Set-up errors analyses in IMRT treatments for nasopharyngeal carcinoma to evaluate time trends, PTV and PRV margins.
BACKGROUND
the aims of this study were to analyze the systematic and random interfractional set-up errors during Intensity Modulated Radiation Therapy (IMRT) in 20 consecutive nasopharyngeal carcinoma (NPC) patients by means of Electronic Portal Images Device (EPID), to define appropriate Planning Target Volume (PTV) and Planning Risk Volume (PRV) margins, as well as to investigate set-up displacement trend as a function of time during fractionated RT course.


METHODS
before EPID clinical implementation, an anthropomorphic phantom was shifted intentionally 5 mm to all directions and the EPIs were compared with the digitally reconstructed radiographs (DRRs) to test the system's capability to recognize displacements observed in clinical studies. Then, 578 clinical images were analyzed with a mean of 29 images for each patient.


RESULTS
phantom data showed that the system was able to correct shifts with an accuracy of 1 mm. As regards clinical data, the estimated population systematic errors were 1.3 mm for left-right (L-R), 1 mm for superior-inferior (S-I) and 1.1 mm for anterior-posterior (A-P) directions, respectively. Population random errors were 1.3 mm, 1.5 mm and 1.3 mm for L-R, S-I and A-P directions, respectively. PTV margin was at least 3.4, 3 and 3.2 mm for L-R, S-I and A-P direction, respectively. PRV margins for brainstem and spinal cord were 2.3, 2 and 2.1 mm and 3.8, 3.5 and 3.2 mm for L-R, A-P and S-I directions, respectively. Set-up error displacements showed no significant changes as the therapy progressed (p>0.05), although displacements >3 mm were found more frequently when severe weight loss or tumor nodal shrinkage occurred.


CONCLUSIONS
these results enable us to choose margins that guarantee with sufficient accuracy the coverage of PTVs and organs at risk sparing. Collected data confirmed the need for a strict check of patient position reproducibility in case of anatomical changes.
Mongioj
Valeria
V
Department of Medical Physics, Fondazione IRCCS Istituto Nazionale Tumori, Milan, Italy. valeria.mongioj@istitutotumori.mi.it
Orlandi
Ester
E
Palazzi
Mauro
M
Deponti
Elena
E
Marzia
Franceschini
F
Stucchi
Claudio
C
Sangalli
Claudia
C
Fallai
Carlo
C
Zonca
Giancarlo
G
Olmi
Patrizia
P
Pignoli
Emanuele
E
D000328:Adult; D000368:Aged; D002277:Carcinoma / Q000532:radiotherapy*; D005260:Female; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D008297:Male; D008875:Middle Aged; D009303:Nasopharyngeal Neoplasms / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated; D013997:Time Factors