Agreement criteria between expected and measured field fluences in IMRT of head and neck cancer: the importance and use of the gamma histograms statistical analysis.
OBJECTIVE
The aim of this study was to analyse our pre-treatment QA data in order to establish uniquely defined agreement criteria between planned and delivered dose distribution for clinical QA practice in IMRT of head and neck (HN) patients.


METHODS
Pre-treatment QA dosimetry using films in combination with ionisation chambers is routinely evaluated for each patient. This evaluation is performed by comparing planned and measured dose distributions in terms of absolute point dose measurements, planar dose verification and gamma function analysis using 4%/3mm values as acceptance criteria. In the current investigation, gamma histograms, calculated on dose levels higher than 10% of the prescription dose (1.8Gy/fraction), were further analysed by considering mean values, gamma values corresponding to Delta=mean+1.5 SD (named as gamma(Delta)) and the % of points with gamma<1, gamma<1.5 and gamma>2. When considering the patient population, the average values of all these parameters and their confidence limits (mean value+1.5 SD) were calculated. The results here presented refer to 57 HN patients treated in the period September 04-April 06.


RESULTS
Better results were found for treatments performed with our newly installed linac (e.g. average gamma(Delta): 0.8 vs. 1.1 for the preexisting one, p<0.001), due to a more accurate dosimetric configuration. Also, confidence limits for the percentage of points with gamma<1, gamma<1.5 and gamma>2 were found to be quite different for the two linacs (95.3%, 98.9% and 0.4% for the newly installed unit, 87.6%, 95.2% and 2.0% for the preexisting one).


CONCLUSIONS
Statistical analyses of gamma evaluation of QA pre-treatment dosimetry are useful to properly define confidence limits of the agreement between expected and measured fluences based on our institutional experience. Our results confirm that the dosimetry configuration of the beam may significantly affect the agreement between planned and measured IMRT beam fluences.
De Martin
Elena
E
IRCCS San Raffaele, via Olgettina 60, Milan, Italy. demartin.elena@hsr.it
Fiorino
Claudio
C
Broggi
Sara
S
Longobardi
Barbara
B
Pierelli
Alessio
A
Perna
Lucia
L
Cattaneo
Giovanni Mauro
GM
Calandrino
Riccardo
R
D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated