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UNIVERSITY COLLEGE LONDON
EXAMINATION FOR INTERNAL STUDENTS
MODULE CODE : MPHY0038
ASSESSMENT Pattern: MPHY0038A6UE
MODULE NAME : Treatment with Ionising Radiation
LEVEL: : Undergraduate
DATE: : 16-May-2023
TIME : 14:30
DURATION : 02:10
Late submission is permitted for Controlled Conditioned exams but late
penalties will apply – any submissions that are up to 40 minutes late will be
penalised, after which no submissions will be accepted under any
circumstances.
You must ensure to allow sufficient time to upload and hand in your work
This paper is suitable for candidates who attended classes for this module in
the following academic year(s):
Year
2022-23
Exam Duration 2 hours
Additional time for converting handwritten
notes to PDF where applicable 10 mins
Upload window 20
Total time
2 Hours 30 mins
Additional material None
Special instructions None
TURN OVER
MPHY0038 / 2022-23 (Main) Page 2 CONTINUES
Answer ALL QUESTIONS
The number in square brackets on the right-hand side after each question indicates
the maximum number of marks allocated for that part of the question. You can use this
as an indication of the extent and depth of the answer expected.
Answers can be hand-written or typed using a word processor, or you can use a
combination of both methods.
All answers must be submitted electronically via the AssessmentUCL platform.
Submissions via email or any other method cannot be accepted.
Everything you submit, including drawings, diagrams, and mathematical
calculations, must be your own work and not copied or reproduced from any other
source.
You must not discuss examination questions or your answers with any other person,
including other UCL students, or obtain assistance from any third party during the
period of the examination.
MPHY0038 / / 2022-23 (Main) Page 3 TURN OVER
QUESTION 1
Dosimetry
An ionisation chamber needs to be calibrated. It is positioned at a depth of 10 cm in a water
tank and at a distance of 100 cm from a Cs-137 source, which emits gamma rays with an
energy of 662 keV. The source has an activity of 6 GBq. Assume that 1 cm of water absorbs
5% of gamma rays at this energy. Mass energy absorption coefficient for air at this energy =
29.2 cm2
kg-1, 1 eV = 1.602×10-19 J and Wair/e = 33.97 J C-1
.
(i) Calculate the total exposure that the ionisation chamber should record during a 30
minute irradiation.
[8]
(ii) State any additional assumptions you have made in your answer to (i).
[1]
QUESTION 2
Radiobiology
The ratio is commonly used in radiobiology.
(i) Explain, with the aid of a diagram, the origin of the terms and .
[2]
(ii) How are they estimated
[1]
(iii) What does it mean for a tumour to have a high ratio And what implications
does that have for radiotherapy treatment
[2]
(iv) A patient is being treated for a rare cancer for which the ratio is unknown. What
are the risks to the patient How could these risks be mitigated against
[4]
MPHY0038 / 2022-23 (Main) Page 4 CONTINUES
QUESTION 3
Proton therapy
(a) Discuss the benefits of proton therapy to the patient compared with x-ray therapy and
any disadvantages.
[4]
(b) What impact do the disadvantages you have mentioned have on the patient and/or the
proton therapy service
[2]
(c) Choose one clinical application where proton therapy is used and explain why this
approach might be preferred instead of using x-ray therapy.
[2]
(d) A proton pencil beam increases with depth. Discuss the impact this has on proton
therapy treatments.
[2]
(e) The Relative Biological Effectiveness is defined as the dose of a standard radiation to
produce a given biological effect divided by the dose of a different radiation to produce
the same effect. This is commonly used in proton therapy to compare with x-ray therapy,
and is usually assigned the value 1.1. Explain why protons and x-rays require a different
dose to give the same biological effect.
In a patient how could biological effect be determined
[8]
QUESTION 4
Treatment planning
(a) One of the aims of radiotherapy planning is to spare dose to healthy tissues. Describe
how this is done for x-ray treatments with a linac. Your answer should consider imaging,
planning and treatment.
[10]
(b) Describe methods that are used in the planning stage to determine whether dose has
been spared and from where. Use one or more diagrams to illustrate your answer.
[4]
MPHY0038 / / 2022-23 (Main) Page 5 END OF PAPER
QUESTION 5
Dosimetry
(a) A patient is receiving radiation treatment for prostate cancer. They are irradiated with 3
beams from different directions. Each beam passes through 10 cm of soft tissue. The
prostate is to receive a total of 70 Gy in 20 fractions. The source is at a distance of 100
cm from the centre of the target. An ionisation chamber is at a distance of 10 cm from
the source. Calculate the total exposure reading that the ionisation chamber should
record during a single fraction from a single beam.
[7]
(b) It is found that the ionisation chamber reading for a patient is higher than the calculated
value. State, with reasons, possible explanations for the different value.
[3]