Scientist with an international reputation for research

John Stewart Orr, emeritus professor of medical physics at the University of London, was well known nationally and internationally for his research work in cell biology, the effects of irradiation on cells and tissues, and for his contributions to clinical radiotherapy, magnetic resonance imaging, and the use of computers in medicine.

Stewart lived in or near Milngavie, near Glasgow, for most of his life. He was the second of three sons of Neil Orr, a well-known Glasgow lawyer. Beginning his schooldays at Atholl School and finishing his school career as proxime accessit at the High School of Glasgow, he went on to receive an honours degree in physics from Glasgow University and rounded off his education with a period of Army national service.

There was one episode during this last period which presaged future leadership. A fire had broken out in the barracks during the night. Badges of rank were not worn on pyjamas and in the ensuing chaos, in which no-one knew who was who, Orr organised everyone, including junior officers, into bucket chains. Once the fire was extinguished, he quietly reverted to his lowly role as Private Orr.

His first professional post was with Barr and Stroud, a Glasgow firm with contracts with the Ministry of Defence. His work was mostly classified as secret and involved infra-red technology and missile guidance. However, some of the fundamental research he did at that time was able to be made public and led to ''Orr's spherule'' entering the physics textbooks.

It was at Barr and Stroud that Stewart met Jean Williamson, and they subsequently married and had four children - a daughter and three sons - to whom he was a supportive, energetic, thoughtful and devoted father.

After six years at Barr and Stroud, Orr was appointed to a post in the medical physics department of the West of Scotland Health Board and was soon placed in charge of its radiotherapy physics division, working mainly at the Western Infirmary and Belvidere Hospital. The division's responsibilities included planning radiotherapy regimes for patients with cancer and ensuring that they were delivered accurately.

Orr also took a keen interest in the fundamental nature of malignancy. Research in this field had been disappointing and he felt that it had concentrated too much on the appearance of cancer cells and their chemical composition to the neglect of their metabolism.

To rectify this, he designed methods which allowed him to calculate the distribution of normal body constituents in the blood and in various tissues and also the movement of such compounds between the blood and various organs. This led to a burst of activity in which he gained the willing help of colleagues in several hospital and university departments.

The work was reported in approximately 60 articles over the next six years. It did not solve the mystery of the fundamental nature of cancer - Orr never thought that it would - but it provided an approach to studying the metabolic changes in cancer cells and how normal cells and cancer cells responded to injury, including that caused by irradiation.

He also applied it to optimising the dosage regimen in particularly vulnerable patients. The work was included in a thesis in which Orr was punctilious in his acknowledgement of the contributions of others. This thesis gained him the degree of DSc from Glasgow University in 1971.

Orr was appointed to the chair of medical physics of the Royal Postgraduate Medical School of the University of London in 1977, and director of the medical physics department of the Hammersmith Hospital.

In addition to his clinical responsibilities, he became deeply involved in computerising many of the hospital activities, and also in developing the use of magnetic resonance imaging. He was involved in the initiation of a project supported by the European Commission which aimed to see whether MRI (and later NMR) could be a quantitative method that could characterise and identify individual tissues in the body.

This pioneering work led to established methods of assuring the performance of MRI systems throughout Europe. Orr's drive and enthusiasm were a major reason for the success of this project.

He was also, at this time, heavily involved in studies funded by the Department of Health to develop methods of assessing the imaging performance of MRI systems.

Among his many contributions to the scientific literature, Orr was co-author of several of the seminal works on MRI.

In 1987, Orr left the Royal Postgraduate Medical School with the title of emeritus professor of medical physics, to return with Jean to Scotland from where he worked as a freelance consultant in medical physics. He continued to contribute in the areas of radiation physics, the European collaborative work on Magnetic Resonance Imaging, and on the application of computers in medicine.

