Primary Pulmonary Tumors in Pediatric Population: Imaging Markers for Predicting Histology.

OBJECTIVES: The objective of the study was to review the imaging features of proven pediatric primary lung tumors, with a purpose of detecting key distinguishing features among the various entities.
MATERIALS AND METHODS: We retrospectively reviewed multidetector computed tomography (CT) images of 17 pediatric patients with primary lung tumors. For each examination, various CT image descriptors were used to characterize the pulmonary nodules/masses; including location, size, number, morphology, cavitation, calcification, intense enhancement, airway involvement, chest wall/pleural involvement, mediastinal/vascular involvement, and nodal enlargement.
RESULTS: The age of the patients ranged from 2 to 18 years (mean age of 9.5 years). Approximately 35.3% of tumors were benign and 64.7% were aggressive/malignant. Nine distinct histopathologic tumor entities were found. Common tumor types were recurrent respiratory papillomatosis (4) and inflammatory myofibroblastic tumor (4) with two endobronchial tumors including carcinoid and mucoepidermoid carcinomas. Besides invasion and nodal enlargement, large size and central location (P < 0.05) were predictors of aggressiveness/malignancy. Multiple lesions and cavitation (P < 0.05), on the other hand, were frequent in benign lesions.
CONCLUSION: On imaging, location and morphological markers can allow diagnosis in majority of the tumors. Copyright:

INTRODUCTION

Primary pediatric lung tumors (PPLTs) are uncommon in children, comprising only 0.2% of all pediatric tumors.[1] Pediatric lung “masses” are more often benign (congenital, infective/inflammatory) and, when aggressive/malignant, more commonly metastatic.[2] However, when talking about PPLTs, aggressive/malignant PPLTs far exceed the number of benign neoplasms, with a ratio of approximately 3:1.[3]

PPLTs have a narrow range of diagnostic possibilities, but the imaging appearances are often nonspecific and may even be mistaken as pneumonia. Hence, familiarity with the imaging features of these entities is crucial to ensure an early diagnosis.

Most of the literature describing the imaging appearances of these tumors exists as case reports. The purpose of this study is to review the imaging features of proven PPLTs, with a purpose of detecting key imaging markers for diagnosis of various entities.

MATERIALS AND METHODS

Subjects

This was a single-institution, retrospective cohort study, approved by the institutional review board. The radiology database was reviewed using the picture archiving and communication system to identify young patients, who had pulmonary masses on chest radiographs or computed tomography (CT), performed during 2015–2018.

Patients who underwent biopsy/surgery and whose final histopathological diagnosis was obtained were included in the study. Patients whose histopathologic reports were not available or those who were eventually diagnosed as chest wall or mediastinal masses with pulmonary extension were excluded from this study. The pathology results were then reviewed along with the medical records. The final group included 17 patients. Due to the retrospective nature of the study, parents/patient informed consent forms was waived.

Two reviewers (ASB and MJ with off 20 and 10 years of experience in pulmonary imaging, respectively) evaluated the scans consensually using the same workstation. The radiologists were blinded to both clinical and histological information.

Data collection and evaluation

Images were volumetrically acquired and thin-section images were evaluated using both standard mediastinal (width, 350 HU; level, 40 HU) and lung (width, 1500 HU; level, −600 HU) window-width and window-level settings. For each examination, various CT descriptors were developed to characterize the pulmonary nodules/masses, and these were classified into 13 categories: (1) location, (2) size, (3) number, (4) morphology, (5) cavitation, (6) calcification, (7) intense enhancement, (8) airway involvement, (9) chest wall involvement, (10) pleural involvement, (11) mediastinal involvement, (12) vascular involvement, and (13) nodal enlargement. Maximum dimensions were recorded in axial and true sagittal planes, only on mediastinal window.

Statistical analysis

Study data tabulation and subsequent analyses were performed using the Statistical Package for the Social Sciences (SPSS, version 25.0; IBM, Armonk, NY, USA). The variables of the groups were analyzed using Fisher's exact test. p < 0.05 was considered as statistically significant.

RESULTS

Nine distinct histopathologic tumor entities were found [Table 1]. Out of the total of 17 tumors, 6 (35.3%) were benign and 11 (64.7%) were aggressive/malignant. Although IMTs are tumors of borderline/ uncertain behaviour, they were considered together with the malignant tumors; as they are locally infiltrative and can even show metastases. Squamous papilloma in the setting of recurrent respiratory papillomatosis (RRP) was the most common benign tumor, whereas the most frequent aggressive/malignant tumor identified was inflammatory myofibroblastic tumor (IMT), comprising 23.5% of cases (4/17) each.

