Optimal Time of Diagnostic and Posttherapeutic Iodine-131 Whole-body Scan in Post-Operative Pediatric and Young Adult Differentiated Thyroid Cancer Patients

Praveen Kumar; Chandrasekhar Bal; Nishikant Avinash Damle

doi:10.4103/ijnm.ijnm_133_24

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Original Article

39 (

); 428-435

doi:

10.4103/ijnm.ijnm_133_24

Optimal Time of Diagnostic and Posttherapeutic Iodine-131 Whole-body Scan in Post-Operative Pediatric and Young Adult Differentiated Thyroid Cancer Patients

Praveen Kumar, Chandrasekhar Bal^,, Nishikant Avinash Damle

Department of Nuclear Medicine, All India Institute of Medical Sciences, New Delhi, India

Address for correspondence: Dr. Chandrasekhar Bal, Department of Nuclear Medicine, All India Institute of Medical Sciences, New Delhi - 110 029, India. E-mail: csbal@hotmail.com

Received: 2024-10-01, Accepted: 2025-01-13, Published: 2025-03

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Disclaimer:
This article was originally published by Wolters Kluwer - Medknow and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Purpose of the Study:

The 2015 American Thyroid Association pediatric differentiated thyroid cancer (DTC) guidelines recommend posttherapy whole body scan (PTS) 4–7 days after (Iodine-131) I-131 activity administration. There is no recommendation of timing of performing a diagnostic whole-body scan (Dx-WBS). However, this 4–7 day time frame for PTS lacks a solid basis and is essentially arbitrary. This is especially crucial as it has the potential to significantly impact patient management. Our primary goal in this study was to establish the optimal timing for both Dx-WBS and PTS in pediatric and young adult patients with DTC.

Methods:

The DTC patients aged ≤21 years underwent serial whole-body scan (WBS) at 24 h, 48 h, and/or 72 h or more after the administration of diagnostic and therapeutic activities of I-131. The utility of Dx-WBS and PTS was assessed based on the identification of new lesions that could potentially influence the prescribed therapeutic activity of I-131. The optimal timing for acquiring Dx-WBS and PTS was determined based on when the first lesion appeared in the I-131 WBS.

Results:

Ninety-five patients (27 males and 68 females) with an average age of 17.9 ± 3 years received a 74 MBq I-131 for Dx-WBS. Ten patients (10.5%) showed no uptake in Dx-WBS, thus, no I-131 therapy was given. The remaining 85 patients received a therapeutic activity of 1.11–5.55 GBq I-131 based on the extent of their disease. The serial Dx-WBS or PTS showed no additional lesions in patients with thyroid remnants. However, additional nodes were detected in 2/32 patients in the ≥48 h Dx-WBS, and 1/32 patients in the known nodal disease patients, which were not clinically relevant. Importantly, 72 h PTS picked up pulmonary metastases in 17.6% (3 out of 17) of patients, which were missed in serial Dx-WBS. However, >72 h PTS did not have additional value.

Conclusion:

I-131 Dx-WBS is best to be performed at 48 h, and PTS at 72 h in pediatric and young adult patients with DTC.

Keywords

Children

differentiated thyroid cancer

radioiodine

whole-body scan

young adults

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Introduction

The purpose of conducting a diagnostic whole-body scan (Dx-WBS) with radioactive iodine in patients with differentiated thyroid cancer (DTC) is to assess the extent of residual disease or surgical removal before the administration of (Iodine-131) I-131 therapy. This scan helps to determine the personalized therapy (Rx) activity, which depends on the disease’s extent. However, there remains a lack of consensus among researchers regarding the necessity of Dx-WBS and the optimal timing after the administration of I-131 activity in DTC patients.

