The design and evaluation of a phantom for the audit of the treatment chain for prostate radiotherapy.
OBJECTIVE
A phantom has been designed and built for a multi-institutional technique audit of the planning and delivery for radiotherapy to the prostate. The phantom was designed to test both the geometric and dosimetric accuracy of each aspect of the process.


METHODS
The phantom consists of two curved water filled perspex tanks either side of a central block of solid water equivalent material. There are two options for the central section; a target defining block and a dose measurement block. The target defining block uses air holes to define a 3-D target volume for imaging via a CT scanner or a simulator. These holes can subsequently be filled with steel pins to allow megavoltage imaging. The dose measurement block allows thimble chamber measurements to be made at pre-selected points in a 5x5mm array. Five dose measurement points, typical for a prostate planning target volume (PTV) were selected. Initial evaluation of the phantom was performed by auditing the prostate radiotherapy planning and treatment chain at one institution.


RESULTS
Agreement between the phantom and planned geometry confirmed that the stages of image acquisition, transfer and manipulation were accurately performed. Agreement within 0.5% was found between phantom and water tank measurements for dose calibration at a reference point. The measured dose delivered was within 2% of the dose calculated by the planning computer for all of the selected measurement points. The target volume marked by the steel pins was visible using electronic portal imaging.


CONCLUSIONS
The phantom is a useful tool for the technique audit of prostate radiotherapy.
Perrin
B A
BA
North Western Medical Physics, Christie Hospital NHS Trust, Wilmslow Road, Withington, Manchester, M20 4BX, UK.
Jordan
T J
TJ
Hounsell
A R
AR
D006801:Humans; D008297:Male; D008485:Medical Audit; D015337:Multicenter Studies as Topic; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted