Selumetinib for a Growing Plexiform Neurofibroma: Starting the Clock on a Lifelong Drug
A 13-year-old with NF1 has a painful, growing plexiform neurofibroma right as puberty accelerates its growth. Selumetinib shrinks these tumors, but stopping it usually lets them regrow — meaning today's decision is really about starting a drug she may take for the rest of her adolescence.
J.O. is 13 years old, plays midfield for her school's soccer team, and has known she has neurofibromatosis type 1 since a routine exam at age 4 found the café-au-lait macules and axillary freckling that led to genetic confirmation. She has done well for most of her childhood — normal cognitive development, no seizures, no optic pathway glioma on surveillance — but a plexiform neurofibroma involving her right brachial plexus, stable on imaging since age 9, has grown measurably on her six-month follow-up MRI. For the first time she is describing real pain, a deep ache that wakes her some nights, and subjective weakness gripping a ball overhead that her exam confirms as mild but real deltoid and biceps weakness.
Two details in her chart place her precisely rather than approximately. The first is timing: plexiform neurofibromas grow fastest during childhood and specifically during puberty, and her growth spurt began roughly four months ago at Tanner stage 3, which lines up closely with when her mother first noticed the pain. That is not a coincidence to be noted and set aside — it means the tumor is growing during the window in which it is expected to grow fastest, and the question of whether to intervene is being asked at the steepest part of the curve. The second is that she matches SPRINT's enrolled population rather than extending it. That trial studied children with inoperable plexiform neurofibromas causing genuine morbidity, not incidental ones; roughly two-thirds achieved a durable partial response with real improvement in pain and function. Her tumor is inoperable given plexus involvement and vessel proximity, and it is now causing both pain and measurable weakness. She is inside the studied group on every axis.
What that does not resolve is duration. Her baseline echocardiogram shows an LVEF of 62 percent and her ophthalmologic exam is clean, which matters less as reassurance than as a starting point: selumetinib's monitored toxicities are LVEF decline and retinal pigment epithelial detachment, and a normal baseline is what makes a later change interpretable rather than ambiguous. Tumors that respond to MEK inhibition have been reported to regrow once the drug stops, so this is very likely not a course with an end. The decision in front of the team functions less like treating a flare and more like deciding whether to commit a 13-year-old to years, possibly the remainder of her adolescence, of a therapy whose chronic toxicity profile is real and only partially understood.
In neuro-oncology clinic, after the six-month MRI
I'd start selumetinib now. SPRINT followed children with inoperable symptomatic plexiform neurofibromas and found roughly two-thirds achieved a durable partial response — real volume reduction, not just stable disease — and reported meaningful improvements in pain and function alongside the imaging. She has new pain and new weakness on a tumor that was stable for four years and is now growing right as she enters puberty, which is exactly when these tumors are known to accelerate. Waiting to see if it keeps growing isn't a neutral choice — it's choosing to let a documented growth-accelerant window pass with the tumor untreated.
If her tumor were still stable and asymptomatic, I would not be making this argument — plenty of NF1 patients with quiescent plexiform neurofibromas are appropriately never started on this drug at all.
You're right that the response data are strong and that the pubertal timing argument is real — I'm not disputing either one. But I want the family to understand what starting this drug actually commits her to before we treat this as a straightforward yes. Selumetinib carries a real chronic toxicity burden: cardiac monitoring for LVEF decline, since a measurable minority of patients develop reduced ejection fraction on treatment; periodic ophthalmologic exams for retinal pigment epithelial detachment; the acneiform rash and paronychia that show up in most patients to some degree; and tumors that respond to MEK inhibition have been reported to regrow once it's stopped, which means this is very likely not a bridge to surgery or a defined course — it's an indefinite commitment for a 13-year-old who is otherwise playing competitive soccer.
The response argument treats 'the tumor is growing' as the whole picture, but a two-thirds response rate also means roughly a third of patients don't get a durable response and are exposed to years of monitoring and side effects for less benefit. I don't think that's a reason not to try it here, given her actual symptoms — but it's a reason to be honest with the family that this isn't a guaranteed fix, it's a real bet with real ongoing cost either way.
I agree we should start MEK inhibition, and I don't think anything I'm about to say changes that conclusion. But I want to name, out loud, that selumetinib isn't the only approved option anymore — mirdametinib was approved more recently for the same indication, in both pediatric and adult NF1-associated plexiform neurofibroma. I don't think we should reach for selumetinib simply because it's the one everyone here has more experience prescribing. If there's a real reason to prefer it for her specifically — and there may well be, given how much more real-world pediatric experience exists with selumetinib right now — I want that to be a stated reason, not an unexamined default.
For what it's worth, I think the deeper real-world track record and the specific dosing familiarity with pediatric patients her age does tip this toward selumetinib for her today — I just don't want that landing as 'the only option' when a family is signing on for years of therapy.
Agreed: start selumetinib, with the family counseled directly, in the pharmacologist's own words, that this is very likely a years-long commitment rather than a defined treatment course, and with baseline cardiac and ophthalmologic surveillance obtained before the first dose. The choice between selumetinib and mirdametinib was made deliberately rather than by default, and documented as such.
Not raised as a disagreement to resolve today, but flagged for the chart: the pharmacologist's point that a third of patients don't achieve a durable response applies to her too, and the team agreed in advance on what would count as an adequate trial period and what findings — continued tumor growth, worsening pain or weakness despite treatment — would prompt reconsidering the drug rather than assuming persistence is always the right answer.