Some authors advocate against Dx-WBS for the various reasons such as (1) Many patients have favorable outcomes without Dx-WBS. (2) Posttherapy whole-body scans (PTS) are more sensitive when conducted with higher I-131 activity than Dx-WBS. (3) There is concern about “stunning” effects caused by the diagnostic 131 I activity. (4) Empiric therapeutic administered activities of I-131 are generally effective in eliminating thyroid remnants. These points were comprehensively discussed by McDougall in a 2009 editorial.[1] In contrast, other authors[2] recommend Dx-WBS as a valuable tool in their studies. The 2015 Pediatric Guidelines of the American thyroid association (ATA)[3] also recommend Dx-WBS but solely for pediatric intermediate- and high-risk patients, excluding pediatric low-risk patients.

Determining the optimal scan timing is crucial for any radiopharmaceutical. Dx-WBS using I-131 is typically performed at or after 24 h, but the ideal timing, especially for children and young adults, has not been established. The latest ATA 2015 Pediatric Guidelines[3] do not specify the optimal Dx-WBS timing. In I-131 scans, there is a risk of insufficient or no uptake in early scans and activity washout in delayed scans, which can impact patient management. Minimizing the I-131 therapy activity is particularly important for children, who have longer life expectancies and are considered more radiosensitive.[4 5 6] Therefore, this study aimed to determine the optimal timing for Dx-WBS based on serial scans conducted after the administration of diagnostic I-131 activity.

Posttherapy scans (PTS) after I-131 therapy assess the uptake of activity in lesions and identify any additional lesions after thyroidectomy that could influence the amount of therapy activity in subsequent cycles. However, despite its extensive use, the optimal timing for PTS remains undetermined. Several studies[7 8 9 10 11] have proposed timing guidelines for PTS, but a consensus is lacking. ATA 2015 Pediatric Guidelines[3] recommend PTS approximately 4–7 days after I-131 therapy for all children. This study aimed to explore the possibility of conducting PTS earlier in patients with DTC to reduce the duration of hospital stays.

Methods

Patient selection

This retrospective study focused on children and young adults (up to 21 years old) diagnosed with DTC. All patients had a thyroid-stimulating hormone (TSH) value of ≥30 µIU/mL, except for those who had undergone hemi-thyroidectomy. They were orally administered a diagnostic activity of 74 MBq (2 mCi) of I-131 in liquid form. This administration took place 4 weeks after thyroidectomy for treatment-naive patients and 4 weeks after discontinuing hormone therapy for follow-up patients. Patients were also instructed to follow a low-iodine diet before receiving I-131. The selection of the therapeutic activity of I-131 was based on Dx-WBS findings, as well as serum thyroglobulin (Tg) and anti-Tg antibody (anti-TgAb) levels of the patients. The Rx activities ranged from 1.11–5.55 GBq (30–150 mCi). Ethical approval for this study was obtained from the institute’s ethical committee, and all patients aged 18 and above provided informed written consent. Parents provided consent for patients under 18. These patients were part of a previously published dosimetric study.[12]

Image acquisition

Serial I-131 whole-body scans (WBS) were performed at three-time points: 24 h, 48 h, and ≥72 h after the administration of both diagnostic and therapeutic activities. A gamma camera (Siemens, Symbia E. Cam) with a high-energy parallel-hole collimator (hole length = 50.8 cm, septal thickness = 2 mm) was used for image acquisition. Anterior and posterior views covering the entire body from head to toe were obtained. The camera settings remained consistent for all scans within a patient and across all patients (photopeak = 364 keV, energy window width = 15%, acquisition matrix = 256 × 1024, scan speed = 15 cm/min, and auto-contouring deactivated). Whenever possible, delayed WBS beyond 72 h (i.e., at ≥96 h) were performed based on patient convenience. In this study, WBS conducted at 24 h were classified as early scans, whereas those done at ≥48 h were categorized as delayed scans.

Image interpretation

All Dx-WBS and PTS scans underwent blind review by two experienced nuclear medicine physicians. Uptake in various lesions, including the intact thyroid lobe, thyroid remnant, nodal metastases, and pulmonary metastases, was recorded for comparison between different WBS. The optimal scan time was determined based on the time point at which a lesion was first observed. For patients with nodal or pulmonary metastases who received multiple cycles of I-131 therapy, the images from serial Dx-WBS and PTS were combined to identify the optimal scan time.

Descriptive statistics, including mean, median, standard deviation, and range, were used to present the data where applicable.

Results

Patient characteristics

A total of 95 patients diagnosed with DTC, comprising 27 males and 68 females, with an average age of 17.9 ± 3 years, were included in this study [Table 1]. All these 95 patients were administered with the diagnostic activity of 74 MBq I-131. Among them, 10 patients (10.5%) exhibited no evidence of disease on Dx-WBS and were considered surgically ablated. These patients, confirmed to be Tg and anti-TgAb negative, were subsequently excluded from the study. Out of the remaining 85 patients, 76 were treatment-naive, whereas the remaining 9 had previously received I-131 therapy before being included in this study. Among 85 patients, 10 had intact thyroid lobe, 26 had thyroid remnant, 32 had nodal metastases (17/32 had thyroid remnants along with nodal metastases), and 17 had pulmonary metastases (1/17 had thyroid remnant; 9/17 had nodal metastases; and 6/17 had both thyroid remnant and along with pulmonary metastases).

Table 1 Demographic profile of patients

Parameters	Number of patients (%)
Total patients studied	95
Included	85/95 (89.5)
Excluded (surgically ablated)	10/95 (10.5)
Age (years), mean±SD	17.9±3
Sex
Male	27/95 (28.4)
Female	68/95 (71.6)
Surgery
Hemithyroidectomy	10/95 (10.5)
Total/near-total/subtotal thyroidectomy	85/95 (89.5)
Histopathology
Papillary	88/95 (92.6)
Follicular	7/95 (7.4)
Staging
Stage-I	68/85 (80)
Stage-II	17/85 (20)
Thyroid cancer risk
Low risk	33/95 (34.7)
Intermediate risk	40/95 (42.1)
High risk	22/95 (23.2)

SD: Standard deviation

Dx-WBS results guided the administration of therapeutic activity, which ranged from 1.11 to 5.55 GBq (30–150 mCi), based on the disease extent. None of the patients had skeletal metastases. Some patients with nodal (5/32) and pulmonary (13/17) metastases received more than one cycle of I-131 therapy. A total of 18/85 patients received more than one cycle of I-131 therapy. Eighty-five patients received a total of 118 cycles of I-131 therapy [refer to Figure 1].

Flow chart showing the number of patients with different extent of disease administered with diagnostic and therapeutic activity of I-131. *Dx-WBS: Diagnostic whole body scan, †RAIT: Radioactive iodine therapy, WBS: Whole-body scan

Diagnostic whole-body scan

Among the 85 patients, 10 had an intact thyroid lobe, and 26 had a thyroid remnant. In these patients, no additional lesions were detected in any of the serial Dx-WBS conducted at 24 h, 48 h, and ≥72 h. Therefore, for patients with an intact thyroid lobe or thyroid remnant, Dx-WBS can be performed at any of these time points (24 h, 48 h, or 72 h) without altering the therapeutic activity.

Among the 32 patients with nodal metastases, two patients showed an additional node in delayed Dx-WBS (i.e., ≥48 h scans) that was not observed in the 24 h Dx-WBS. One patient displayed a washout of activity in nodes during the 48 h and 72 h Dx-WBS, whereas it was visualized in the 24 h Dx-WBS. However, these additional nodes did not impact the prescribed I-131 therapy activity, as the activity remained the same regardless of the number of nodes. Nevertheless, to maximize the detection of nodal lesions, it is recommended to perform Dx-WBS at 48 h for patients with nodal metastases.

