This retrospective single-centre review investigates the schedule of and intervals between surveillance scanning of children with presumed or confirmed WHO grade 1 gliomas. In addition, we investigated the duration to tumour recurrence or progression in the context of attempting to identify whether reducing the number of surveillance MRI scans, along with increasing the interval between such scans, would permit a balance between identifying tumour relapse or progression requiring intervention with the burden and risks of performing excessive surveillance imaging.
Even from our single centre, the results demonstrate significant variation in clinical practice as to the approach to surveillance MRI imaging, with varying number of and interval between surveillance scans. Our data suggests that the default interval between MRI scans is 6 months (which is in keeping with the SIOP-E-BTG guidance), although this varied depending on tumour location and degree of surgical resection. However, the large number of surveillance scans compared to the relatively small number of instances of recurrence/progression suggest that there may be scope to reduce the volume and frequency of surveillance imaging.
Our results support previous published evidence demonstrating that completely resected cerebellar grade 1 astrocytomas rarely recur , with only one of 25 (4%) patients in our cohort recurring. For this group of patients in particular, we therefore suggest that a reduced frequency of surveillance imaging and a shorter overall surveillance period is likely to be adequate and to achieve a better balance for patients. McAuley et al. suggest a protocol for completely resected cerebellar astrocytomas of imaging at 6, 18, and 30 months . In our cohort, this would have led to a delayed diagnosis in our single recurrence at 13 months but whether this would have changed outcome is difficult to determine retrospectively. However, the absence of recurrence at longer intervals suggests that it would be prudent to limit the overall surveillance period and McAuley’s suggestion of 30 months seems reasonable according to our data.
There is very limited data regarding the optimum surveillance imaging schedule in non-cerebellar tumours, as well as in tumours that are non-operatively managed or incompletely resected. Stevens et al. highlight that 75% of the tumours in their systematic review of surveillance imaging were cerebellar and the majority completely resected (which is likely to explain why our recurrence/progression rate of 35% is higher than their reported overall rate of 24%) .
The progressions that occurred furthest from resection were in an optic pathway tumour which had been debulked and received chemotherapy, at 78 months, and a tectal plate tumour (not histologically confirmed but considered to be grade 1 based on imaging and treated with chemotherapy) at 64 months but the remaining cases of progression or recurrence occurred within 4 years. Kim et al. suggest an imaging strategy of MRI at 3 months and 1, 2, 5, and 10 years . In the context of incomplete resection or non-operative management, it may not be possible to conclusively determine when surveillance imaging should be discontinued. Our results would appear to favour a lengthened interval—imaging at 6 and then 12 months and then annually up to 5 years would have reduced the number of MRI scans whilst minimising the delay to diagnosis for the majority of patients in our cohort. However, until further cohorts support these findings, we feel it is prudent to maintain 6 monthly imaging surveillance for non-cerebellar (or incompletely resected cerebellar) tumours currently.
A more recent study examined the detection efficacy and cost of surveillance imaging in a retrospective analysis of 517 patients with low-grade gliomas using an algorithmic approach to define a more cost-effective surveillance programme . This is an interesting and useful approach, but no distinction was made between the location of the tumours and our data suggests there is a difference in recurrence/progression risk depending on the location of the tumour and as such the generalisability is difficult to determine.
Overall, our data would suggest that the available guidelines are fairly arbitrary and lead to a very low detection rate with a large burden on the patient, family, and hospital. Multiple studies suggest a shorter and less frequent surveillance protocol; we would suggest based on these findings that this should be tailored to the location of the tumour as well as the degree of initial resection (where relevant), and that the protocol outlined by McAuley et al.  seems most appropriate for cerebellar tumours and would have avoided missing instances of recurrence or progression in our cohort. However, more data is required on surveillance patterns in non-cerebellar tumours.
The use of gadolinium-based contrast agents (GBCAs) in surveillance imaging has become more controversial given the recent published demonstration of deposition of gadolinium in the brain. It has previously been suggested that post-contrast T1 is the most sensitive sequence for tumour recurrence . However, in our cohort of 13 patients with an isolated change in contrast enhancement, this justified discussion in the neuro-oncology MDT meeting but was never sufficient alone to indicate treatment escalation without other symptoms or evidence of tumour progression.
This finding is echoed by recent published evidence specifically examining low-grade optic pathway tumours. Maloney et al. report that in their cohort of children with isolated optic pathway gliomas, treatment escalation was only instigated in the context of increased contrast enhancement with concomitant increase in tumour size visible on T2-weighted images, and thus propose use of a non-contrast surveillance protocol . An unenhanced surveillance protocol has subsequently been shown by Marsault et al. to have satisfactory sensitivity and specificity, based primarily on assessment of tumour volume variation . We believe that the use of GBCAs could be limited in the setting of all low-grade paediatric astrocytomas but this requires further investigation in other tumour locations, as performing repeat MRI surveillance scans for GBCA administration is not ethical when it often involves risk of sedation or anaesthesia.
We note that the recent Response Assessment in Paediatric Neuro-Oncology (RAPNO) working group recommendations highlight the evidence that contrast enhancement may be less reliable than T2 signal in assessing response, but currently recommend it as a core sequence for follow-up . It is also important to note the additional value of GBCAs in detecting disseminated disease at recurrence, which if not nodular may be difficult to identify on unenhanced imaging. The risk of this in low-grade tumours is relatively small, but not insignificant and is higher in infants. In our cohort, there was no dissemination of disease at recurrence (including on post-contrast imaging), but we note previous studies demonstrate a 4.3% incidence of dissemination in all low-grade gliomas  and increased incidence in infants under one .
There are limitations to the conclusions made from interpretation of our retrospective dataset. Firstly, we have included multiple tumours not histologically confirmed as grade 1 astrocytomas. This was deemed necessary given that optic pathway gliomas are often not amenable to surgical access for biopsy or resection and yet fall within the surveillance purview of low-grade gliomas. Despite minimising the risk of confounding results by excluding any tumours with high-grade radiological features, in the absence of a tissue diagnosis, we cannot confidently conclude that all patients included did have low-grade tumours.
Secondly, the variation in treatment and management options for non-cerebellar astrocytomas (with respect to surgical debulking or resection, chemotherapy, and radiotherapy options) means that the surveillance data for non-cerebellar tumours is more challenging to objectively analyse, and therefore only limited conclusions can be drawn from the data as a whole. The retrospective methodology may have limited our data analysis, with variable patient follow-up durations as many of our patients transferred to adult services or alternate regional hospitals closer to home throughout the surveillance period. Finally, our interpretation of the conclusions made at multidisciplinary team meetings is somewhat subjective, particularly with regard to the utility of GBCAs, and this should therefore be interpreted with caution. On the other hand, it could be deemed that multiple oncologists and neuro-radiologists concluding non-tumour progression in the presence of progressive contrast enhancement strengthens our conclusions.