I-C. Hsu

Evaluation of ultrasound-based prostate localization for image-guided radiotherapy.

To evaluate the use of the ultrasound-based BAT system for daily prostate alignment. Prostate alignments using the BAT system were compared with alignments using radiographic images of implanted radiopaque markers. The latter alignments were used as a reference. The difference between the BAT and marker alignments represents the displacements that would remain if the alignments were done using ultrasonography. The inter-user variability of the contour alignment process was assessed. On the basis of the marker alignments, the initial displacement of the prostate in the AP, superoinferior, and lateral direction was -0.9 +/- 3.9, 0.1 +/- 3.9, and 0.2 +/- 3.4 mm respectively. The directed differences between the BAT and marker alignments in the respective directions were 0.2 +/- 3.7, 2.7 +/- 3.9, and 1.6 +/- 3.1 mm. The occurrence of displacements >/=5 mm was reduced by a factor of two in the AP direction after the BAT system was used. Among eight users, the average range of couch shifts due to contour alignment variability was 7, 7, and 5 mm in the antero-posterior (AP), superoinferior, and lateral direction, respectively. In our study, the BAT alignments were systematically different from the marker alignments in the superoinferior, and lateral directions. The remaining random variability of the prostate position after the ultrasound-based alignment was similar to the initial variability. However, the occurrence of displacements >/=5 mm was reduced in the AP direction. The inter-user variation of the contour alignment process was significant.

Components of a hyperthermia clinic: Recommendations for staffing, equipment, and treatment monitoring

Abstract Like other technically sophisticated medical endeavours, a hyperthermia clinic relies on skilled staffing. Physicians, physicists and technologists perform multiple tasks to ensure properly functioning equipment, appropriate patient selection, and to plan and administer this treatment. This paper reviews the competencies and tasks that are used in a hyperthermia clinic.

Dosimetric equivalence of nonstandard HDR brachytherapy catheter patterns

PURPOSE To determine whether alternative high dose rate prostate brachytherapy catheter patterns can result in similar or improved dose distributions while providing better access and reducing trauma. MATERIALS AND METHODS Standard prostate cancer high dose rate brachytherapy uses a regular grid of parallel needle positions to guide the catheter insertion. This geometry does not easily allow the physician to avoid piercing the critical structures near the penile bulb nor does it provide position flexibility in the case of pubic arch interference. This study used CT datasets with 3 mm slice spacing from ten previously treated patients and digitized new catheters following three hypothetical catheter patterns: conical, bi-conical, and fireworks. The conical patterns were used to accommodate a robotic delivery using a single entry point. The bi-conical and fireworks patterns were specifically designed to avoid the critical structures near the penile bulb. For each catheter distribution, a plan was optimized with the inverse planning algorithm, IPSA, and compared with the plan used for treatment. Irrelevant of catheter geometry, a plan must fulfill the RTOG-0321 dose criteria for target dose coverage (V100(Prostate) > 90%) and organ-at-risk dose sparing (V75(Bladder) < 1 cc, V75(Rectum) < 1 cc, V125(Urethra) << 1cc). RESULTS The three nonstandard catheter patterns used 16 nonparallel, straight divergent catheters, with entry points in the perineum. Thirty plans from ten patients with prostate sizes ranging from 26 to 89 cc were optimized. All nonstandard patterns fulfilled the RTOG criteria when the clinical plan did. In some cases, the dose distribution was improved by better sparing the organs-at-risk. CONCLUSION Alternative catheter patterns can provide the physician with additional ways to treat patients previously considered unsuited for brachytherapy treatment (pubic arch interference) and facilitate robotic guidance of catheter insertion. In addition, alternative catheter patterns may decrease toxicity by avoidance of the critical structures near the penile bulb while still fulfilling the RTOG criteria.

