I WAS invigilating a GCSE examination in a classroom at school. Bare walls, white shirts, silence, heat, and an awareness that future lives depended on events in that room. All of it vividly recalled the long summer afternoon I spent in the recovery ward at Woodlands, the Birmingham Royal Orthopaedic Hospital, three years ago.

The quiet of the ward, the heat outside, the intensity of watching the fragile figure on the bed, created a curious sense of timelessness. My son's arm, which I could barely bring myself to look at, was lightly covered. Tubes were pumping in blood and fluids; tubes were draining from the wound. An oxygen mask half covered his pale face. The whiteness of the sheet and the still head, bald from the chemotherapy, made him seem newborn.

It was a week after Gregory's 13th birthday. He had just had a replica titanium 'bone' put into his left arm. Photographs taken especially for him during the operation revealed the ugly lump of the bone cancer, the osteogenic sarcoma, which deformed the upper section of his humerus. From about an inch and a half above his left elbow they removed all the bone to his shoulder and replaced it with a tailor-made titanium prosthesis. A subsequent picture shows this gleaming replacement fitted into the shoulder socket, nestling into the tissues of the opened limb. The Woodlands is one of only two hospitals in the country where such specialised surgery is performed. Today, limbs are saved that once would have been amputated.

An osteogenic sarcoma is a rare form of bone cancer found mainly in older children and teenagers, especially boys. More usually it occurs in the leg. Greg's operation involved severing the muscles at the top of the arm, so although his arm has been saved, he has limited use of it.

As I sat at the back of the examination room and scanned the pupils I could pick him out because his left shoulder is spikier than the right and shows beneath his shirt as narrower and less muscular. Sometimes he looked up to daydream for a moment and his thick hair caught his collar. Since he had missed two-thirds of the course it was a miracle that he was taking the exams at all.

But it is much more of a miracle that he is alive to do so. His shirt hides the livid scar that curls over his shoulder and rips down to the elbow. It also hides another that slashes along the line of his shoulder blade, and six more tiny scars which are scattered over his left ribs and make him look like an unintended victim caught at the edge of machine gun fire.

Although Greg's cancer did not return to adjacent bone tissue, secondary tumours did appear in his lungs, a favourite place in this type of cancer. This happened twice, once 16 months after the surgery to his arm and again nine months later.

Greg had two thoracotomies at Papworth Hospital in Cambridgeshire to remove parts of his lung containing the tumours. During the first, his left lung developed an 'air leak' and would not clear properly through the drainage tubes. These are normally inserted at the end of the operation while the patient is under general anaesthetic. But twice subsequently Greg had to have the tubes reinserted from the outside to stop his lung collapsing. This involved hacking a hole between the ribs, through muscle until the wall of his lung was reached, and then pushing in a plastic tube - all under local anaesthetic. I talked to him while I helped out in the treatment room, to try to mask the crunching sound made by the surgical scissors.

All possible chemotherapy has now been tried. He knows all the horrors: the endless mouth ulcers, the collapsed vein that makes it impossible for the drip to flow; damage to the wall of the vein caused by the cancer drugs themselves; the terrors of leakage and having to resite the needle; the waking fear of blood-soaked sheets and a sodden drip bandage. Greg has had transfusions to repair the ravages to his blood, a side-effect of his chemotherapy. Twice he has coped with the trauma of losing his hair.

There were times when I wondered how much more I could ask him to take. Occasionally, watching clusters of white-coated experts gather round my son, raising him on metal plates, throwing radiation at him, shining bright lights, wielding instruments and poisons, I have been reminded of films of torture. Yet these specialists have given life to a child who stastically should be dead.

As I watched him writing fast and fluently, pictures drifted into my head: with his sister, Josephine, dangling their feet into choppy waves from the front of a small boat, she with her arm supporting him as his left was in a sling, 10 days after his prosthetic replacement operation; an emaciated skeleton bursting into a comic dance routine to a pop record in the doorway before collapsing into his bed after chemotherapy; a fragile youth holding himself stiffly but putting a golf ball determinedly round the garden, four days after his second thoracotomy.

I knew that when the examination ended Greg would burst out into the sun to join his friends. They would cycle home, change, and race to the lake to swim. Or they would thump a tennis ball about or practise their golf swings - the game he was told his damaged arm would prevent him from playing. He will be scratched, dirty and bruised - not with the insidious marking of internal bleeding caused by the inability of his damaged blood to clot, but with the wonderful purple batterings of romping. He is thin, but he is tall, noisy and hungry. He grabs at life, consumes it, paces himself and wastes nothing. Oh, the miracles of medicine; the courage of youth.

He is on three-monthly check- ups; we know the word 'cure' does not feature in the cancer specialist's vocabulary, but with a fighter such as Greg who could fail to hope for it?

Gregory Taylor, who has had less than six months of formal schooling in the past three years, gained one A, two Bs, two Cs and a D in his GCSE examinations.

(Photograph omitted)