Francis van Gils

Critical assessment of intramodality 3D ultrasound imaging for prostate IGRT compared to fiducial markers

PURPOSE A quantitative 3D intramodality ultrasound (US) imaging system was verified for daily in-room prostate localization, and compared to prostate localization based on implanted fiducial markers (FMs). METHODS Thirteen prostate patients underwent multiple US scans during treatment. A total of 376 US-scans and 817 matches were used to determine the intra- and interoperator variability. Additionally, eight other patients underwent daily prostate localization using both US and electronic portal imaging (EPI) with FMs resulting in 244 combined US-EPI scans. Scanning was performed with minimal probe pressure and a correction for the speed of sound aberration was performed. Uncertainties of both US and FM methods were assessed. User variability of the US method was assessed. RESULTS The overall US user variability is 2.6 mm. The mean differences between US and FM are: 2.5 ± 4.0 mm (LR), 0.6 ± 4.9 mm (SI), and -2.3 ± 3.6 mm (AP). The intramodality character of this US system mitigates potential errors due to transducer pressure and speed of sound aberrations. CONCLUSIONS The overall accuracy of US (3.0 mm) is comparable to our FM workflow (2.2 mm). Since neither US nor FM can be considered a gold standard no conclusions can be drawn on the superiority of either method. Because US imaging captures the prostate itself instead of surrogates no invasive procedure is required. It requires more effort to standardize US imaging than FM detection. Since US imaging does not involve a radiation burden, US prostate imaging offers an alternative for FM EPI positioning.

IN VIVO DOSIMETRY USING A LINEAR MOSFET-ARRAY DOSIMETER TO DETERMINE THE URETHRA DOSE IN 125I PERMANENT PROSTATE IMPLANTS

PURPOSE In vivo dosimetry during brachytherapy of the prostate with (125)I seeds is challenging because of the high dose gradients and low photon energies involved. We present the results of a study using metal-oxide-semiconductor field-effect transistor (MOSFET) dosimeters to evaluate the dose in the urethra after a permanent prostate implantation procedure. METHODS AND MATERIALS Phantom measurements were made to validate the measurement technique, determine the measurement accuracy, and define action levels for clinical measurements. Patient measurements were performed with a MOSFET array in the urinary catheter immediately after the implantation procedure. A CT scan was performed, and dose values, calculated by the treatment planning system, were compared to in vivo dose values measured with MOSFET dosimeters. RESULTS Corrections for temperature dependence of the MOSFET array response and photon attenuation in the catheter on the in vivo dose values are necessary. The overall uncertainty in the measurement procedure, determined in a simulation experiment, is 8.0% (1 SD). In vivo dose values were obtained for 17 patients. In the high-dose region (> 100 Gy), calculated and measured dose values agreed within 1.7% +/- 10.7% (1 SD). In the low-dose region outside the prostate (< 100 Gy), larger deviations occurred. CONCLUSIONS MOSFET detectors are suitable for in vivo dosimetry during (125)I brachytherapy of prostate cancer. An action level of +/- 16% (2 SD) for detection of errors in the implantation procedure is achievable after validation of the detector system and measurement conditions.

In Vivo Dosimetry With a Linear MOSFET Array to Evaluate the Urethra Dose During Permanent Implant Brachytherapy Using Iodine-125

PURPOSE To develop a technique to monitor the dose rate in the urethra during permanent implant brachytherapy using a linear MOSFET array, with sufficient accuracy and without significantly extending the implantation time. METHODS AND MATERIALS Phantom measurements were performed to determine the optimal conditions for clinical measurements. In vivo measurements were performed in 5 patients during the (125)I brachytherapy implant procedure. To evaluate if the urethra dose obtained in the operating room with the ultrasound transducer in the rectum and the patient in treatment position is a reference for the total accumulated dose; additional measurements were performed after the implantation procedure, in the recovery room. RESULTS In vivo measurements during and after the implantation procedure agree very well, illustrating that the ultrasound transducer in the rectum and patient positioning do not influence the measured dose in the urethra. In vivo dose values obtained during the implantation are therefore representative for the total accumulated dose in the urethra. In 5 patients, the dose rates during and after the implantation were below the maximum dose rate of the urethra, using the planned seed distribution. CONCLUSION In vivo dosimetry during the implantation, using a MOSFET array, is a feasible technique to evaluate the dose in the urethra during the implantation of (125)I seeds for prostate brachytherapy.

Psychometric properties of the Dutch version of the Mental Adjustment to Cancer scale in Dutch cancer patients

Objectives: The measurement of adjustment to cancer is relevant for research purposes and daily practice. In this study, the psychometric properties of the original five subscales and the two recently proposed summary scales of the Mental Adjustment to Cancer (MAC) scale were examined in Dutch cancer patients.

Influence of trace elements in human tissue in low-energy photon brachytherapy dosimetry

The aim of this paper is to determine the dosimetric impact of trace elements in human tissues for low-energy photon sources used in brachytherapy. Monte Carlo dose calculations were used to investigate the dosimetric effect of trace elements present in normal or cancerous human tissues. The effect of individual traces (atomic number Z = 11-30) was studied in soft tissue irradiated by low-energy brachytherapy sources. Three other tissue types (prostate, adipose and mammary gland) were also simulated with varying trace concentrations to quantify the contribution of each trace to the dose distribution. The dose differences between cancerous and healthy prostate tissues were calculated in single- and multi-source geometries. The presence of traces in a tissue produces a difference in the dose distribution that is dependent on Z and the concentration of the trace. Low-Z traces (Na) have a negligible effect (<0.3%) in all tissues, while higher Z (K) had a larger effect (>3%). There is a potentially significant difference in the dose distribution between cancerous and healthy prostate tissues (4%) and even larger if compared to the trace-free composition (15%) in both single- and multi-sourced geometries. Trace elements have a non-negligible (up to 8% in prostate D(90)) effect on the dose in tissues irradiated with low-energy photon sources. This study underlines the need for further investigation into accurate determination of the trace composition of tissues associated with low-energy brachytherapy. Alternatively, trace elements could be incorporated as a source of uncertainty in dose calculations.

