Seren was diagnosed in July with a type of brain cancer called ependymoma, following a family holiday in Devon.
Her father, Mark, recounted how Seren appeared "a little out of sorts and wobbly" after their trip, which prompted a hospital visit that led to the discovery of her brain tumour.
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Seren underwent an MRI scan at Southampton General Hospital which confirmed the tumour could be operated on. (Image: UCLH NHS Foundation Trust)
Seren, from Bournemouth, was rushed to University Hospital Southampton's specialist children's neurological ward, and an MRI scan in Southampton General Hospital confirmed the tumour could be operated on.
Her mother, Clare, said: "It was a complete whirlwind - one minute Seren was a healthy three-year-old having fun on holiday and the next we were being transferred to Southampton with a really serious diagnosis.
"Although we were extremely worried, we felt fortunate that the tumour was operable."
The surgery took place on July 8, with the team managing to remove the tumour over 12 hours.
Subsequent tests confirmed that the tumour was cancerous, indicating the need for follow-up treatment.
Seren was discharged from University Hospital Southampton in mid-August, having undergone weeks of physiotherapy and support to help her recover from her operation, including learning to walk and swallow again.
The family travelled to London at the end of August for six weeks of proton beam therapy treatment at University College London Hospital's Proton Beam Therapy Centre.
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Dr Jenny Gains, consultant oncologist and Seren's doctor, said: "Every patient is different but for Seren, PBT is an ideal treatment to follow up on the surgery that removed her tumour.
"As she is only three, her brain is still developing and she obviously has a lot of growing still to do."
Mum Clare said: "We feel incredibly fortunate that Seren has been able to have proton beam therapy in the UK as we know that only a few years ago people had to travel abroad for this treatment.
"The fact that she is the 1000th NHS patient to be treated here is really special."
Seren's parents said she is slowly getting back to herself, and is looking forward to seeing her friends and starting school next year.