[en] Purpose: A disadvantage of ovoid shields in a Fletcher-type applicator is that these shields cause artifacts on post implant CT images. CT images, however, make it possible to calculate the dose distribution in the rectum and the bladder. To be able to estimate the possible advantage of having CT information over the use of ovoid shields without having CT information, we investigated the influence of shielding segments in a Fletcher-type Selectron-LDR applicator on the dose distribution in rectum and bladder. Methods and Materials: Contours of rectum and bladder were delineated on transaxial CT slices of 15 unshielded applications. Of the volumes contained within these structures dose-volume histograms (DVHs) were calculated. In a similar way, DVHs of simulated shielded applications were calculated. The reduction, due to shielding, of the dose to the 2 cm³ (D₂) and 5 cm³ (D₅) volume of the cumulative DVHs of rectum and bladder, were determined. An isodose pattern in the sagittal plane through the center of each applicator was plotted to compare the location of the shielded area with the location of maximum dose in rectum and bladder in the unshielded situation. In two cases local dose reductions to the rectal wall were determined by calculating the dose in points at 10-mm intervals on the rectal contours. Results: For the rectum, the reduction of D₂ ranged from 0 to 11.1%, with an average of 5.0%; the reduction of D₅ ranged from 2.3 to 12.1%, with an average of 6.4%. The reduction of D₂ and D₅ for the bladder ranged from 0 to 11.9% and from 0 to 11.6%, with average values of 2.2 and 2.6%, respectively. In 8 out of 15 cases the rectal maximum dose was located inferior to the shielded area. In all cases except one the bladder maximum dose was located superior to the shielded area. Local dose reductions on the rectal wall can be as high as 30% or more in an optimally shielded area. Conclusions: Reductions of D₂ and D₅ to rectum and bladder due to shielding are rather small, because the shielded area does usually not coincide with the high dose region and even if it does, the shielded area is too small to result in large reductions of these values. Because local dose reductions vary largely, one should proceed with caution when calculating the dose in just one rectal or bladder reference point. Because large overall dose reductions cannot be achieved with shielding, it is safe to use an unshielded applicator when post implant CT images are used to realize optimized dose distributions

[en] Background and Purpose: To investigate possible relationships between the dose to the sub-segments of the lower urinary tract and lower urinary tract symptoms (LUTS) after brachytherapy of the prostate. Materials and Methods: This study involved 225 patients treated for prostate cancer with I-125 seeds. Post-implant dose-volume histograms of the prostate, urethra, bladder wall, bladder neck and external sphincter were determined. Endpoints were the mean and the maximum International Prostate Symptom Score (IPSS) during the first 3 months after the treatment. For binary analysis the patients were stratified in a group with enhanced LUTS and a group with non-enhanced LUTS. Results: The dose to 0.5 cm³ of the bladder neck ‘D_0.5cc-blne’ (p = 0.002 and p = 0.005), the prostate volume prior to treatment ‘V_pr-0’ (p = 0.005 and p = 0.024) and the pre-treatment IPSS (both p < 0.001) were independently correlated with mean and maximum IPSS, respectively. Of the patients with a D_0.5cc-blne ⩾ 175 Gy and a V_pr-0 ⩾ 42 cm³, 68% suffered from enhanced LUTS, against just 30% of the other patients (p < 0.0001). Conclusions: Pre-treatment IPSS, prostate volume and dose to the bladder neck are correlated with post-implant IPSS. A combination of a large prostate and a high dose to the bladder neck is highly predictive for enhanced early LUTS

[en] To study dose-effect relations of prostate implants with I-125 seeds, accurate knowledge of the dose distribution in the prostate is essential. Commonly, a post-implant computed tomography (CT) scan is used to determine the geometry of the implant and to delineate the contours of the prostate. However, the delineation of the prostate on CT slices is very cumbersome due to poor contrast between the prostate capsule and surrounding tissues. Transrectal Ultrasound (TRUS) on the other hand offers good visualization of the prostate but poor visualization of the implanted seeds. The purpose of this study was to investigate the applicability of combining CT with 3D TRUS by means of image fusion. The advantage of fused TRUS-CT imaging is that both prostate contours and implanted seeds will be well visible. In our clinic, post-implant imaging was realized by simultaneously acquiring a TRUS scan and a CT scan. The TRUS transducer was inserted while the patient was on the CT couch and the CT scan was made directly after the TRUS scan, with the probe still in situ. With the TRUS transducer being visible on both TRUS and CT images, the geometrical relationship between both image sets could be defined by registration on the transducer. Having proven the applicability of simultaneous imaging, the accuracy of this registration method was investigated by additional registration on visible seeds, after preregistration on the transducer. In 4 out of 23 investigated cases an automatic grey value registration on seeds failed for each of the investigated cost functions, and in 2 cases for both cost functions, due to poor visibility of the seeds on the TRUS scan. The average deviations of the seed registration with respect to the transducer registration were negligible. However, in a few individual cases the deviations were significant and probably due to movement of the patient between TRUS and CT scan. In case of a registration on the transducer it is important to avoid patient movement in-between the TRUS and CT scan and to keep the time in-between the scans as short as possible. It can be concluded that fusion of a CT scan and a simultaneously made TRUS scan by means of a three-dimensional (3D) transducer is feasible and accurate when performing a registration on the transducer, if necessary, fine-tuned by a registration on seeds. These fused images are likely to be of great value for post-implant dose distribution evaluations

