Marcel J. Steggerda

Inter-observer variation in delineation of bladder and rectum contours for brachytherapy of cervical cancer

BACKGROUND AND PURPOSE In 3D treatment planning of low dose rate brachytherapy of cervical carcinoma the dose in bladder and rectum can be estimated from dose-volume histograms (DVHs). In this study, the influence of inter-observer variation in delineation of bladder and rectum on DVHs and dose at specific bladder and rectum points was investigated. MATERIALS AND METHODS Three observers delineated bladder and rectum on axial CT images of ten patients. The highest minimum dose in bladder and rectum was determined for, respectively, 2 cm(3) (D(2)) and 5 cm(3) (D(5)), as well as the dose at specific points placed on the bladder and rectum wall. RESULTS The inter-observer variation in D(2) was 10% (1 average relative SD) in bladder and 11% (1 SD) in rectum. In D(5) the variation was 8% (1 SD) in bladder and 11% in rectum. The variation in the bladder point was 13% (1 SD) and in the rectum point 11% (1 SD). Differences in delineation among the observers were caused by unclear organ boundaries on the CT images. CONCLUSIONS Taking the inter-observer variation caused by delineation differences into account, dose in bladder and rectum can be determined within an accuracy of about 10% (1 SD).

Comparison of dose-volume histograms and dose-wall histograms of the rectum of patients treated with intracavitary brachytherapy.

The correlation between dose values from dose-volume histograms (DVHs) and dose values from dose-wall histograms (DWHs) of the rectum tissue of patient with uterine cervix cancer was determined. The minimum dose in 2 cm3 in the high-dose region of the DVH is a good estimate of the dose in the rectum wall.

An analysis of the effect of ovoid shields in a selectron-LDR cervical applicator on dose distributions in rectum and bladder☆

PURPOSE A disadvantage of ovoid shields in a Fletcher-type applicator is that these shields cause artifacts on postimplant 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.

The applicability of simultaneous TRUS-CT imaging for the evaluation of prostate seed implants.

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.

A method to improve the dose distribution of interstitial breast implants using geometrically optimized stepping source techniques and dose normalization.

BACKGROUND AND PURPOSE The standard linear source breast implant of our institution was compared with alternative linear source implant geometries and a stepping source implant, to evaluate the possibility of minimizing the treated volume. Normalization to a higher isodose than the conventional 85% of the mean central dose (MCD) was investigated for the stepping source implant to reduce the thickness of the treated volume and to increase dose uniformity. The purpose of this study was to develop an implant geometry yielding a high conformity and a more uniform dose distribution over the target volume. MATERIALS AND METHODS The dose distributions of four implant geometries were compared for a planning target volume (PTV) of 48 cm(3). Implants #1 (standard) and #2 had linear sources arranged in a triangular pattern of equal lengths and lengths adapted to the shape of the PTV. Implants #3 and #4 were squared pattern arranged implants with linear sources and a stepping source with geometric optimized dwell times. The active lengths were adapted to the shape of the PTV. Using implant #4 for PTVs of different volumes, the reference dose (RD) was normalized to 85 and 91% of the MCD. RESULTS Comparing implants #2, #3, and #4 with #1, the treated volume (V(100)) encompassed by the reference isodose was reduced by 22, 35, and 37%, respectively. The volumes receiving a dose of at least 125% (V(125)) of the reference dose was reduced by 16, 30, and 30%, respectively. The conformation number increased being 0.30, 0.39, 0.47, and 0.48 for implants #1, #2, #3, and #4, respectively. The average reduction of V(125) when the dose was normalized to 91% compared with 85% of the MCD was 18%. CONCLUSIONS A conformal treatment to a PTV could be best achieved with a geometrically optimized stepping source plan with needles arranged in a squared pattern. Reduction of high dose volumes within the implant was obtained by normalizing the RD to 91% instead of 85% of the MCD.

Establishing implantation uncertainties for focal brachytherapy with I-125 seeds for the treatment of localized prostate cancer.

Abstract Background. The efficacy of focal continuous low dose-rate brachytherapy (CLDR-BT) for prostate cancer requires that appropriate margins are applied to ensure robust target coverage. In this study we propose a method to establish such margins by emulating a focal treatment in patients treated with CLDR-BT to the entire gland. Material and methods. In 15 patients with localized prostate cancer, prostate volumes and dominant intra-prostatic lesions were delineated on pre-treatment magnetic resonance imaging (MRI). Delineations and MRI were registered to trans-rectal ultrasound images in the operating theater. The patients received CLDR-BT treatment to the total prostate volume. The implantation consisted of two parts: an experimental focal plan covering the dominant intra-prostatic lesion (F-GTV), followed by a plan containing additional seeds to achieve entire prostate coverage. Isodose surfaces were reconstructed using follow-up computed tomography (CT). The focal dose was emulated by reconstructing seeds from the focal plan only. The distance to agreement between planned and delivered isodose surfaces and F-GTV coverage was determined to calculate the margin required for robust treatment. Results. If patients had been treated only focally, the target volume would have been reduced from an average of 40.9 cm3 for the entire prostate to 5.8 cm3 for the focal plan. The D90 for the F-GTV in the focal plan was 195 ± 60 Gy, the V100 was 94% [range 71–100%]. The maximum distance (cd95) between the planned and delivered isodose contours was 0.48 cm. Conclusions. This study provides an estimate of 0.5 cm for the margin required for robust coverage of a focal target volume prior to actually implementing a focal treatment protocol.

