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OBJECTIVEThe aim of this study was to compare 3 different methods to assess the geometrical distortion of two 1.5-T and one 3-T magnetic resonance (MR) scanners and to evaluate co-registration accuracy. The overall uncertainty of each particular method was also evaluated.METHODSThree different MR phantoms were used: 2 commercial CIRS skull phantoms and PTGR known target phantom and 1 custom cylindrical Perspex phantom made in-house. All phantoms were fixed in the Leksell stereotactic frame and examined by a Siemens Somatom CT unit, two 1.5-T Siemens (Avanto and Symphony) MRI systems, and one 3-T Siemens (Skyra) MRI system. The images were evaluated using Leksell GammaPlan software, and geometrical deviation of the selected points from the reference values were determined. The deviations were further investigated for both definitions including fiducial-based and co-registration–based in the case of the CIRS phantom images. The same co-registration accuracy assessment was also performed for a clinical case. Patient stereotactic imaging was done on 3-T Skyra, 1.5-T Avanto, and CT scanners.RESULTSThe accuracy of the CT scanner was determined as 0.10, 0.30, and 0.30 mm for X, Y, and Z coordinates, respectively. The total estimated uncertainty in distortion measurement in one coordinate was determined to be 0.32 mm and 0.14 mm, respectively, for methods using and not using CT as reference imaging. Slightly more significant distortions were observed when using the 3-T than either 1.5-T MR units. However, all scanners were comparable within the estimated measurement error. Observed deviation/distortion for individual X, Y, and Z stereotactic coordinates was typically within 0.50 mm for all 3 scanners and all 3 measurement methods employed. The total radial deviation/distortion was typically within 1.00 mm. Maximum total radial distortion was observed when the CIRS phantom was used; 1.08 ± 0.49 mm, 1.15 ± 0.48 mm, and 1.35 ± 0.49 mm for Symphony, Avanto, and Skyra, respectively. The co-registration process improved image stereotactic definition in a clinical case in which fiducial-based stereotactic definition was not accurate; this was demonstrated for 3-T stereotactic imaging in this study. The best results were shown for 3-T MR image co-registration with CT images improving image stereotactic definition by about 0.50 mm. The results obtained with patient data provided a similar trend of improvement in stereotactic definition by co-registration.CONCLUSIONSAll 3 methods/phantoms used were evaluated as satisfactory for the image distortion measurement. The method using the PTGR phantom had the lowest uncertainty as no reference CT imaging was needed. Image co-registration can improve stereotactic image definition when fiducial-based definition is not accurate.

Aims. Curative sphincter sparing radiotherapy is a treatment option for early rectal cancer. There are many methods developed for fertility preservation in young patients treated with pelvic radiotherapy. Pregnancy rates after radiotherapy are dependent on the radiation dose to ovaries and uterus. Data on outcomes of total body irradiation suggest a pregnancy is possible following 12-14 Gy TBI, despite elevated rates of preterm deliveries and other complications. Methods. We report a case of full-term delivery of twins after curative chemoradiotherapy for anorectal adenocarcinoma T2 N0 M0 with the total dose 58.6 Gy. The patient underwent laparoscopic laterocranial ovarian transposition before radiotherapy. Results. Long term complete remission was achieved after treatment. Although a spontaneous conception was not successful, the patient underwent an in vitro fertilisation procedure with donor eggs and conceived twins 10 years after the radiotherapy treatment. The mean dose to the uterus was 16 Gy and to the uterine cervix 35 Gy. She reached a full-term pregnancy and delivered two healthy babies by caesarean section at a gestational age of 38 weeks, weighing 2420 g and 2220 g. Conclusion. This is the first case report of the successful pregnancy following sphincter sparing curative pelvic radiotherapy for rectal cancer. Furthermore it allows us to propose an increased limit dose to the uterus enabling fertility sparing beyond the limits achieved from total body irradiation series with 12-14 Gy and accept 16 Gy as uterine body (35 Gy for uterine cervix) limit for IMRT treatment planning in young patients asking for maintaining fertility potential.