Use of radioiodine-131 scan to measure influence of surgical discipline, practice, and volume on residual thyroid tissue after total thyroidectomy for differentiated thyroid carcinoma

A practical individualized radiation precaution based on the dose rate at release time after inpatient 131I ablation therapy

Introduction

Retained radioactivity of ¹³¹I after ablation therapy largely differs in each patient according to factors including the amount of remnant thyroid tissue, renal function, and use of recombinant human thyroid-stimulating hormone. To reduce unnecessary restriction of patient’s daily life after inpatient ¹³¹I ablation therapy, we propose a practical individualized method for radiation precaution based on dose rate at release time.

Methods

We evaluated 215 patients with differentiated thyroid cancer who underwent inpatient ¹³¹I ablation therapy following total thyroidectomy. Effective dose equivalent rates at 1-m distance were measured upon release (EDR_R) on day 2 and during delayed whole-body scan (EDR_D) visits on day 6‒8 after ¹³¹I administration. The biexponential model was designed to estimate total effective dose equivalent to others. To assess conservativeness of our model, EDR_D estimated by our model was compared with measured EDR_D. EDR_R-based periods of precaution not to receiving 1 mSv of radiation exposure were estimated and compared with those based on administered radioactivities on American Thyroid Association (ATA) recommendations.

Results

The EDR_R ranged from 1.0–48.9 μSv/hr. The measured EDR_D were equal to or lower than estimated EDR_D in all patients, except for one, indicating that our model is sufficiently conservative. According to our model, no subjects needed additional daytime restriction after release. The maximum permissible times for public transportation use were longer in all patients compared with those based on administered radioactivities. Nighttime restriction periods were significantly shorter than those based on administered radioactivity; median periods requiring sleeping apart were 0 (range, 0‒5), 4 (range, 1‒14), and 3 (range, 2‒13) days after release in patients treated with radioactivity doses of 2.96, 5.50, and 7.40 GBq, respectively, needing 8, 16, and 19 additional days, respectively, based on administered radioactivity.

Conclusions

Radiation safety instructions using proposed method based on EDR_R of individual patient could safely reduce the burden of radiation precaution.

The Value of Pre-Ablative I-131 Scan for Clinical Management in Patients With Differentiated Thyroid Carcinoma

BACKGROUND

A diagnostic I-131 (Dx) scan is used to detect a thyroid remnant or metastases before treatment of differentiated thyroid cancer (DTC) with I-131. The aim of this study is to specify in which patients with DTC a Dx scan could have an additional value, by studying the effect of the Dx scan on clinical management.

METHODS

Patients with DTC, treated with I-131 after thyroidectomy were included in this retrospective cohort study. Twenty-four hours after administration of 37 MBq I-131 a whole body Dx scan and an uptake measurement at the original thyroid bed were performed. Outcomes of the Dx scan and the subsequent changes in clinical management, defined as additional surgery or adjustment of I-131 activity, were reported. Risk factors for a change in clinical management were identified with a binary logistic regression.

RESULTS

In 11 (4.2%) patients clinical management was changed, including additional surgery (n=5), lowering I-131 activity (n=5) or both (n=1). Risk factors for a change in clinical management were previous neck surgery (OR 5.9, 95% CI: 1.4-24.5), surgery in a non-tertiary center (OR 13.4, 95% CI: 2.8 - 63.8), TSH <53.4 mU/L (OR 19.64, 95% CI: 4.94-78.13), thyroglobulin ≥50.0 ng/L (OR 7.4, 95% CI: 1.6-34.9) and free T4 ≥4.75 pmol/L (OR 156.8, 95% CI: 128.4-864.2).

CONCLUSION

The Dx scan can potentially change clinical management before treatment with I-131, but the yield is low. A Dx-scan should only be considered for patients with a high pre-scan risk of a change in management, based on patient history and prior center-based surgical outcomes.

Postoperative thyroid remnants for differentiated thyroid cancer may not affect the outcome of high-dose radioiodine therapy

OBJECTIVES

This study aims to provide a way to estimate the volume of the thyroid remnant and determine its relationship with the outcome of radioiodine (RAI) therapy in depth.

MATERIALS AND METHODS

A retrospective analysis was performed on patients who underwent initial RAI therapy between January 2010 and January 2016. The patients were divided into five groups based on the thyroid remnant estimated by post-therapy whole-body scan(post-Rx WBS), thyroid scintigraphy and ultrasonography. The relationship between the volume of thyroid remnant and the outcome of RAI therapy were evaluated by univariate analysis and multivariate analysis.

RESULTS

Of 703 patients, the majority could be found different size of thyroid remnants using the three imaging methods, and only few patients(2.1%) could reach no thyroid remnant. There was no association between the volume of thyroid remnant and the outcome of RAI therapy in univariate analysis (χ₂ = 1.633, P = 0.652) and multivariate analysis (P > 0.05). In the subgroup of patients with high-risk factors, there was still no significant difference (intermediate risk subgroup: P = 0.338 vs high risk subgroup: P = 0.263).

CONCLUSION

Different sizes of thyroid remnants were left after surgery. However, in high radioiodine activity, the volume of thyroid remnants may not affect the outcome of RAI therapy even in patients with some high-risk factors, so the high radioiodine activities may resolve the the problem caused by thyroid remnants in some cases.

Negative remnant 99mTc-pertechnetate uptake predicts excellent response to radioactive iodine therapy in low- to intermediate-risk differentiated thyroid cancer patients who have undergone total thyroidectomy

OBJECTIVE

Negative ^99mTc-pertechnetate uptake of the thyroid bed indicates the absence or a small volume of remnant thyroid tissue (RTT) after total thyroidectomy (TT). The aim of this study is to evaluate the predictive value of negative ^99mTc-pertechnetate scintigraphy for excellent response (ER) to radioactive iodine therapy (RIT) in low- to intermediate-risk differentiated thyroid cancer (DTC) patients.

PATIENTS

One-hundred and eighty-nine low- to intermediate-risk DTC patients who underwent TT, RIT with a single dose of 30 mCi and suppressive therapy with thyroid-stimulating hormone (TSH) from July 2015 to February 2016 in our hospital were retrospectively evaluated. ^99mTc-pertechnetate thyroid scintigraphy was performed just before RIT and images were reported dichotomously as negative or positive. The response of patients was assessed for 23.2 ± 3.8 months after RIT and dichotomized as excellent response (ER) or non-excellent response (NER). ^99mTc-pertechnetate uptake, age at diagnosis, gender, multifocality, T stage, N stage, preablative stimulated thyroglobulin (ps-Tg), and TSH were explored as potential predictors for ER.

RESULTS

80.68% (71/88) of patients with negative ^99mTc-pertechnetate uptake achieved ER. When patients were evaluated according to different ps-Tg levels, we found that 94.83% (55/58) of patients with ps-Tg < 1 ng/ml and negative ^99mTc-pertechnetate uptake achieved ER. Multivariate Cox regression analysis revealed that ps-Tg (P = 0.0001) and ^99mTc-pertechnetate uptake (P = 0.0473) were independent predictors for ER.

CONCLUSIONS

In addition to ps-Tg, negative ^99mTc-pertechnetate uptake is also a significant independent predictor for an excellent response in low- to intermediate-risk patients. It may be possible to omit RIT in patients with ps-Tg < 1 ng/ml and concurrent negative ^99mTc-pertechnetate uptake.