Re-distribution of brachytherapy dose using a differential dose prescription adapted to risk of local failure in low-risk prostate cancer patients

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(Prostate)) and organs at risk delineation. The GTV contour was transferred to US images after US/MRI registration. An intermediate-risk target volume (CTV(Prostate)) and a high-risk target volume (CTV(HR)=GTV+5 mm margin) were defined. Two virtual dose plans were made: Plan(risk-adapt) consisted of a de-escalated dose of minimum 125 Gy to the CTV(Prostate) and an escalated dose to 145-250 Gy to the CTV(HR); Plan(ref) included the standard clinical dose of minimum 145 Gy to the CTV(Prostate). Dose-volume-histogram (DVH) parameters were expressed in equivalent 2 Gy fractionation doses. RESULTS The median D(90%) to the GTV and CTV(HR) significantly increased by 44 Gy and 17 Gy, respectively when comparing Plan(risk-adapt) to Plan(ref). The median D(10%) and D(30%) 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(2.0cm(3)) had a significant decrease of 4 Gy, while the median rectal D(0.1cm(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.

Minimizing the number of implantation needles for prostate 125I brachytherapy: An investigation of possibilities and implications

PURPOSE Reduction of the number of implantation needles for prostate brachytherapy will shorten the duration of implantation procedures and possibly reduce trauma-related morbidity. The purpose of this study was to investigate possibilities for the minimization of the number of needles and to investigate the consequences for the dose distribution. METHODS AND MATERIALS A planning study for six different prostate volumes was performed. The number of needles was minimized by changing fixed 1cm interseed spacing to free interseed spacing within the needles and by increasing the seed activity. Dose-volume parameters of prostate and organs at risk (OAR) bladder, rectum, and urethra were determined. For plans with different needle and seed configurations, the sensitivity for random seed placement inaccuracies was tested. Dose distributions of realized implants based on fixed (n=21) and free interseed spacing (n=21) were compared. RESULTS The average number of needles (±1 standard deviation) could be reduced from 18.8±3.6 to 12.7±2.9 (-33%) when changing from fixed interseed spacing to free interseed spacing and subsequently to 7.3±1.0 (-42%) by increasing the seed strength from 0.57U to 1.14U. These needle reductions resulted in increased dose inhomogeneity within the prostate and increased sensitivity of dose-volume parameters of the OAR for random geometrical inaccuracies. Introduction of free interseed spacing in our clinic resulted in very satisfactory dose coverage of the prostate (D(90)=172±17Gy), while the average number of needles was reduced by 30%. CONCLUSIONS Substantial reduction of the number of implantation needles is possible without compromising adequate dose coverage of the prostate. However, the chance of an unpredicted high dose to the OAR increases as fewer needles are used.

Replacement corrections of a Farmer-type ionization chamber for the calibration of Cs-137 and Ir-192 sources in a solid phantom

Calibrating Cs-137 and Ir-192 brachytherapy sources in a solid phantom has the advantage over calibration in air that the positioning of the sources is very accurate and straightforward. In order to determine the air kerma rate at the point of measurement it is, however, necessary to take the replacement of the phantom material by the ionization chamber into account. The replacement correction factor pr of a Farmer-type ionization chamber has been determined for a few types of 137Cs and 192Ir sources at a source to chamber distance of 5 cm. For spherical 137Cs sources the replacement correction was determined by means of measurements with chambers with decreasing diameter and length. Additional measurements were performed for some other source configurations in order to determine pr for 137Cs micro-seed trains. For an 192Ir-HDR source pr was determined for a source chamber configuration equal to that for spherical 137Cs sources by comparing measurements in-phantom with measurements free in-air. Finally, measurements were performed with source configurations that yielded pr values for 192Ir seed trains and 192Ir wires. The resulting replacement correction factors are all within +/- 2% of unity. It can be concluded that, although the dose nonuniformity over the chamber volume caused by each source is rather substantial, the replacement correction that has to be applied is rather small.

OC-0157: Prostate fiducial markers detection with the use of multiparametric-MRI

Purpose or Objective: Introducing an MRI-only workflow into the radiotherapy clinic, requires that MR-images can be used both for treatment planning calculations and for patient positioning. The two-fold aim of this study was to evaluate the use of MR-images with respect to 1) the accuracy of treatment planning dose calculations, and 2) the reliability of fiducial marker identification for patient positioning.