He became involved with the National Centre for Clinical Coding and Classification at the commencement of the Read Version 3 codes project, initially as chair of the clinical working group on radiotherapy terms. It soon became clear to the centre that he had much more to offer than his expertise in the field of medical physics and he was offered a post to help manage the project, in particular to co-ordinate the clinical working groups.

This he did well, with his clarity of thought, strategic approach to problem solving, and his skill in managing people. Whilst taking a broad view on the one hand, Orr never lost sight of the need for the Read codes to have practical applications in a clinical environment: he was a champion of the potential users of the Read codes.

When it was apparent to him that the project was drifting away from its purpose, Orr tried hard to bring it back on course.

In this, unfortunately, he failed. He did, however, succeed in alerting the wider clinical community to the shortcomings and forced a major change in direction. No-one will ever know how much resource this saved the National Health Service.

Orr held senior office of the Hospital Physics Association (later to become the Institute of Physics and Engineering in Medicine) continually from 1969-1982, including its presidency, and played a major role in drawing up a constitution for the fledgling European Federation of Medical Physicists.

In 1983 he was invited to sit on the Black Committee, an independent advisory group set up in response to the increased incidence of cancer in West Cumbria. When the committee reported a year later it noted that no standing advisory group existed, which lead to Comare - Committee on Medical Aspects of Radiation in the Environment - being set up.

Orr was a member of the main committee from its inception in 1985 until 1990, and continued to serve on a sub-group until 1996.

He was president of the British Academy of Forensic Science from 1997-1998 and continued to serve on the editorial board. He was elected to fellowship of the Royal Society of Edinburgh, to the Institute of Physics and to the Institute of Physics and Engineering in Medicine.

It is difficult to provide a comprehensive summary of Orr's achievements because, although he obtained great satisfaction from his career, he was a modest man and rarely talked to his friends about his work.

Described recently as ''a gifted scientist, who inspired others'', he was also a keen educator and an entertaining lecturer. His ability to bring groups of individuals to agreement, always with good humour and consideration for all, was universally recognised. This rare skill, along with his integrity, his imaginative approach to problem solving, and his infectious enthusiasm, together with a dogged determination, were the attributes which probably contributed most to his highly successful career.

Some of these characteristics were also apparent in other areas of Orr's life.

He was, from childhood, a keen climber and hill walker. As a student he joined the Glasgow University Mountaineering (GUM) Club and climbed extensively in Scotland and also in the Alps, Pyrenees, and Norway. As well as the physical capability, Orr possessed those intellectual qualities which are as necessary for successful climbing as for a successful career - courage, integrity, clear communication and determination tempered by sound judgment.

In his younger days he pioneered several difficult routes in Scotland, including Stewart Orr's Variation on Chir Mhor, and one in Norway. An enjoyment of the mountains and the friendships forged there were lifelong. In the year of the 50th anniversary of the GUM club, he helped to organise a weekend meeting in Glencoe for past and present members, and this has since been an annual event.

In recent years Orr organised a number of walking tours for friends and family, and he was already planning one in the Lake District for next year. He had walked the West Highland Way numerous times and had started to walk it individually with his grandchildren.

Sailing was another of Orr's interests. Although not naturally competitive, he enjoyed the mental stimulation of friendly dinghy racing. He was a very competent yachtsman and took great pleasure in sailing regularly on the West Coast of Scotland with one or more friends, often in chartered yachts, among the Western Isles and even to St Kilda.

His marriage broke up in 1997, since when he lived in Shropshire with a new partner, Pam Frost, who is also a retired physicist. He continued to return to Scotland regularly to walk, to sail and to visit friends and family, particularly his nine grandchildren in whom he took a great interest and enormous pride.

Orr made friends very easily and kept them.

He had a puckish sense of humour and was always good value at social gatherings where he would often play his piano accordion. He shared generously his great energy and enjoyment of life. He will be sorely missed.

Professor John Stewart Orr, BSc, DSc, FInstP, FIPEM, FRSE; born August 10, 1930, died October 21, 2001.