A hospital uses radioactive isotopes for both diagnosis and treatment of disease. Fig. 51.1 shows two different procedures. For diagnosis, a small amount of...
Question 1 Report
A hospital uses radioactive isotopes for both diagnosis and treatment of disease. Fig. 51.1 shows two different procedures. For diagnosis, a small amount of technetium-99m (half-life 6 hours, gamma emitter) is injected into the patient. A gamma camera detects the gamma rays and builds an image of the organ. For treatment, a focused beam of gamma radiation from a cobalt-60 source (half-life 5.27 years) is directed at a tumour inside the patient's body. The beam is rotated around the patient so that the tumour receives a high dose but surrounding healthy tissue receives a lower dose. The hospital physicist is responsible for ensuring that both procedures are carried out safely and that radiation doses to staff and patients are kept as low as reasonably achievable.
(a) Explain why a short-half-life gamma emitter is used for diagnosis but a long-half-life gamma source is used for treatment. [3]
(b) Explain why alpha or beta sources would not be suitable for either procedure. [2]
(c) Describe two safety precautions that hospital staff should follow during these procedures. [2]
(d) State two harmful effects of ionising radiation on living cells and explain why the treatment beam is rotated around the patient. [2]
Answer Details
(a) Why short half-life for diagnosis but long half-life for treatment [3]
Diagnosis: A short half-life (6 hours for Tc-99m) means the tracer decays quickly after the imaging procedure is complete, minimising the total radiation dose to the patient [1]. The patient is not exposed to radiation for an extended period.
Treatment: A long half-life (5.27 years for Co-60) means the source maintains a high, constant activity over many treatment sessions spanning weeks or months, without needing frequent replacement [1].
Both procedures use gamma: Gamma radiation is highly penetrating. For diagnosis, the gamma rays emitted by the tracer inside the body can escape through tissue to reach the external camera. For treatment, the gamma beam can penetrate deep into the body to reach internal tumours [1].
(b) Why alpha or beta sources are unsuitable [2]
Alpha particles would be absorbed by body tissue almost immediately (within micrometres). They would not penetrate far enough to reach the organ being imaged or the deep tumour being treated [1]. Beta particles would also be largely absorbed by tissue within a few millimetres and would not be detectable externally for imaging, nor would they reach deep tumours effectively for treatment [1].
(c) Two safety precautions for staff [2]
- During treatment, staff should leave the room or stand behind thick lead shielding to avoid exposure to the high-intensity gamma beam [1].
- When handling the tracer for diagnosis, staff should wear gloves, use syringes with shielding, and minimise the time spent in contact with the radioactive material [1].
(d) Harmful effects and why the beam is rotated [2]
Ionising radiation can kill cells directly and can cause mutations in DNA that may lead to cancer [1]. The treatment beam is rotated around the patient so that the tumour, at the centre of rotation, receives a concentrated dose from all angles, while each surrounding section of healthy tissue is only briefly in the beam's path and receives a much smaller dose. This maximises damage to the tumour while minimising harm to healthy tissue [1].
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