In 17 patients with pulmonary metastases, Dx-WBS missed pulmonary metastases in 23.5% (4/17) of patients at 24 h and 17.6% (3/17) of patients at 48 h and ≥72 h [detailed in Table 2]. The 48 h Dx-WBS offered an advantage in 5.9% (1/17) of patients [Figure 2]. Therefore, for patients with pulmonary metastases, Dx-WBS at 48 h is recommended.

The serial diagnostic whole-body scan (Dx-WBS), (a) and posttherapy whole body scan (PTS), (b) of a 17-year-old male patient with papillary thyroid cancer administered with diagnostic activity of 74 MBq (2 mCi) followed by therapeutic activity of 5.55 GBq (150 mCi) I-131 showing no convincing lung uptake on 24 h Dx-WBS and 24 h PTS but showing bilateral diffuse lung uptake at ≥48 h Dx-WBS and ≥48 h PTS. Dx: Diagnostic, PTS: posttherapy whole body scan, MBq: Megabecquerel, GBq: gigabecquerel, mCi: millicurie

Table 2 I-131 diagnostic whole-body scan and posttherapy scan findings of differentiated thyroid cancer patients with pulmonary metastases done at 24 h, 48 h and ≥72 h

Patient number	Age	Sex	Baseline Tg (ng/mL)	Dx-WBS			PTS
Patient number	Age	Sex	Baseline Tg (ng/mL)	24 h	48 h	≥72 h	24 h	48 h	≥72 h
1	17	Male		−	+	+	−	+	+
2	19	Male		−	−	−	−	−	+
3	20	Female		+	+	+	+	+	+
4	19	Male		+	+	+	+	+	+
5	14	Male		+	+	+	+	+	+
6	11	Female	>300	+	+	+	+	+	+
7	20	Female	152	−	−	−	−	+	+
8	19	Female	127	−	−	−	+	+	+
9	19	Female	20	+	+	+	+	+	+
10	14	Female	>300	+	+	+	+	+	+
11	14	Female	>300	+	+	+	+	+	+
12	6	Female	>300	+	+	+	+	+	+
13	15	Male	270	+	+	+	+	+	+
14	19	Female	>300	+	+	+	+	+	+
15	18	Female	58	+	+	+	+	+	+
16	18	Male	25	+	+	+	+	+	+
17	15	Male	105	+	+	+	+	+	+
Total WBS negative patients				4	3	3	3	1	0

+: Presence of pulmonary metastases, −: No evidence of disease, WBS: Whole-body scan, Dx-WBS: Diagnostic-WBS, PTS: posttherapy whole body scan

Overall, the results suggest that Dx-WBS can be performed at 24 h for patients with an intact thyroid lobe or thyroid remnant and at 48 h for patients with nodal or pulmonary metastases. However, it is often challenging to decide the optimal timing of Dx-WBS before assessing the disease extent. A delay in initiating I-131 therapy after Dx-WBS may lead to the stunning effect, whereas performing the scan too early at 24 h may result in missed lesions due to a lower target-to-background ratio. Therefore, if only one Dx-WBS is to be conducted for a patient, it is advisable to perform it universally at 48 h for all pediatric DTC patients, regardless of disease extent.

Posttherapeutic whole-body scan

Among patients with intact thyroid lobes, thyroid remnants, and nodal metastases, no additional lesions were found in serial PTS conducted at 24 h, 48 h, and ≥72 h. Therefore, for these patient categories, PTS can be performed at any of these time points (24 h, 48 h, or 72 h) without affecting clinical management.

However, in patients with pulmonary metastases, PTS missed pulmonary metastases in 17.6% (3/17) of patients at 24 h, 5.9% (1/17) at 48 h, and interestingly, no pulmonary metastases were missed in any patient at 72 h [Table 2]. Consequently, it is recommended to conduct PTS at 72 h for patients with pulmonary metastases. Notably, in 17.6% (3/17) of patients, pulmonary metastases were not detected in any of the serial Dx-WBS performed at 24 h, 48 h, and 72 h but were visualized later in the PTS between 24 h and 72 h [as shown in a patient in Figure 3].

