Etienne Lessard

INVERSE PLANNING FOR INTERSTITIAL GYNECOLOGIC TEMPLATE BRACHYTHERAPY: TRULY ANATOMY-BASED PLANNING

PURPOSE Commercially available optimization schemes generally result in an undesirable dose distribution, because of the particular shapes of tumors extending laterally from the tandem. Dose distribution is therefore manually obtained by adjusting relative dwell time values until an acceptable solution is found. The objective of this work is to present the clinical application of an inverse planning dose optimization tool for the automatic determination of source dwell time values in the treatment of interstitial gynecologic templates. METHODS AND MATERIALS In cases where the tumor extends beyond the range of the tandem-ovoid applicator, catheters as well as the tandem are inserted into the paravaginal and parametrial region in an attempt to cover the tumor volume. CT scans of these patients are then used for CT-based dose planning. Dose distribution is obtained manually by varying the relative dwell times until adequate dose coverage is achieved. This manual planning is performed by an experienced physician. In parallel, our in-house inverse planning based on simulated annealing is used to automatically determine which of all possible dwell positions will become active and to calculate the dwell time values needed to fulfill dose constraints applied to the tumor volume and to each organ at risk. To compare the results of these planning methods, dose-volume histograms and isodose distributions were generated for the target and each organ at risk. RESULTS This procedure has been applied for the dose planning of 12 consecutive interstitial gynecologic templates cases. For all cases, once the anatomy was contoured, the routine of inverse planning based on simulated annealing found the solution to the dose constraints within 1 min of CPU time. In comparison, manual planning took more than 45 min. The inverse planning-generated plans showed improved protection to organs at risk for the same coverage compared to manual planning. CONCLUSION This inverse planning tool reduced the planning time significantly and produced improved plans with reduced dose to the organs at risk. Furthermore, the inverse planning approach improves the physicians control over treatment. The focus becomes the physicians prescription to the target and his or her compromise due to dose to normal structures.

Early clinical experience with anatomy-based inverse planning dose optimization for high-dose-rate boost of the prostate

PURPOSE To present an exhaustive dosimetric comparison between three geometric optimization methods and our inverse-planning simulated annealing (IPSA) algorithm, with two different prescriptions for high-dose-rate (HDR) boost of the prostate. The objective of this analysis was to quantify the dosimetric advantages of the IPSA algorithm compared with more standard geometric optimizations. METHODS AND MATERIALS Between September 1999 and June 2001, 34 patients were treated to a dose of 40-44 Gy by external pelvic fields, followed by an HDR boost of 18 Gy in 3 fractions. The first 4 patients were treated with HDR using geometric optimization, and anatomy-based inverse-planning dose optimization was used for the remaining 30 patients. We retrospectively used the data from these 30 patients to create HDR dose distributions according to five different dose optimization protocols, including our IPSA algorithm. The various geometric optimization procedures differed in the way the dwell positions were activated and plan normalization was performed. Dose-volume histograms from all these plans were analyzed and multiple implant quality indexes extracted. RESULTS The IPSA algorithm provided better clinical tumor volume prescription dose coverage than did the geometric optimizations. The average prostate volume receiving 100% of the prescribed dose (V100) was 96.3% and 94.5% for IPSA with two different prescriptions compared with 92.1%, 92.6%, and 88.8% for the three geometric optimization schemes. The average urethra V150 value was 0.0% and 0.7% for IPSA with two different prescriptions, and the three geometric optimization protocols generated average values of 22.9%, 33.9%, and 38.8%. The bladder and rectal dose-volume histograms were similar, although the latest version of the IPSA algorithm slightly decreases the dose to these organs at risk because of organ-specific dose constraints included in the objective function. CONCLUSION We found that planning an HDR prostate boost could be performed in a fast, secure, and effective manner with the IPSA algorithm. We demonstrated that our inverse-planning algorithm produces superior HDR plans than more conventional geometric optimizations for adenocarcinoma of the prostate. The organs at risk protection included in the objective function is a major feature of the algorithm and should allow us to escalate the HDR dose to the prostate without increasing undesirable side effects.

Optimization of HDR brachytherapy dose distributions using linear programming with penalty costs

Prostate cancer is increasingly treated with high-dose-rate (HDR) brachytherapy, a type of radiotherapy in which a radioactive source is guided through catheters temporarily implanted in the prostate. Clinicians must set dwell times for the source inside the catheters so the resulting dose distribution minimizes deviation from dose prescriptions that conform to patient-specific anatomy. The primary contribution of this paper is to take the well-established dwell times optimization problem defined by Inverse Planning by Simulated Annealing (IPSA) developed at UCSF and exactly formulate it as a linear programming (LP) problem. Because LP problems can be solved exactly and deterministically, this formulation provides strong performance guarantees: one can rapidly find the dwell times solution that globally minimizes IPSAs objective function for any patient case and clinical criteria parameters. For a sample of 20 prostates with volume ranging from 23to103cc, the new LP method optimized dwell times in less than 15s per case on a standard PC. The dwell times solutions currently being obtained clinically using simulated annealing (SA), a probabilistic method, were quantitatively compared to the mathematically optimal solutions obtained using the LP method. The LP method resulted in significantly improved objective function values compared to SA (P=1.54×10-7), but none of the dosimetric indices indicated a statistically significant difference (P<0.01). The results indicate that solutions generated by the current version of IPSA are clinically equivalent to the mathematically optimal solutions.

Class solution for inversely planned permanent prostate implants to mimic an experienced dosimetrist

The purpose of this paper is to present a method for the selection of inverse planning parameters and to establish a set of inverse planning parameters (class solution) for the inverse planning included in a commercial permanent prostate implant treatment planning system. The manual planning of more than 750 patients since 1996 led to the establishment of general treatment planning rules. A class solution is tuned to fulfill the treatment planning rules and generate equivalent implants. For ten patients, the inverse planning is compared with manual planning performed by our experienced physicist. The prostate volumes ranged from 17 to 51cc and are implanted with low activity I-125 seeds. Dosimetric indices are calculated for comparison. The inverse planning needed about 15s for each optimization (400000 iterations on a 2.5GHz PC). In comparison, the physicist needed about 20min to perform each manual plan. A class solution is found that consistently produces dosimetric indices equivalent or better than the manual planning. Moreover, even with strict seed placement rules, the inverse planning can produce adequate prostate dose coverage and organ at risk protection. The inverse planning avoids implant with seeds outside of the prostate and too close to the urethra. It also avoids needles with only one seed and needles with three consecutive seeds. This reduces the risk of complication due to seed misplacement and edema. The inverse planning also uses a smaller number of needles, reducing the cause of trauma. The quality of the treatment plans is independent of the gland size and shape. A class solution is established that consistently and rapidly produces equivalent dosimetric indices as manual planning while respecting severe seed placement rules. The class solution can be used as a starting point for every patient, dramatically reducing the time needed to plan individual patient treatments. The class solution works with inverse preplanning, intraoperative inverse preplanning, and intraoperative real-time planning. This technology is not intended to replace the physicist but to accelerate the planning process, making intraoperative treatment planning more effective.

Inverse planning simulated annealing for magnetic resonance imaging-based intracavitary high-dose-rate brachytherapy for cervical cancer

PURPOSE To develop a technique using exclusively magnetic resonance imaging (MRI) to perform dwell position identification, targets and organs at risk delineation, and to apply inverse planning dose optimization to high-dose-rate brachytherapy for cervical cancer. METHODS AND MATERIALS We included 15 consecutive women treated with high-dose-rate (HDR) brachytherapy for cervical cancer. All patients underwent MRI after placement of tandem and ring applicator containing a gadodiamide-filled dummy marker. This technique allowed direct visualization of the source pathway and precise definition of the intra-applicator source positions. For each patient, we delineated gross target volume (GTV), high-risk clinical target volume (HR-CTV), and organs at risk on MRI, according to the European Gynecological GEC-ESTRO Working Group definitions. We performed inverse planning simulated annealing (IPSA) and analyzed the dose-volume histograms with the following endpoints: D(90), D(100), and V(100) for GTV and HR-CTV; D0.1 cc, D1 cc, D2 cc for bladder, rectum, and bowel; and dose at Point A. RESULTS The intra-applicator source pathway was easily visualized on MRI using the gadodiamide-filled marker. IPSA provided excellent target coverage. The mean D(90) and V(100) for HR-CTV were 103+/-5% and 92+/-3%, respectively. IPSA provided excellent bladder sparing. D1 cc and D2 cc of bladder were 73+/-10% and 67+/-10%, respectively. CONCLUSIONS We developed a novel technique that allows direct visualization of the intra-applicator source pathway on MRI. Using this technique, we successfully performed inverse planning directly from MRI.

Investigating the clinical advantages of a robotic linac equipped with a multileaf collimator in the treatment of brain and prostate cancer patients

The purpose of this study was to evaluate the performance of a commercially available CyberKnife system with a multileaf collimator (CK‐MLC) for stereotactic body radiotherapy (SBRT) and standard fractionated intensity‐modulated radiotherapy (IMRT) applications. Ten prostate and ten intracranial cases were planned for the CK‐MLC. Half of these cases were compared with clinically approved SBRT plans generated for the CyberKnife with circular collimators, and the other half were compared with clinically approved standard fractionated IMRT plans generated for conventional linacs. The plans were compared on target coverage, conformity, homogeneity, dose to organs at risk (OAR), low dose to the surrounding tissue, total monitor units (MU), and treatment time. CK‐MLC plans generated for the SBRT cases achieved more homogeneous dose to the target than the CK plans with the circular collimators, for equivalent coverage, conformity, and dose to OARs. Total monitor units were reduced by 40% to 70% and treatment time was reduced by half. The CK‐MLC plans generated for the standard fractionated cases achieved prescription isodose lines between 86% and 93%, which was 2%–3% below the plans generated for conventional linacs. Compared to standard IMRT plans, the total MU were up to three times greater for the prostate (whole pelvis) plans and up to 1.4 times greater for the intracranial plans. Average treatment time was 25 min for the whole pelvis plans and 19 min for the intracranial cases. The CK‐MLC system provides significant improvements in treatment time and target homogeneity compared to the CK system with circular collimators, while maintaining high conformity and dose sparing to critical organs. Standard fractionated plans for large target volumes (>100 cm3) were generated that achieved high prescription isodose levels. The CK‐MLC system provides more efficient SRS and SBRT treatments and, in select clinical cases, might be a potential alternative for standard fractionated treatments. PACS numbers: 87.56.nk, 87.56.bd

Dose uncertainty due to computed tomography (CT) slice thickness in CT‐based high dose rate brachytherapy of the prostate cancer

In computed tomography (CT)-based high dose rate (HDR) brachytherapy, the uncertainty in the localization of the longitudinal catheter-tip positions due to the discrete CT slice thickness, results in a delivered dose uncertainty. Catheter coordinates were extracted from five patients treated for prostate cancer, and three simulation scenarios were followed to mimic the longitudinal imprecision of the catheter tips, hence the dwell positions. All catheters were displaced (1) forward, (2) backward, or (3) randomly distributed within the space defined by one CT slice thickness, for thicknesses ranging from 2 to 5 mm. Average and standard deviation values of the relative dose variations are reported for the various catheter displacement scenarios. Also, the dose points were grouped according to their relative position in the prostate, inner, peripheral and outer area of prostate and base, median and apex zones, in order to estimate the spatial sensitivity of the dose errors. For scenarios (1) and (2), the dose uncertainties due to the finite slice thickness increase linearly with the slice spacing, from 3% to 8% for the slice thickness values ranging from 2 to 5 mm, respectively. The more realistic scenario (3) yields average errors ranging from 0.7% to 1.7%. The apex and the base show larger dose errors and variability of dose errors than the median of the prostate. No statistical difference was observed among different transversal sections of the prostate. A CT slice thickness of 3 mm appears to be a good compromise showing an acceptable average dose uncertainty of 1%, without unduly increasing the number of slices.