Model-based feasibility assessment and evaluation of prostate hyperthermia with a commercial MR-guided endorectal HIFU ablation array

PURPOSE Feasibility of targeted and volumetric hyperthermia (40-45 °C) delivery to the prostate with a commercial MR-guided endorectal ultrasound phased array system, designed specifically for thermal ablation and approved for ablation trials (ExAblate 2100, Insightec Ltd.), was assessed through computer simulations and tissue-equivalent phantom experiments with the intention of fast clinical translation for targeted hyperthermia in conjunction with radiotherapy and chemotherapy. METHODS The simulations included a 3D finite element method based biothermal model, and acoustic field calculations for the ExAblate ERUS phased array (2.3 MHz, 2.3 × 4.0 cm(2), ∼1000 channels) using the rectangular radiator method. Array beamforming strategies were investigated to deliver protracted, continuous-wave hyperthermia to focal prostate cancer targets identified from representative patient cases. Constraints on power densities, sonication durations and switching speeds imposed by ExAblate hardware and software were incorporated in the models. Preliminary experiments included beamformed sonications in tissue mimicking phantoms under MR temperature monitoring at 3 T (GE Discovery MR750W). RESULTS Acoustic intensities considered during simulation were limited to ensure mild hyperthermia (Tmax < 45 °C) and fail-safe operation of the ExAblate array (spatial and time averaged acoustic intensity ISATA < 3.4 W/cm(2)). Tissue volumes with therapeutic temperature levels (T > 41 °C) were estimated. Numerical simulations indicated that T > 41 °C was calculated in 13-23 cm(3) volumes for sonications with planar or diverging beam patterns at 0.9-1.2 W/cm(2), in 4.5-5.8 cm(3) volumes for simultaneous multipoint focus beam patterns at ∼0.7 W/cm(2), and in ∼6.0 cm(3) for curvilinear (cylindrical) beam patterns at 0.75 W/cm(2). Focused heating patterns may be practical for treating focal disease in a single posterior quadrant of the prostate and diffused heating patterns may be useful for heating quadrants, hemigland volumes or even bilateral targets. Treatable volumes may be limited by pubic bone heating. Therapeutic temperatures were estimated for a range of physiological parameters, sonication duty cycles and rectal cooling. Hyperthermia specific phasing patterns were implemented on the ExAblate prostate array and continuous-wave sonications (∼0.88 W/cm(2), 15 min) were performed in tissue-mimicking material with real-time MR-based temperature imaging (PRFS imaging at 3.0 T). Shapes of heating patterns observed during experiments were consistent with simulations. CONCLUSIONS The ExAblate 2100, designed specifically for thermal ablation, can be controlled for delivering continuous hyperthermia in prostate while working within operational constraints.

Targeted hyperthermia in prostate with an MR-guided endorectal ultrasound phased array: patient specific modeling and preliminary experiments

Feasibility of hyperthermia delivery to the prostate with a commercially available MR-guided endorectal ultrasound (ERUS) phased array ablation system (ExAblate 2100, Insightec, LTD) was assessed through computer simulations and ex vivo experiments. The simulations included a 3D FEM-based biothermal model, and acoustic field calculations for the ExAblate phased array (2.3 MHz, 2.3x4.0 cm2) using the rectangular radiator method. Array beamforming strategies were investigated to deliver 30-min hyperthermia (<41 °C) to focal regions of prostate cancer, identified from MR images in representative patient cases. Constraints on power densities, sonication durations and switching speeds imposed by ExAblate hardware and software were incorporated in the models. T<41 °C was calculated in 14-19 cm3 for sonications with planar or diverging beam patterns at 0.9-1.2 W/cm2, and in 3-10 cm3 for curvilinear (cylindrical) or multifocus beam patterns at 1.5-3.3 W/cm2, potentially useful for treating focal disease in a single posterior quadrant. Preliminary experiments included beamformed sonications in tissue mimicking phantom material under MRI-based temperature monitoring at 3T (GRE TE=7.0 ms, TR=15 ms, BW=10.5 kHz, FOV=15 cm, matrix 128x128, FA=40°). MR-temperature rises of 2-6 °C were induced in a phantom with the ExAblate array, consistent with calculated values and lower power settings (~0.86 W/cm2, 3 min.). Conformable hyperthermia may be delivered by tailoring power deposition along the array length and angular expanse. MRgERUS HIFU systems can be controlled for continuous hyperthermia in prostate to augment radiotherapy and drug delivery. [FUS Foundation, NIH R01 122276, 111981].

