Evaluation of treatment planning system of brachytherapy according to dose to the rectum delivered

Abstract

The aim of this study was to assess the actual dose delivered to the rectum and compare it with the treatment planning system (TPS) reports. In this study, the dose delivered to the rectum was measured by semiconductor diode detectors (PTW, Germany). The factors that influence diode response were investigated as well. Calibration factors of diodes were measured weekly to investigate the effect of time interval on the accuracy of calibration. Then 40 applications of patients with cervix carcinoma were evaluated. Rectum dose was measured by means of rectal dosemeter and compared with the TPS-calculated dose. In this research, the differences between the measured and the calculated dose were investigated. The mean difference between the TPS-calculated dose and the measured dose was 6.5% (range: -22 to +39) for rectum. The TPS-calculated maximum dose was typically higher than the measured maximum dose. The study showed that the main reason for the difference was due to the movements of the patient and applicator shift in the elapsed time between the imaging and treatment stage. It is recommended that in vivo dosimetry should be performed in addition to treatment planning computation. In vivo dosimetry is a reliable solution to compare the planned and actual dose delivered to organs at risk.

Measurement of rectum dose by in vivo alanine/ESR dosimetry in gynecological 192Ir HDR brachytherapy

“…Furthermore, variations of À40% to 40% were found by Eich et al (1998) using a semiconductor dosimeter and analyzing rectal dose by graphical approximation. Allahverdi et al (2011) found differences between planned and measured doses of À22% to 39%, using semiconductor diodes dosimeters, also with difficulties related to organ movement. Tanderup et al (2006) analyzed the geometric uncertainties of the applicator and the rectal probe and obtained a displacement average of 0.8 mm (±0.5 mm) in all directions in HDR BT.…”

In vivo dosimetry in brachytherapy

“…Typical sites for IVD are rectum, [37][38][39][40] and bladder or urethra. 32,33,41 Noninvasive detector positioning far from the source, e.g., on the skin, make the measurements insensitive to error detection because of low signal and difficulties to know precisely the relation between the source(s) and detector(s).…”

In vivodosimetry: trends and prospects for brachytherapy

“…Diodes have been incorporated for IVD during BT, 12,13,[62][63][64] in the bladder (3-mm outer probe diameter) and/ or rectum (7-mm outer probe diameter) manufactured by PTW Fruiberg, Freiberg, Germany. Compared with diode-based dosemeters, the MOSFET detector is a relatively new dosemeter technology in RT.…”

“…For instance, discrepancies .30% between measured and expected dose rates have been observed during IVD 14,19,21,62,63 and have been attributed to poor correspondence between the dosemeter position during the patient image acquisition and that during the treatment delivery owing to patient movements during the transfer between imaging and treatment locations. 19,21,63 Since BT dose distributions are characterized by steep gradients, the uncertainties in the detector position may generate substantial dose uncertainties. Positional uncertainties ,1.5 mm for the dosemeter and individual source positions are required in order to be able to detect source displacement errors of 5 mm.…”

“…Compromised dosemeter positions, where the position during the treatment delivery did not correspond to that acquired from patient images, have been indicated during IVD in the urethra, 12,19 the rectum, 12,13,62,63,84 and source applicators. 14,21 Compromised dosemeter positions may be caused by erroneous reconstructions owing to poor image quality and/or by dosemeter shifts from their initial placement with respect to the adjacent tissue, for example, owing to patient movements during transport between locations.…”