Development and clinical introduction of an inverse planning dose optimization by simulated annealing (IPSA) for high dose rate brachytherapy

High dose rate brachytherapy is a promising radiation treatment modality that uses temporarily implanted catheters to deliver the curative dose directly in the tumor. A programmable robotic device (the afterloader) moves a single tiny radioactive source (192Ir) along the catheters using a flexible cable attached to the source. With this flexible system, a wide variety of dose distributions can be generated from a given implant simply by adjusting the length of time (dwell time) that the source dwells at any location within the implanted catheters (dwell position). The challenge is to select the optimal sequence of dwell times related to the unique clinical situation of each patient. This treatment-planning problem can be formalized as a combinatorial optimization problem. The optimization algorithm presented in this thesis is conceived to perform this task. An inverse planning (IP) approach has been adopted to guide the optimization process. This means that the optimization is guided by clinical objectives described by means of dose constraints specified to each digitized anatomical structure. A simulated annealing (SA) optimization engine has been designed to solve this particular problem in a short time for clinical applications (about 30 s ). This inverse planning by simulated annealing (IPSA) algorithm has been successfully implanted in four institutions: UCSF(1), CHUQ(2), NIH(3), CAV(4). At the moment of writing this thesis, more than 300 patients have been treated at these institutions for a wide variety of anatomical sites. Clinical studies performed by clinicians using IPSA demonstrated that the algorithm produces superior treatment plans from a dosimetric point of view than the conventional method using geometrical optimization. IPSA improves the target dose coverage while minimizing the dose delivered to organs at risk and provides consistent results from one patient to another. Both dosimetric indices and overall procedure time were improved with the clinical introduction of IPSA.

TH-C-AUD A-03: A New Approach for Afterloading Brachytherapy Inverse Planned Dose Optimization Based On the Accurate Monte Carlo Method

Purpose: In brachytherapy, considering the perturbations from the heterogeneities in the planning system will give a better dose conformity for specific sites. The goal of this work is to demonstrate the feasibility of replacing the TG43 analytical approach by a Monte Carlo (MC)dose calculation engine in the optimization process. Method and Materials: The novel method is based on pre‐computed 3D dose kernels. The CT clinical images and the dwell positions (DWP) are loaded from the DICOM‐RT files to create a voxel based simulation of the treatment. MCdose calculation is used to create the dose kernel specific for each possible DWP. Density and tissue compositions are fully taken into account in MC. The Inverse Planning Simulated Annealing (IPSA) algorithm is used for the optimization process. IPSA reads and analyzes the MCdose kernels before the beginning of the optimization process; it replaces the TG43 parameterization. A breast interstitial HDR plan is used to demonstrate the approach. Results: Computation of precise 3D‐kernels is the most time consuming portion and is proportional to the number of DWP. However, the optimization process itself takes the same amount of time as a standard (TG43) optimization. The breast, TG43/MC plan shows an underdosage in the CTV relative to the TG43/TG43 plan by 4.3 % on D90 and 3.2 % on D50. For the surgical bed, the difference is 4.2 % and 3.5 % for D90 and D50 respectively. This was corrected in the MC/MC plan, with a minimal dose increase of the skin.Conclusion:MCoptimization improved the dose conformity. The method presented is straightforward and can be applied to any site and any afterloading process (HDR or PDR) using various type of sources, from Ir‐192 to micro‐XRay devices, as long as a precise MC model is made.

Clinical Benefits of Inverse Planning for High Dose Rate Prostate Brachytherapy

The purpose of this paper is to report the clinical benefits of our in house inverse planning routine (IPSA) for the treatment planning of prostate high dose rate brachytherapy (HDR). IPSA has been designed for 3D image guided brachytherapy treatment planning. The optimization process is guided by dose objectives define for each organ extracted from medical imaging. IPSA takes into account multiple targets (prostate, boost) and multiple organs at risk (urethra, rectum, bladder, etc.). IPSA is setup to maximize the prostate dose coverage while taking into account other clinical objectives like the dose homogeneity and the organs at risk protection. A simulated annealing algorithm optimizes the dwell times to fulfill the dose objectives in less than 1 minute. The dose volume histograms of 30 consecutive prostate cancer patients were computed and analyzed for this study. The prostate volumes range from small to large and the number of implanted catheters vary accordingly. The dosimetric indices are excellent independently of the gland size and shape. IPSA reduced significantly the treatment planning time and produced improved treatment plans with reduced dose to the organs at risk compare to conventional treatment planning methods. This anatomy based optimization achieved conformal dose coverage to the prostate opening the possibility to deliver higher dose to the prostate while reducing the dose deliver to the urethra and normal tissues. Wide adaptation of this technology should improve quality of patient care.