TU‐D‐BRB‐03: Enforcing Maximum Dwell Times in High Dose Rate Brachytherapy Highlights the Tradeoff between Small Dwell Time Gradients and Dose Coverage

Purpose Applicator‐based dose optimization (forward planning) emphasizes dwell time homogeneity to avoid regions of exceptionally high dose concentration. Anatomy‐based dose optimization (inverse planning) relaxes this constraint in order to obtain a dose distribution that is more conformal the anatomy. We developed a version of inverse planning constrained by maximum dwell times. The relationship between dwell time homogeneity and the dose delivered to cancerous and healthy organs is examined. Methods and Materials A software method was implemented within the IPSA inverse planning algorithm to allow for (1) imposition of a user‐defined global maximum dwell time at any given dwell position and (2) homogeneity of dwell times within any given catheter. In‐silico studies were performed on six previously‐treated‐patient image datasets using the contours and catheter digitization of the clinically‐used plan. Two each of tandem‐and‐ring gynecological, tandem‐and‐ovoid gynecological, and prostate were examined. The plan used for treatment served as the control and had no restriction on dwell times. New plans were optimized by imposing either a global (Study 1) or catheter‐specific maximum dwell times (Study 2) for different time limits. Dosimetric indices were compared with the control plans. Results Clinically acceptable plans were achievable with modest restriction of the dwell times, but severe restriction lead to unacceptable plans. Across all studies, restriction of dwell times correlated with an increase of the final objective function value. A higher objective function means less adherence of the optimization to the dosimetric requirements imposed on the optimization by the user. Conclusion Imposing restrictions on a global maximum dwell and intra‐catheter dwell time homogeneity necessarily results in a worsening of the dosimetric optimization objective function. However, less severe restrictions can allow for some tailoring of the dose distribution to avoid intra‐catheter dwell time heterogeneity with only a small decrease in target coverage. Research sponsored in part by Nucletron.

TU-C-141-07: Limitations of Summing Dose Across Fractions: A Simple Test to Identify Failure

Purpose: With the increased availability of clinical deformable image registration tools, summing dose distributions calculated over different CT volumes is a tempting option. However, visual inspections of image registrations are insufficient to assess accuracy, and no patient‐specific QA methods exist to‐date. If the plan quality of the resulting summed dose is poor, it is difficult to discern whether the degradation results from organ motion or registration inaccuracies. We present a straightforward patient‐specific test that identifies invalid dose registrations. Methods: We applied the proposed test to assess dose registration accuracy for an illustrative 2‐fraction HDR brachytherapy prostate case. One plan per fraction was optimized using a unique CT acquired and contoured for each fraction and MIMVista was used to deformably register the clinical dose and contours. The resulting deformed and target CTs were compared. DVHs for the original dose distributions were calculated on their corresponding contours and for the deformed dose using both the target and warped contours. The four resulting DVH sets were compared and analyzed. Results: Though the deformed and target CTs match closely, V100 for the deformed dose plan is 42.0% as compared to 88.6% and 89.6% for the first and second day plans. Computing the deformed dose prostate DVH on the warped contours instead produces a target DVH with V100 of 82.6%, which differs greatly from the deformed V100 calculated on the target contours. Conclusion: When assessing a deformed dose, it is impossible to determine a priori whether degradation of the plan DVHs has resulted from organ motion or registration inaccuracies. If contoured and registered properly, the DVHs of the deformed dose should be similar regardless of whether they are computed over the deformed or target CT contours. Large differences between the two are a strong indication that the warped dose is unusable.

