Review
The role of imaging in the initial investigation of paediatric renal tumours

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Summary

Imaging has a key role in the assessment of paediatric renal tumours, especially when the initial treatment approach is to proceed to standard chemotherapy without histological confirmation. In Europe, according to the International Society of Paediatric Oncology guidelines, core needle biopsy is not routinely done unless the child is older than 10 years. Between age 6 months and 9 years, the child is treated with a standard regimen of preoperative chemotherapy unless there are concerns about non-Wilms' tumour pathology. Atypical imaging findings could therefore stratify a child into a different treatment protocol, and can prompt the need for pretreatment histology. This review details the latest protocols and techniques used in the assessment of paediatric renal tumours. Important imaging findings are discussed, especially the features that might prompt the need for a pretreatment biopsy. Local radiology practices vary, but both MRI and CT are widely used as routine imaging tests for the assessment of paediatric renal tumours in Europe. Advances in imaging technology and MRI sequences are facilitating the development of new techniques, which might increase the utility of imaging in terms of predicting tumour histology and clinical behaviour. Several of these new imaging techniques are outlined here.

Introduction

The diagnosis and treatment pathways for childhood renal tumours differ according to geography. In North America, the Children's Oncology Group argues for an upfront nephrectomy to obtain a definitive histological diagnosis and stratify treatment on the basis of a chemo-naive tumour. Preoperative chemotherapy is reserved for complex tumours.1 The most recent guidelines of the International Society of Paediatric Oncology Renal Tumours Study Group (SIOP-RTSG),2 which are largely followed in Europe, argue in favour of preoperative chemotherapy to downstage the tumour, reduce the risk of rupture, and stratify postoperative treatment on the basis of the histological response and tumour stage. Previously, SIOP-RTSG advocated for the use of standard chemotherapy preoperatively, without histological confirmation in the form of a biopsy. All children older than 6 months with typical clinical findings for Wilms' tumour are treated according to this approach, since 90% of childhood renal tumours beyond early infancy are Wilms' tumours. Biopsy is reserved for cases with clinical diagnostic uncertainty or children older than 10 years, in whom the relative likelihood of a non-Wilms' tumour diagnosis starts to increase. The subjective nature of deciding when imaging appearances are typical for a Wilms' tumour probably underlies the variation in reported biopsy rates of 8–26% between different countries and European regions.3 When the UK and Irish Children's Cancer and Leukaemia Group (CCLG) joined the SIOP-RTSG Wilms' tumour 2001 study, they continued their national practice of performing a percutaneous biopsy before beginning chemotherapy in nearly all children (97%) with a renal tumour. The aim was to prevent children with non-Wilms' tumour from receiving sub-optimal preoperative chemotherapy or unnecessary treatment. In June, 2019, the CCLG reviewed the diagnostic accuracy of paediatric renal tumour biopsies in UK children, who were treated according to the SIOP-RTSG Wilms' tumour 2001 guidelines.4 It was concluded that for children aged 6 months to 10 years with Wilms' tumour as the most likely diagnosis, the risk of biopsy outweighs the clinical benefits. In this age group, a biopsy result rarely leads to a substantial change in management from the standard SIOP-RTSG approach. This observation has led to a change in CCLG guidelines and biopsy is now reserved for patients with clinical or radiographic features that suggest a non-Wilms' tumour. In this context, imaging is an important part of the preoperative assessment. In particular, imaging needs to identify cases in which radiological uncertainty is sufficient to warrant preoperative histology. The SIOP-RTSG has developed a new protocol for the diagnosis and treatment of childhood renal tumours, the UMBRELLA SIOP-RTSG 2016.2 A specific research question that involves imaging is the independent prognostic value of the volume of the blastemal component of a Wilms' tumour that has survived preoperative chemotherapy.2 Imaging has an advantage over biopsy in this regard because the whole tumour volume can be evaluated. However, there is currently insufficient evidence to suggest that this evaluation of whole tumour volume can be achieved with clinically validated imaging techniques.

Key messages

  • •

    Core needle biopsy of a renal mass is no longer recommended by UK and European guidelines for children aged between 6 months and 10 years with clinical and radiological features of a Wilms' tumour

  • •

    Ultrasound should be the first imaging method used to investigate any abdominal mass in a child

  • •

    MRI or CT scans can be used interchangeably to assess a known renal mass; MRI is recommended in the latest UMBRELLA research trial

  • •

    It is important to know the imaging features of a renal mass that indicate a non-Wilms' tumour.

  • •

    Imaging cannot yet differentiate tumour types or Wilms' tumour stage, but this area of imaging might advance in the future

In this paper, we review the European recommendations for abdominal imaging in paediatric renal tumours. We discuss the necessary parameters for optimisation of MRI protocols and focus on key cross-sectional imaging features that might prompt consideration for core needle biopsy. There are several advanced techniques aimed at assessing tumour biology—in the case of Wilms' tumours, these are used to assess the degree of anaplasia, necrosis, and blastemal content. A few of these techniques are available in clinical practice, but there is no evidence to support their effectiveness. We will briefly summarise these techniques, which could be interesting areas for future research.

Section snippets

Renal tumours in children

In western industrialised countries, primary malignant kidney tumours account for 4–7% of cancers diagnosed in children under the age of 15 years. Wilms' tumour, also known as nephroblastoma, is the most frequent histological type, accounting for 87% of these tumours.5 At ages 1–9 years, Wilms' tumours account for 92–95% of all renal malignant tumours, this figure drops to 81–89% in children younger than 1 year old, and to 54–67% for children 10–14 years of age.4, 6, 7 The most common

Imaging of paediatric tumours

To recommend a specific imaging protocol for any condition is challenging because of rapid advances in technology and variation in local equipment and practice. The table shows an example of an MRI protocol for an abdominal tumour as recommended for use at UK centres. An ultrasound should be the initial imaging method in any suspected abdominal mass in a child.15 MRI or CT scans are used as the primary cross-sectional modalities, once the initial diagnosis of a mass has been made by an

Imaging features

In this section, we discuss the important imaging features that the radiologist should evaluate as part of image analysis. CT or MRI can be used to perform this evaluation where necessary, and we indicate which of the two modalities might be preferred. As well as a general review of the tumour characteristics—such as location, size, and signal density—paediatric renal tumour imaging review should include an assessment for several important features.

Advanced imaging in Wilms' tumours

In this section, we review techniques that are being used in research studies with the aim of improving the capabilities of imaging in oncology. Some techniques have been specifically used in paediatric renal tumours; others are being used more generally in oncology imaging. This is not an exhaustive list. Most of these techniques are not in routine clinical use. Implementation will require resources to provide these techniques in a clinical setting in terms of software and hardware, as well as

Conclusion

A no-biopsy approach to the management of paediatric renal masses places additional emphasis on the capabilities of preoperative imaging for accurate diagnosis and risk stratification to ensure appropriate preoperative care. High quality MRI or CT imaging can show important features to assist in predicting histological diagnosis in combination with clinical features and inform the choice of initial treatment approaches (ie, empirical preoperative chemotherapy, biopsy, or immediate nephrectomy).

Search strategy and selection criteria

Data for this review were identified from searches of MEDLINE, PubMed, and references from relevant articles using the search terms “children” or “infants” or “paediatric” or “pediatric”, and “Wilms' tumour” or “clear cell sarcoma” or “rhabdoid tumour” or “cystic partially differentiated nephroblastoma” or “cystic nephroma” or “multiloculated cystic nephroma” or “mesoblastic nephroma”, and “MRI” or “diffusion weighted imaging” or “CT” or “positron emission tomography” or “ultrasound”. Abstracts

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