.5 versus 75.9 months; P=0.007).

CONCLUSIONS

VS60 and VS45 of the thyroid should be considered important dose constraints against hypothyroidism without compromising target coverage during IMRT optimisation for NPC.