SU‐D‐108‐04: An Expeditious Method of Constructing Low‐Cost, Durable, CT‐and US‐Compatible Gynecological Training Phantoms for Instruction of Medical Residents in High Dose Rate Brachytherapy Clinical Practice

PURPOSE During gynecological (GYN) brachytherapy (BT), suturing the cervix and inserting the applicator into the uterine canal are highly skill dependent tasks. Medical residents often have to practice these techniques in the operating room; this can be sub-optimal for the patient. We present a fast and low-cost method of building realistic, gelatin based GYN phantoms which can be used to train physicians new to GYN BT.METHODSThe phantoms include a rectal cavity large enough to accommodate a standard transrectal US probe, a vaginal cavity, a uterus, a cervix, and a cervical canal, all embedded in a gelatin matrix. The uterus and cervix were coated with latex to create a suturing surface and to provide US contrast. The inner gelatine surface of the rectal cavity was hardened to make it resilient to probe insertions. The gelatine uterus was softened to give it more anatomically correct tactile properties. Construction times and materials costs were recorded for each phantom. RESULTS Active participation build time is 8 hours; however, due to the time necessary for curing between construction steps, the total build time is approximately 3 days. Re-usable parts allows for a reduction in time for subsequent phantoms (10 hours total, with an active participation time of 4 hours). Anatomical structures were easily distinguishable in CT and US. The cervix was sutured by a practicing brachytherapist who judged it as effective and realistic. CONCLUSION A method and recipe for quickly constructing low cost, durable US-and CT-compatible GYN BT practice phantoms was developed. These phantoms will be integrated into the training of our medical residents during their GYN brachytherapy rotation.

Model-based feasibility assessment and evaluation of prostate hyperthermia with a commercial MRI-guided endorectal high intensity focused ultrasound ablation array

Numerical simulations were conducted to devise methods for targeted and protracted hyperthermia (40–46 °C, 30–60 min) to the prostate with a commercial MR-guided endorectal ultrasound phased array (2.3 MHz, ExAblate, InSightec). The intention is to fast-track clinical implementation of this FDA approved ablation system for delivering targeted hyperthermia in conjunction with radiation or chemotherapy. Conformable hyperthermia to focal tumors in posterior and hemi-gland prostate was simulated through 3D patient-specific biothermal models and beamformed acoustic patterns that incorporated the specific constraints imposed on the ExAblate array: irregular element spacing, switching speeds, operating power and short pulse duration. Simulations indicated that diverging and iso-phase sonications could treat (T > 41 °C, max < 46 °C) 13–23 cm3 with ~1.1 W/cm2, multi-focused patterns could treat 4.0 cm3 with 3.4 W/cm2, and curvilinear patterns could treat 6.5 cm3 with 0.8 W/cm2 while avoiding rectum, urethra, pubic...

SU‐GG‐T‐63: Determination of Dose Objective Parameters and Dose Evaluation during Inverse Planned HDR Brachytherapy Based on a Global DVH‐Based Statistical Comparison

Purpose: Since brachytherapydose distribution contains steep dose gradients, plan evaluation over the whole DVH (rather than a few dose points) may be beneficial. We developed a rapid dose evaluation procedure to determine the optimization parameters, facilitate the planning process, and complete the dose distribution evaluation simultaneously for all regions of interest. Materials and Methods: After each dose calculation for a prostate implant, the DVH are automatically compared to the clinically validated reference data for prostate, urethra, rectum, bladder and bulb. The reference data was generated for each DVH from 50 HDR patients planned with IPSA, separated by volumes into three groups. Upper and lower 95% and 99% confidence intervals and the mean were computed for each structure from 10% to 200% of the prescribed dose. The data were entered into the OncentraBrachy planning system. Results: The observation of the color‐coded deviations provide an immediate visual cue indicating which organ(s) and what DVH dose intervals are above or below the preset reference range and require attention, and therefore the optimization parameters to be adjusted, such as the minimum or maximum dose objective values. The process is completed when all dose intervals for all structures are within the normal range in relation to the reference data. This approach is now clinically implemented. Conclusion: The similarity of the DVH, over a wide range of prostate volumes and shapes, illustrates the consistency of the dose distributions when IPSA is used. A global DVH‐based statistical procedure has been developed to quickly determine whether, and for which structure, a dose modification is needed during the dose optimization planning process. In addition to rapidly identifying which optimization parameters need adjustment, the comparison with clinically validated reference values provides an automatic, immediate, and global quantitative quality assurance. Research sponsored by Nucletron and DOD‐PCRP‐W81XWH‐04‐1‐0262 research contracts.