[en] Purpose: Lower urinary tract symptoms are frequently observed after I-125 seed implantation of the prostate. More knowledge about causes and predictors is necessary to be able to develop less toxic implantation techniques. The aim of this study was to identify implantation related factors that contribute to post-implant urinary morbidity. Materials and methods: Analysed was a group of 72 patients that filled in a symptom score questionnaire before, 3 months and 6 months after implantation as well as a group of 15 patients that suffered from acute urinary retention. Several dose-volume parameters of prostate, urethra and bladder wall were determined based on a post-implant TRUS-CT scan. Results: The dose to a 1 cm³ hotspot in the bladder wall (D1cc-bl) as well as the prostate volume were independently correlated with urinary morbidity symptom scores at 3 months (p = 0.006 and p = 0.005, respectively) and at 6 months (p = 0.001 and p = 0.015, respectively) after implantation. The number of implanted seeds and the D1cc-bl were significant discriminators (p < 0.001 and p = 0.015, respectively) for either mild or severe early urinary morbidity. Conclusion: Bladder hotspot dose appears to be an important dosimetric predictor for urinary morbidity both at 3 months and at 6 months after implantation. Other predictors are prostate volume, or equivalently, the number of implanted seeds

[en] Purpose: After prostate implantation, dose calculation is usually based on a single imaging session, assuming no geometrical changes occur during the months of dose accumulation. In this study, the effect of changes in anatomy and implant geometry on the dose distribution was investigated. Materials and methods: One day, 1 month and 312 months after seed implantation, a combined TRUS-CT scan was made of 13 patients. Based on these scans changes in dose rate distribution were determined in prostate, urethra and bladder and a 'geometry corrected' dose distribution was estimated. Results: When based on the day-1 scan, parameters representing high dose volumes in prostate and urethra were largely underestimated: V150 of the prostate 18+/-10% and V120 of the urethra 47+/-32%. The dose to a 2cm³ hotspot in the bladder wall (D2cc), however, was overestimated by 31+/-35%. Parameters based on scans 1 month post-implant or later were all within +/-5% of geometry corrected values. Conclusion: Values meant to indicate the adequacy of dose coverage of the prostate, V100 and D90, were not influenced by geometrical changes and were independent of the post-implant scan date. Other parameters representing high dose volumes changed strongly within the first month after implantation

[en] Background and purpose: We investigated the application of a differential target- and dose prescription concept for low-dose-rate prostate brachytherapy (LDR-BT), involving a re-distribution of dose according to risk of local failure and treatment-related morbidity. Material and methods: Our study included 15 patients. Multi-parametric MRI was acquired prior to LDR-BT for gross tumor volume (GTV) delineation. Trans-rectal ultrasound (US) images were acquired during LDR-BT for prostate gland- (CTV_P_r_o_s_t_a_t_e) and organs at risk delineation. The GTV contour was transferred to US images after US/MRI registration. An intermediate-risk target volume (CTV_P_r_o_s_t_a_t_e) and a high-risk target volume (CTV_H_R = GTV + 5 mm margin) were defined. Two virtual dose plans were made: Plan_r_i_s_k_-_a_d_a_p_t consisted of a de-escalated dose of minimum 125 Gy to the CTV_P_r_o_s_t_a_t_e and an escalated dose to 145–250 Gy to the CTV_H_R; Plan_r_e_f included the standard clinical dose of minimum 145 Gy to the CTV_P_r_o_s_t_a_t_e. Dose-volume-histogram (DVH) parameters were expressed in equivalent 2 Gy fractionation doses. Results: The median D_9_0_% to the GTV and CTV_H_R significantly increased by 44 Gy and 17 Gy, respectively when comparing Plan_r_i_s_k_-_a_d_a_p_t to Plan_r_e_f. The median D_1_0_% and D_3_0_% to the urethra significantly decreased by 9 Gy and 11 Gy, respectively and for bladder neck by 18 Gy and 15 Gy, respectively. The median rectal D_2_._0 _c_m_"3 had a significant decrease of 4 Gy, while the median rectal D_0_._1 _c_m_"3 showed an increase of 1 Gy. Conclusions: Our risk adaptive target- and dose prescription concept of prescribing a lower dose to the whole gland and an escalated dose to the GTV using LDR-BT seed planning was technically feasible and resulted in a significant dose-reduction to urethra and bladder neck