Table 1

Distribution of benign and aggressive/malignant lesions (n=17)

Tumor type	n (%)
Benign
RRP	4 (23.5)
FLH	1 (5.9)
Hemangioma	1 (5.9)
Total	6 (35.3)
Aggressive/malignant
IMT	4 (23.5)
PPB	2 (11.7)
MEC	2 (11.7)
Carcinoid	1 (5.9)
Anaplastic lymphoma	1 (5.9)
RMS	1 (5.9)
Total	11 (64.7)

RRP: Recurrent respiratory papillomatosis, FLH: Fibroleiomyomatous hamartoma, IMT: Inflammatory myofibroblastic tumor, PPB: Pleuropulmonary blastoma, MEC: Mucoepidermoid carcinoma, RMS: Rhabdomyosarcoma

The mean age at diagnosis was 9.5 years and ranged from 2 to 15 years, whereas most of the patients in the age group of 11–18 years had a aggressive/malignant tumor (5/6); in the age group of 6–10 years, there was an equal distribution of benign and aggressive/malignant tumors.

Comparison of imaging features of benign versus aggressive/malignant lung neoplasms

All the aggressive/malignant lesions presented as solitary masses, whereas 83.3% of benign lesions were multiple in number (4 cases of squamous papilloma and one infantile hemangioma). Majority of benign tumors (83.3%) had smaller size (<5 cm in greatest dimension); most of the aggressive/malignant tumors (81.2%) had larger size.

About 27.3% of aggressive/malignant tumors involved multiple lobes; and 36.4% involved entire lung parenchyma including two cases of pleuropulmonary blastoma (PPB) and one case each of IMT and pulmonary rhabdomyosarcoma [Table 2]. Majority of aggressive/ malignant tumors were centrally located. None of the benign tumors were central/endobronchial in location.

Table 2

Location and extent of benign and aggressive/malignant lesions (n=12)

Parameter	Types	Benign (n=6), n (%)	Aggressive/malignant (n=11), n (%)	P
Location	Central/endobronchial location	0	7 (63.7)	0.034
	Peripheral	6 (100)	4 (36.4)
Extent	Single lobe	0	2 (18.2)	0.485
	Multiple lobes/entire lung	6 (100)	7 (63.7)
Adjoining invasion	Mediastinal/vascular/pleural/chest wall	1 (16.7)	8 (72.7)	0.049
Nodal enlargement		0	2 (18.2)	0.514

Among the aggressive/malignant neoplasms, 63.6% showed mass effect and mediastinal invasion. One case of IMT showed pleural-based nodules, with invasion of chest wall muscles and rib erosions.

Calcification was present in 33.3% (2/6) of benign lesions (fine, speck-like calcifications in squamous papilloma; coarse, chunky calcifications in fibroleiomyomatous hamartoma) [Table 3]. Dense and coarse calcifications were identified in the two IMTs.

Table 3

Comparison of various imaging morphologic characteristics of benign and aggressive/malignant lesions

Imaging features	Benign (n=6)	Aggressive/malignant (n=11)	P
Size (cm)
<5	5 (83.3)	2 (18.2)	0.034
>5	1 (16.7)	9 (81.2)
Multiplicity	5 (83.3)	0 (0)	0.001
Intense enhancement	1 (16.7)	2 (18.2)	1.0
Cavitation	4 (66.7)	0 (0)	0.006
Cystic component	0 (0)	2 (18.2)	0.51
Calcifications	2 (33.3)	2 (18.2)	0.58

Cavitation was present in multiple squamous papillomas in 4 (66.7%) cases of RRP, whereas none of the aggressive/malignant tumors showed cavitary changes. Two cases of PPB showed fluid-filled cystic areas within the tumor. Intense enhancement pattern was observed in a case of infantile hemangioma (16.7%) and in two aggressive/malignant tumors (18.2%; carcinoid and mucoepidermoid carcinomas [MEC]).

Imaging features of individual histopathologic tumor types are detailed in Table 4.

Table 4

Imaging features of different histopathological entities

Imaging	IMT	PPB	Carcinoid	MEC	Lymphoma	RMS	RRP	Hemangioma	FLH
Number of cases	4	2	1	2	1	1	4	1	1
Size (cm)
<5	0	0	1	2	0	0	4	1	0
>5	4	2	0	0	1	1	0	0	1
Multiplicity	0	0	0	0	0	0	4	1	0
Central location	3	0	1	2	0	0	0	0	0
Calcification	2	0	0	0	0	0	1	0	1
Cavitation	0	0	0	0	0	0	4	0	0
Cystic component	0	2	0	0	0	0	0	0	0
Intense enhancement	0	0	1	1	0	0	0	1	0

RRP: Recurrent respiratory papillomatosis, FLH: Fibroleiomyomatous hamartoma, IMT: Inflammatory myofibroblastic tumor, PPB: Pleuropulmonary blastoma, MEC: Mucoepidermoid carcinoma, RMS: Rhabdomyosarcoma

DISCUSSION

Most PPLTs are aggressive/malignant with a ratio of approximately 3:1.[3] In our study, we found 64.7% aggressive/malignant and 35.3% benign tumors, with common tumors being IMT, PPB, and squamous papilloma.

So far in the literature, there is no large series describing the imaging features of PPLTs, though there are studies on histopathological patterns. In their review of the literature in 1982, Hartman and Shochat[4] described 230 tumors, with 151 aggressive/malignant tumors with 16 histopathologic subtypes.