The serial diagnostic whole-body scans (Dx-WBS), (a) and posttherapy whole-body scans (PTS) (b) of a 19-year-old male patient with papillary thyroid cancer administered with diagnostic activity of 74 MBq (2 mCi) followed by therapeutic activity of 5.74 GBq I-131, showing no lung uptake in any Dx-WBS, and at 24 h and 48 h PTS but showing bilateral diffuse lung uptake only in 68 h PTS. Dx: Diagnostic, PTS: posttherapy whole body scan, MBq: Megabecquerel, GBq: gigabecquerel, mCi: millicurie

Serum Tg and anti-TgAb levels serve as important markers for thyroid carcinoma. In this study, patients with low Tg levels (<10 ng/mL) were more likely to have thyroid remnants, whereas those with high Tg levels (≥10 ng/mL) had a higher likelihood of nodal or pulmonary metastases. The minimum baseline Tg level among pulmonary metastase patients was 20 ng/mL. Moreover, only 12.5% (9/72) of patients tested positive for anti-TgAb. None of the 12 patients with pulmonary metastases had a positive anti-TgAb result, i.e., ≤125 IU/mL (normal range = 0–125 IU/mL) [as detailed in Table 3].

Table 3 Baseline thyroid-stimulating hormone stimulated thyroglobulin and antithyroglobulin antibody values of children and young adult differentiated thyroid cancer patients

Disease extent	Tg		Anti-TgAb
Disease extent	Median (range) (ng/mL); n	Tg ≥10 ng/mL, percentage (number of patients; n)	Median (range) (IU/mL)	Anti-TgAb positive, (percentage/number of patients; n)
Thyroid remnant and intact thyroid lobe	3 (0–113); 35	11.4% (4/35)	28 (0–506); 34	17.6% (6/34)
Nodal metastases	4.5 (0–290); 27	33.3% (9/27)	32 (12–518); 26	11.5% (3/26)
Lung metastases	211 (20–>300); 12	100% (12/12)	35.5 (24–112); 12	0.00% (0/12)
Total		33.8% (25/74)*		12.5% (9/72)^†

*Tg values available for 74/85 patients, ^†Anti-TgAb values available for 72/85 patients. Anti-TgAb normal range=0–125 IU/mL. Anti-TgAb: Anti-thyroglobulin antibody, Tg: Thyroglobulin

Discussion

Diagnostic whole-body scan

Dx-WBS, although not impacting patient management in a high percentage of cases, is a widely conducted procedure. However, a study by Van Nostrand et al.[2] in adult DTC patients demonstrated that Dx-WBS altered patient management in a substantial 53% of cases. The ATA 2015 Pediatric Guidelines[3] provide recommendations for Dx-WBS, suggesting that pediatric low-risk patients should undergo follow-up with TSH-suppressed Tg alone, whereas pediatric intermediate- and high-risk patients should have TSH-stimulated Tg and Dx-WBS.

In this study, 10.5% (10/95) of young patients were found to be surgically ablated based on Dx-WBS results, thus avoiding radiation exposure from therapeutic activity. Hence, we find Dx-WBS useful in low-risk patients. In addition, 2/24 (8.3%) thyroid remnant patients who were initially supposed to receive higher I-131 activity based on surgical and clinicopathological findings had their activity reduced based on Dx-WBS results. These patients had nodal metastases on histopathological findings, and Tg values were 25 and 113 ng/mL but did not had nodal metastases on Dx-WBS. The therapy activity in these patients was decreased from 1.85 to 3.7 GBq (50–100 mCi) to 1.11 GBq (30 mCi) based on Dx-WBS findings according to our departmental protocol. This single cycle of I-131 therapy showed complete response in both of these patients. Moreover, there were 5/32 (15.6%) nodal metastase patients where nodal metastases were not seen in histopathology, and Tg was <10 ng/mL but were noted in Dx-WBS. These patients were administered with higher I-131 therapy activity of 1.85 GBq (50 mCi) instead of 1.11 GBq (30 mCi) that showed complete response from a single dose in all five patients. Therefore, Dx-WBS proved to be beneficial in children and young adults, who are considered more radiation-sensitive.

