I AM sitting by a pond, watching ducks, listening to the whistlings of two black swans. It is warm and sunny. I am writing a postcard and I am tempted to say: 'Having a lovely time; wish you were here.' Instead I write: 'Greg has just gone into theatre and I am sitting in the grounds of Papworth Hospital . . .'

I pause, and think back a fortnight. I am in an office in Addenbrooke's Hospital looking at a set of CT scan pictures. The tall youth standing straight and still beside me is my son, Greg, and he stares, unflinching, at the picture of his own right lung; the secondary tumour shows up clearly, white against dark lung tissue, for this is a tumour containing calcium from the bone cancer he first began to fight when he was 12 years old.

He is not a child now; he is 17 and an old hand at this. Already there have been three major operations: one to remove the original tumour in his left arm and replace the humerus with a titanium replica bone; two to remove secondary tumours from his left lung, the last nearly two years ago. So now there is no panic, only a question and careful attention to the answer: no more radiotherapy, no more chemotherapy, 'just' an operation. This is what he wants to hear.

Looking at the scans, I am awed by the power of this thing. It has defied three separate sessions of destructive chemotherapy. It has lurked and struggled and finally gained enough strength to grow into a tumour two inches across. It has already stolen the end of his childhood and much of his adolescence. He is just starting to select university courses; don't let it wreck his young manhood, too.

I search my recent memory for clues. Greg says he had a 'bad feeling' about this check-up, but everyone else has said how strong and fit he looks. Has there been a lessening of energy? It is such an insidious disease, a betrayal of the body that nurtures it.

The terror of additional chemotherapy is so great for him, because of the side-effects, that the immediate response to the operation is one of relief.

Greg genuinely regards both his surgical hospitals, the Royal Orthopaedic at Birmingham and Papworth in Cambridgeshire, almost as holiday places. He will leave on his feet, not in a wheelchair. Papworth is even designed like a hotel, having originally been for tuberculosis patients, with rooms and wards opening out on to tree-studded grounds. Nurses welcome him with cheerful recognition; his anaesthetist is an old family friend; his surgeon greets him warmly. Twice now he has given life back to this boy.

This time he will get to the lungs by sawing through the sternum (breast bone) to remove the tumour. The sternum will be repaired with stainless steel 'staples' which will help it to mend like any broken bone, and which, Greg hopes, will set off the metal detectors at airports.

He was strangely calm before the operation, and I am calm as I look at him in the recovery room. I need to touch him. I avoid the tubes, lines and machines, and the long wound that zips up his chest, to rest my hand against his head. Sitting and watching with him in this quiet, intense room is the most comforting thing I can do for both of us. All is well.

Two or three hours later he is returned to the ward. A young nurse is busy rearranging equipment which has been temporarily switched off, and I am about to settle down next to him. He is very white. Even his lips are white. I murmur: 'Poor Greg. He looks like death.' As I say the word we both stare at him. He does look dead.

He has stopped breathing.

Nurses multiply. One constantly calls: 'Greg] Come on, Greg] Wake up]' A doctor runs in, an anaesthetist arrives from theatre, his surgeon appears. Horror engulfs me. How can we have gone through all this, and then watch him die on the bed? His eyeballs roll up under the lids. His pupils react to nothing. His face is white, white, white. Minutes pass. Oh God, Greg, breathe.

And he does. A sudden choking panic and a fight against the oxygen mask, a call for me . . . and he settles. It is decided that he had a reaction to the painkiller and this is changed.

This is the most terrifying moment of the last four and a half years. I pull myself together. 'At least your face is more like the colour of your pink wash now than your pillow,' I say. A plaintive, muffled voice is heard inside the mask: 'Don't . . . make me . . . laugh.'

Recovery is rapid. Tubes out; drains out. The next day he is on his feet for a little walk. He eats, and is not sick. His sister, Jo, brings him a prawn cocktail from the local pub and a Chinese take-away. His room is filled with laughter; tears are there, too, but we keep them under control.

Four days later he walks to the car. Slowly he will pick up his battered life from where he left off.

We never properly thank the hospital staff, never dare offer a tribute, for it seems too final, too certain that we will not return. We try, these days, not to look for signs and symbols.

It will take time: the scar will be painful; the sternum will split apart if he strains his chest. The threat of yet another recurrence will overshadow all he does, as the routine of checks and X-rays punctuates his A-level lessons and exams. But he will consume life with all the intellectual, emotional and physical energy he can muster. With his quick grin and sharp wit, if anyone is a survivor, he is.

(Photograph omitted)