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"My blood pressure dropped and my temperature rose. My body was fighting the infection. I got flu-like symptoms. You're kind of living with the flu, that's what the stuff does. It proves the body is fighting cancer." That was early last year.
Mr Boughtwood, 60, has now had more than 20 infusions of atezolizumab - an antibody aimed at limiting the ability of tumours to inhibit the immune system's anti-cancer activity.
"It's been like a lifesaver to me. I have been just so fortunate to get on the trial."
He was diagnosed with cancer in January 2013. Several months earlier he and his wife Cath had caught a cold and Mr Boughtwood was left with a persistent cough.
"My GP sent me to North Shore Hospital for a chest x-ray in December and the following day they phoned back and said there was a mark on my right lung."
The next month a CT scan showed a 4cm-wide tumour in the lung, with cancer cells having spread to lymph nodes around the lung, says Mr Boughtwood, who quit smoking after his diagnosis.
He went on carboplatin chemotherapy for several months, lost his hair and felt like he had a constant hangover, yet there was no change in the cancer.
On the trial, however, CT scans have shown progressive improvements, although without a biopsy uncertainty remains.
"The primary tumour has reduced by 30 per cent. They regard what's left as the debris ... it's scarring. Basically the cancer cells have for the most part been wiped out.
"I would class myself as well on the way to living a bit longer."
Before he joined the trial, Mr Boughtwood and his wife were planning for how she would carry on without him.
Now, with his energy returning, he hopes to return to doing more work at the family owned printing business, and taking "nice long walks on the beach".
Trials of atezolizumab are under way or planned in cancer of the kidneys, breast and bladder.
The Series
Monday: Breast Cancer
Yesterday: Bowel Cancer
Today: Lung Cancer
Thursday: Melanoma
Friday: Prostate Cancer