The ATA 2015 Pediatric Guidelines[3] recommend follow-up with TSH-suppressed Tg alone and do not recommend Dx-WBS in pediatric low-risk patients. However, this study found similar TSH-stimulated Tg levels in surgically ablated patients and patients with thyroid remnants or intact thyroid lobes. Consequently, the decision of whom to administer Rx activity becomes challenging in these two patient categories. Therefore, based on our results, we recommended to perform Dx-WBS in all patients regardless of their risk category. However, caution should be exercised to use the lowest possible activity of I-131 for diagnostic purposes to avoid the stunning effect.

The ATA 2015 Pediatric Guidelines[3] favor the use of I-123 in children for diagnostic scans due to its superior imaging quality and lower absorbed doses to tissues.[13 14] However, due to its cost and limited availability, many centers continue to use I-131, despite the potential for stunning. This risk can be minimized using the lowest possible activity of I-131, as stunning is not observed with 37–74 MBq (1–2 mCi) I-131 or any activity of I-123.[15 16 17] EANM[18] recommends 10–185 MBq (0.27–5 mCi) I-131 or 40–200 MBq (1.08–5.41 mCi) I-123 to minimize the risk of stunning.

The optimal time for Dx-WBS has not been established in the literature, including the ATA 2015 Pediatric Guidelines.[3] However, this study found that the optimal time for Dx-WBS was 48 h, where there is the maximum chance of lesion detection and the least chance of I-131 activity washout. Median serum Tg levels were lower in patients with thyroid remnants (3 ng/mL) and nodal metastases (4.5 ng/mL) but higher in those with lung metastases (211 ng/mL). However, a high Tg level does not necessarily indicate lung metastases. About 11.4% (4/35) of patients with thyroid remnants or intact thyroid lobes and 33.3% (9/27) of patients with nodal metastases had Tg levels ≥10 ng/mL [Table 3]. Relying solely on Tg levels may result in overdosing pediatric patients. Computed tomography (CT) scans of the thorax are typically used to look for pulmonary metastases. However, diffuse types of lung metastases, common in children, may not be clearly visible on CT scans. In this study, 88.2% (15/17) of patients had diffuse pulmonary metastases, highlighting the importance of Dx-WBS in children. Couto et al.[19] in their study evaluated and determined the cutoff value for pulmonary metastases in adults (94 patients) to be 117.5 ng/mL. In the present study of children and young adults, among patients diagnosed with pulmonary metastases, 66.7% (8/12) had Tg >117.5 ng/mL. Moreover, there were no patient with intact thyroid lobe or thyroid remnant patient who had Tg >117.5 ng/mL. However, 11.1% (3/27) of nodal metastases patient had Tg >117.5 ng/mL.

In I-131 Dx-WBS, partial or complete washout of activity from metastatic sites (nodal or pulmonary) is often observed, potentially leading to missed lesions on delayed Dx-WBS. Conversely, some patients may exhibit slow uptake in lesions, causing early scans to miss the lesions. To address these challenges of activity washout or slow uptake, Tg levels should be checked in conjunction with Dx-WBS. Filesi et al.[20] in their study also concluded that both Dx-WBS and Tg measurements should always be performed postthyroidectomy.

