Recent advancements in linear accelerator and x-ray collimation technology have given the possibility to
produce photon radiation fields of few millimetres even at very high photon energy. This equipment has
been introduced recently in industry and clinical practice for irradiation of small targets. The use of MV
range photon beams in small target irradiation presents a challenge for a physicist which intends to
measure the dose distribution generated by the beam in water.
Bouchard et al. published in 2015 a remarkable publication where the theoretical concepts associated
with small field dosimetry are discussed and explored.
Part A
Using the arguments explained in this publication and in other documents of your selection (please refer
to the documents appropriately), write an essay of 2000 words answering the following questions:
1. When does a field become “small”?
2. What the factors are which make small field dosimetry different from standard field dosimetry?
3. What main parameters or characteristics a detector should have to be used in small field dosimetry?
Part B – case scenario
A 6MV industrial linear accelerator is used to irradiate small objects made of rubber (approximately
water equivalent) for sterilisation purposes. The nominal target dose distribution is 100 Gy delivered to
a sphere of 1cm diameter at 2 cm depth. The objects are rotated 360 degree to be uniformly irradiated.
The beam is 1×1 cm2 of area at isocentre and the only accessible point of measurement is at a depth of 2
cm inside the object. The physicist in charge of verifying the dose delivered to these objects has to
provide the value in real-time and has the following dosimeters available:
a. Ionisation Chamber CC013 from PTW
b. TLD carbon loaded 2x2mm2
(5mm thick)
c. Stereotactic diode EDGE from Sun Nuclear
d. PinPoint Ionisation Chamber 31014 from PTW
e. Microdiamond from PTW
Write a 1000 words essay to motivate your choice with the aim of theory, knowledge of the detectors’
working principles and drawings.