Dose reduction in LDR brachytherapy by implanted prostate gold fiducial markers.

PURPOSE The dosimetric impact of gold fiducial markers (FM) implanted prior to external beam radiotherapy of prostate cancer on low dose rate (LDR) brachytherapy seed implants performed in the context of combined therapy was investigated. METHODS A virtual water phantom was designed containing a single FM. Single and multi source scenarios were investigated by performing Monte Carlo dose calculations, along with the influence of varying orientation and distance of the FM with respect to the sources. Three prostate cancer patients treated with LDR brachytherapy for a recurrence following external beam radiotherapy with implanted FM were studied as surrogate cases to combined therapy. FM and brachytherapy seeds were identified on post implant CT scans and Monte Carlo dose calculations were performed with and without FM. The dosimetric impact of the FM was evaluated by quantifying the amplitude of dose shadows and the volume of cold spots. D(90) was reported based on the post implant CT prostate contour. RESULTS Large shadows are observed in the single source-FM scenarios. As expected from geometric considerations, the shadows are dependent on source-FM distance and orientation. Large dose reductions are observed at the distal side of FM, while at the proximal side a dose enhancement is observed. In multisource scenarios, the importance of shadows appears mitigated, although FM at the periphery of the seed distribution caused underdosage (<prescription dose). In clinical cases, the FM reduced the dose to some voxels by up to 50% and generated shadows with extents of the order of 4 mm. Within the prostate contour, cold spots (<95% prescription dose) of the order of 20 mm(3) were observed. D(90) proved insensitive to the presence of FM for the cases selected. CONCLUSIONS There is a major local impact of FM present in LDR brachytherapy seed implant dose distributions. Therefore, reduced tumor control could be expected from FM implanted in tumors, although our results are too limited to draw conclusions regarding clinical significance.

Why determine only the total prostate-specific antigen, if the free-to-total ratio contains the information?

Abstract Background Total prostate-specific antigen (tPSA) is the best available test for the detection of prostate cancer but it lacks specificity. The free-to-total ratio (F/T ratio) is used to increase specificity in the range of tPSA of 4–10 µg/L. Materials and methods Four hundred and seven biopsy results and quantitative tPSA and F/T ratio data were combined. Using the histological determination, normal/hyperplasia versus malignant as a gold standard, receiver operating characteristic (ROC) curves as well as the areas under the curve (AUC) for tPSA and F/T ratio were determined. The differences between the two AUCs were considered for various tPSA ranges and specificities of F/T ratio and tPSA were calculated. Results In the total group, there was a gain of specificity of 11% (from 23% to 34%) when the sensitivity was 92% (using a cut-off >0.28 for the F/T ratio and a cut-off >4 µg/L for tPSA). When considering the group of patients for which the F/T ratio is currently used (4–10 µg/L), the gain of specificity was 27% (from 2% to 29%). This implicates that the number of unnecessary biopsies taken will be reduced by 27%. Moreover, the AUC of the F/T ratio was significantly higher at an even broader range of tPSA, i.e. up to 40 µg/L. Conclusions This study demonstrates that the F/T ratio has better diagnostic performance than tPSA, not only in the grey zone of tPSA, but also outside the grey zone, i.e. up to 40 µg/L.

WE-D-220-04: Operator Uncertainties in a Soft Tissue 3D Ultrasound Image Guided Radiotherapy System

Purpose: Image Guided Radiotherapy(IGRT) is a necessity for accurate radiotherapy.Ultrasound(US)imaging is a frequently used diagnostic technique for qualitative imaging of soft tissues. Recently, a quantitative 3D US system was introduced (Clarity system, Resonant Medical, Canada) which can assess the position of soft tissue in absolute space. Before introducing the device into daily practice, user variability, during both image acquisition and matching procedures must be determined. In this study we determined the inter‐ and intra‐operator variability of 3D US matching in prostate cancer patients. Moreover, we studied the influence of the scan variability on patient setup corrections, and the influence of probe pressure on the prostate position while using a strict bladder filling protocol. Methods: For 12 prostate patients multiple US scans are acquired by one or two operators during treatment. The repeated scans are matched to the reference US‐scan by a single user (variability for scanning). The remaining scans are matched three times by different users, and for each patient one single scan is matched five times by the same user (variability for matching). Results: In all three directions the mean intra‐operator difference ranges from 1.5 to 2.4 mm, with a standard deviation of approximately 1.7 mm. The mean inter‐operator difference is of the same order, 1.7 to 2.3 ± 1.8 mm. The prostate displacement due to the probe pressure varies from patient to patient and is not limited to one direction. Only the superior/inferior displacement seemed significant for high pressure, which was not needed to obtain good image quality with our bladder filling protocol. The total uncertainty is conservatively estimated to be 4 mm. Conclusions: The uncertainty of the 3D USIGRT system is comparable to the uncertainty of the current standard IGRT for prostate: electronic portal imager in combination with fiducial markers.