Posttherapeutic whole-body scan

The use of PTS in patients with metastatic uptake on Dx-WBS has been a topic of debate. While some authors believe that despite detecting additional lesions, PTS has little influence on the clinical approach, others argue that it should be limited to patients with negative Dx-WBS and high Tg levels. However, this study observed that there are patients with high Tg levels who tested positive for nodal metastases and negative for pulmonary metastases on Dx-WBS. In such cases, determining tumor, node, metastasis staging, which guides the Rx activity, becomes challenging. Therefore, it is recommended to perform PTS in all patients regardless of Dx-WBS findings and Tg values. The ATA 2015 Pediatric guidelines[3] also recommend PTS for all children after 4–7 days. The findings of this study suggest an earlier time for PTS. The ideal time for PTS in these patients is 72 h, where no remnant or metastatic lesions (nodal or pulmonary) are missed. This finding aligns with the EANM guidelines,[18] which recommend not performing PTS sooner than 72 h. Lee et al.[9] compared early (3^rd day) and delayed (10^th day) PTS in 81 adult DTC patients. Although they found additional lesions in only 5% of patients, they did not mention the clinical benefit. Importantly, they found that 38% (3/8) of patients with a negative early PTS but high Tg values showed additional lesions on delayed PTS. However, in this study, PTS was typically performed at 72 h in most patients, preventing a direct comparison with their study. Delaying PTS for too long increases hospital stay duration, which can be uncomfortable, especially for out-of-town patients. Since this study suggests that PTS at 72 h does not miss any metastatic lesions, it is recommended to perform PTS in all children at 72 h.

In a study by Mazzaferri et al.,[21] adult patients with Tg >10 ng/mL during hypothyroidism were more likely to present with metastatic lesions on PTS. This finding was supported by the present study, where 21/39 (53.8%) (nodal metastases = 9/27; pulmonary metastases = 12/12) patients had Tg levels ≥10 ng/mL [Table 3]. However, another study by Iwano et al.[22] observed an even lower Tg level (>1.5 ng/mL) in adult patients more likely to exhibit metastatic accumulation. This finding differs from the present study, where 66% (23/35) of patients with baseline Tg levels >1.5 ng/mL did not have nodal or pulmonary metastatic lesions but only had thyroid remnants or intact thyroid lobes.

In this study, no additional lesions were found in patients with intact thyroid lobes and thyroid remnants in PTS. In one of the 32 patients with nodal metastases, an additional node was observed in PTS but was not clinically relevant. However, in 17.6% (3/17) of patients with pulmonary metastases, additional lesions were seen in PTS that were not detected in Dx-WBS, leading to changes in the therapeutic approach. Several authors conducted similar studies in adult patients and found a lower percentage of patients with changes in management due to additional lesions detected in PTS. Sherman et al.[23] found additional lesions in 27% of PTS cases but clinical relevance in only 10% of metastatic disease patients. Grigsby[24] found additional lesions in 10% of PTS cases. Fatourechi et al.[25] reported 13% additional lesions in PTS but clinical relevance in 9% of patients. In a study by Souza Rosário et al.,[26] 26% of patients with metastatic disease showed clinically relevant information on PTS, with changes in therapeutic approach in 15% of patients and disease stage changes in 8.3% of patients.

PTS was typically performed up to 72 h in most patients. However, in 18/85 patients (intact thyroid lobe = 4/10, thyroid remnant = 4/26, nodal metastases = 8/32, l and ung metastases = 2/17), scan was performed after 3^rd day of therapy (4^th day – 15 patients, 5^th day – two patients, and 6^th day – one patient). Moreover, in these patients, no additional lesions were found in >72 h PTS as compared to <72 PTS in any patient that could change the patient management.

This study is not without limitations. Since, the sample size was smaller, especially in lung metastase patients making it difficult to compare early PTS with much-delayed PTS, especially after 5–10 days, as recommended by some authors.[8 10 11]

Conclusion

Based on our data, 24 h and 48 h posttherapy images may miss metastatic disease in pediatric patients with pulmonary metastases. Both the Dx-WBS and PTS demonstrate their utility in pediatric and young adult patients with DTC and best to be performed at 48 h and 72 h, respectively.

Conflicts of interest

There are no conflicts of interest.

Nil.

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