The use of penumbrae to define and analyse non-standard fields

Charles, P, Kairn, T, Crowe, S, & Trapp, J (2014) The use of penumbrae to define and analyse non-standard fields. In 2014 Combined Scientific MeetingImaging and Radiation in Personalised Medicine, 4-7 Sep 2014, Melbourne Convention and Exhibition Centre, Melbourne, Vic.

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A new method of penumbral analysis is implemented which allows an unambiguous determination of field size and penumbra size and quality for small fields and other non-standard fields. Both source occlusion and lateral electronic disequilibrium will affect the size and shape of cross-axis profile penumbrae; each is examined in detail.


A new method of penumbral analysis is implemented where the square of the derivative of the cross-axis profile is plotted. The resultant graph displays two peaks in the place of the two penumbrae. This allows a strong visualisation of the quality of a field penumbra, as well as a mathematically consistent method of determining field size (distance between the two peak’s maxima), and penumbra (full-widthtenth-maximum of peak). Cross-axis profiles were simulated in a water phantom at a depth of 5 cm using Monte Carlo modelling, for field sizes between 5 and 30 mm. The field size and penumbra size of each field was calculated using the method above, as well as traditional definitions set out in IEC976. The effect of source occlusion and lateral electronic disequilibrium on the penumbrae was isolated by repeating the simulations removing electron transport and using an electron spot size of 0 mm, respectively.


All field sizes calculated using the traditional and proposed methods agreed within 0.2 mm. The penumbra size measured using the proposed method was systematically 1.8 mm larger than the traditional method at all field sizes. The size of the source had a larger effect on the size of the penumbra than did lateral electronic disequilibrium, particularly at very small field sizes.


Traditional methods of calculating field size and penumbra are proved to be mathematically adequate for small fields. However, the field size definition proposed in this study would be more robust amongst other nonstandard fields, such as flattening filter free. Source occlusion plays a bigger role than lateral electronic disequilibrium in small field penumbra size.

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ID Code: 75957
Item Type: Conference Paper
Refereed: Yes
Additional Information: Abstract published in Journal of Medical Radiation Sciences V.61 (S1) pp.208
Additional URLs:
Keywords: small field, field size, radiotherapy, stereotactic radiosurgery
DOI: 10.1594/ranzcr2014/R-0131
Subjects: Australian and New Zealand Standard Research Classification > PHYSICAL SCIENCES (020000) > OTHER PHYSICAL SCIENCES (029900) > Medical Physics (029903)
Divisions: Current > Schools > School of Chemistry, Physics & Mechanical Engineering
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Deposited On: 08 Sep 2014 04:12
Last Modified: 05 May 2017 03:53

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