Evaluating deviations in prostatectomy patients treated with IMRT.
OBJECTIVE
To evaluate the deviations in prostatectomy patients treated with IMRT in order to calculate appropriate margins to create the PTV.


BACKGROUND
Defining inappropriate margins can lead to underdosing in target volumes and also overdosing in healthy tissues, increasing morbidity.


METHODS
223 CBCT images used for alignment with the CT planning scan based on bony anatomy were analyzed in 12 patients treated with IMRT following prostatectomy. Shifts of CBCT images were recorded in three directions to calculate the required margin to create PTV.


CONCLUSIONS
The mean and standard deviation (SD) values in millimetres were -0.05 ± 1.35 in the LR direction, -0.03 ± 0.65 in the SI direction and -0.02 ± 2.05 the AP direction. The systematic error measured in the LR, SI and AP direction were 1.35 mm, 0.65 mm, and 2.05 mm with a random error of 2.07 mm; 1.45 mm and 3.16 mm, resulting in a PTV margin of 4.82 mm; 2.64 mm, and 7.33 mm, respectively.


CONCLUSIONS
With IGRT we suggest a margin of 5 mm, 3 mm and 8 mm in the LR, SI and AP direction, respectively, to PTV1 and PTV2. Therefore, this study supports an anisotropic margin expansion to the PTV being the largest expansion in the AP direction and lower in SI.
Radiotherapy Department, Lisbon School of Health Technology, Polytechnic Institute of Lisbon, Avenida D. João II, lote 4.69.01, 1990-096 Lisbon, Portugal.
Ana Cravo
AC
Sá
Radiotherapy Department, Lisbon School of Health Technology, Polytechnic Institute of Lisbon, Avenida D. João II, lote 4.69.01, 1990-096 Lisbon, Portugal.
Ana
A
Peres
Radiotherapy Department, Lisbon School of Health Technology, Polytechnic Institute of Lisbon, Avenida D. João II, lote 4.69.01, 1990-096 Lisbon, Portugal.
Mónica
M
Pereira
Radiotherapy Department, Lisbon School of Health Technology, Polytechnic Institute of Lisbon, Avenida D. João II, lote 4.69.01, 1990-096 Lisbon, Portugal.
Carina Marques
CM
Coelho
Radiotherapy Department, Lisbon School of Health Technology, Polytechnic Institute of Lisbon, Avenida D. João II, lote 4.69.01, 1990-096 Lisbon, Portugal.
Fátima
F
Monsanto
Mathematic Department, Lisbon School of Health Technology, Polytechnic Institute of Lisbon, Avenida D. João II, lote 4.69.01, 1990-096 Lisbon, Portugal.
Ana
A
Macedo
Dosimetry Department, Meixoeiro Hospital, Meixoeiro s/n, 36200 Vigo, Spain.
Adrian
A
Lamas
Cone beam computed tomography; Geometric uncertainty; Intensity modulated radiotherapy; Planning target volume margin; Prostate tumour