The most common aggressive/malignant tumor identified in our study was IMT (4 cases, 23.5%). IMT, previously known as inflammatory pseudotumor, was considered the most common primary benign lung tumor in children. However, the current WHO histologic classification of lung tumors (2015) reclassified IMT as Neoplasm of borderline or uncertain behaviour due to the occurrence of aggressive features such as local invasion and distant metastasis.[5] However, for discussion, we considered IMTs in the malignant/ aggressive neoplasm group. On CT, solitary large masses (>5 cm) with heterogeneous enhancement, no cavitation, and coarse calcifications were seen in the majority [Figure 1]. We found a high incidence of adjacent organ invasion in IMT [Figure 1a and b], which is similar to the results found by Oguz et al.[6] Distant metastasis was not found in any of the cases.

Figure 1

(a-i) Imaging features of primary pulmonary neoplasms. Inflammatory myofibroblastic tumor (a and b). Large calcified mass with mediastinal invasion in an 18-year-old girl (a), and noncalcified infiltrative large central mass causing bronchial narrowing (arrow in b) in a 6-year-old boy. Recurrent respiratory papillomatosis (c). Multiple cavitary nodules in a 3-year-old girl. Endobronchial tumors (d and e). Typical carcinoid in the left main bronchus (arrow in d), and mucoepidermoid carcinoma in the right lower lobe bronchus (arrow in e) in another patient. Pleuropulmonary blastoma (f-h). Huge cystic mass with solid components in a 2-year-old girl (f). Solid-cystic mass (asterisk) in the left hemithorax in a 5-year-old boy (g and h). Infantile hemangioma (i) in a 4-year-old girl, seen as hyperenhancing mass (asterisk) centered in the right lower lobe, with dilated pulmonary artery branch and draining pulmonary vein

In our study, squamous papilloma comprised the most common benign neoplasm (4 cases, 23.5%). Usual imaging appearance is masses/nodules in the larynx/upper airway and multiple of pulmonary parenchymal papillomas.[7] In our cases, papillomas appeared as multiple nodules (most of them <3 cm) scattered in both lungs with or without cavitation [Figure 1c]. All patients also had concomitant laryngeal papillomas. In a study done in 2009, twenty cases of RRP with lung involvement in children were reviewed and similar imaging characteristics were reported.[8]

Another traditionally used term that has been reclassified by the WHO is bronchial adenoma.[4] Historically, bronchial adenomas encompassed carcinoid, MEC, and adenoid cystic tumors. Currently, these tumors are categorized as either carcinoid or salivary gland tumors. In two reviews conducted in 1993,[39] it was reported that carcinoid tumors accounted for 11%–13% of all PPLTs. Our study reported a single case of endobronchial typical carcinoid (5.9% of total tumors), which appeared as well-circumscribed, hyperenhancing mass within the bronchus [Figure 1d], oriented along the axis of the bronchus (parallel sign) with no extraluminal component or calcifications.

There were two cases of MEC: one purely endobronchial [Figure 1e] and another with additional extrabronchial component. Unlike in adults, MECs tend to be low grade in children and remain localized for a long time with low metastatic potential. They arise in the main bronchus or proximal lobar bronchus and show variable enhancement with low-grade tumors having more marked enhancement.[101112] Our study also demonstrated a marked enhancement in a case of well-circumscribed endobronchial MEC. Multidetector CT (MDCT) findings of bronchial carcinoid tumors are similar to those of MECs. Hence, in general, differentiating between these two diseases on CT can be difficult. They reported punctate calcifications in 25% cases; however, our case did not show calcifications within the mass.[12]

We concluded in our study that large solitary masses are more likely to be aggressive/malignant, whereas multiple nodules/masses favored primary benign neoplasm, after excluding metastases as the cause of multiplicity. Large heterogeneous masses (>5 cm) included the entities such as IMT, PPB [Figure 1f-h], lymphoma, and rhabdomyosarcoma. Among these, the central location of the tumor and calcifications were noted only in IMT, whereas cystic fluid-filled spaces with the absence of calcifications favored PPB (Types 1 and 2) [Figure 1f]. Lymphoma and rhabdomyosarcoma did not show any specific imaging features.

Among the smaller masses (<5 cm), majority were benign, including papillomas and infantile hemangiomas. These two entities were easily differentiated on imaging with papillomas appearing as multiple nodular solid or cavitating masses with few showing punctate calcifications, whereas hemangiomas seen as hyperenhancing masses [Figure 1i] without cavitation or calcifications.

Besides invasion and nodal enlargement, large size (P = 0.034) and central location (P = 0.034) were predictors of aggressiveness/ malignancy. Multiple lesions (P = 0.001) and cavitation (P = 0.006), on the other hand, were frequent in benign lesions. In conclusion, PPLTs show fairly characteristic imaging features on MDCT which can allow near histospecific diagnosis in most cases.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.