High Dose Rate Brachytherapy Plans Research Articles

A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system

To develop a calibration phantom for (192)Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system. A phantom, herein designated BrachyPhantom, consists of a Solid Water™ 8-cm high cylinder with a diameter of 14 cm cavity in its axis that allows the positioning of an A1SL ionization chamber with its reference measuring point at the midheight of the cylinder's axis. Inside the BrachyPhantom, at a 3-cm radial distance from the chamber's reference measuring point, there is a circular channel connected to a cylindrical-guide cavity that allows the insertion of a 6-French flexible plastic catheter from the BrachyPhantom surface. The PENELOPE Monte Carlo code was used to calculate a factor, P(sw)(lw), to correct the reading of the ionization chamber to a full scatter condition in liquid water. The verification of dose calculation of a HDR brachytherapy treatment planning system was performed by inserting a catheter with a dummy source in the phantom channel and scanning it with a CT. The CT scan was then transferred to the HDR computer program in which a multiple treatment plan was programmed to deliver a total dose of 150 cGy to the ionization chamber. The instrument reading was then converted to absorbed dose to water using the N(gas) formalism and the P(sw)(lw) factor. Likewise, the absorbed dose to water was calculated using the source strength, Sk, values provided by 15 institutions visited in this work. A value of 1.020 (0.09%, k = 2) was found for P(sw)(lw). The expanded uncertainty in the absorbed dose assessed with the BrachyPhantom was found to be 2.12% (k = 1). To an associated Sk of 27.8 cGy m(2) h(-1), the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. The deviation between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans and using the Sk values did not exceed ± 3% and ± 1.6%, respectively. The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system with a priori threshold level to spot problems of 2% and ± 3%, respectively, and in the long run save time for the medical physicist.

TCP-Driven Biological Planning for High-Dose-Rate Brachytherapy

Purpose/Objective(s): Tumor control probability (TCP) measures the probability that no malignant cells are left in the affected organ. It is thus an important clinical metric for measuring treatment success. Zaider and Minerbo provide a complex yet important TCP formalism which links the treatment duration, dose deposit, radiobiology of tumor cells and cell life characteristics. We perform feasibility tests on TCP-driven biological planning for high dose rate brachytherapy (HDR) where the objective maximizes TCP of the resulting plans. Robustness of the planning is tested, and plan quality and potential outcome significance are evaluated. Materials/Methods: CT and 18F-fluorodeoxyglucose based-PET images are obtained for 15 cervical cancer patients. All received prior external beam radiation. The CTV prescribed dose is 5 Gy for 4 fractionations, with the PET-identified pockets prescribed an escalated dose ranging from 7.59.0 Gy per fraction. The treatment model determines the dwell time and seed location that maximizes TCP while constraining PTV coverage, the lower/upper dose, and dose-volume shape for organs-at-risk and PTV. Two alternative plans are contrasted: standard HDR plan and PET-pocket escalated plan. Results: TCP for standard plans ranges from 48-63% whereas TCP for PET-guided escalated plans ranges from 82-99%. The increase is most significant with the smallest PET-identified pockets. Uniformly, bladder and rectum receive 5-8% reduced dose. There is a marginal difference in PTV dose between the standard and escalated plans. All resulting TCPdriven plans are clinically acceptable. Conclusions: TCP can be a very important objective for treatment plan optimization. With advances in biological/functional imaging, there is an urgent need to incorporate radiobiological parameters and TCP within the planning process. This study marks the first use of TCP as the driving objective for treatment planning. Although the optimization problem is very difficult to solve and requires computational breakthroughs, the resulting plans improve significantly the overall local tumor control, and also reduce dose to organs at risk. Rush University began clinical studies in July 2011. A long term outcome study must be carried out to gauge the overall impact. Author Disclosure: E.K. Lee: None. F. Yuan: None. A. Templeton: None. R. Yao: None. K. Kiel: None. J. Chu: None.

SU-D-500-01: TCP-Driven Biological Planning for High-Dose Rate Brachytherapy

Purpose: Tumor control probability(TCP) measures the probability that no malignant cells are left in the affected organ. It is an important clinical metric for measuring treatment success. Zaider and Minerbo provide a complex yet important TCP formalism which links the treatment duration, dose deposit, radiobiology of tumor cells and cell life characteristics. We perform feasibility tests on TCP-driven biological planning for high-dose-rate brachytherapy(HDR) where the objective maximizes TCP of the treatment plans. Plan robustness is tested, and plan quality and potential outcome significance are evaluated. Methods: CT and 18F-fluorodeoxyglucose based-PET images are obtained for 15 cervical cancer patients. All received prior external-beam radiation. The CTV prescribed dose is 5Gy for 4 fractionations, with the PET-identified pockets prescribed an escalated-dose ranging from 7.5–9.0Gy per fraction. The treatment model determines the dwell time and seed location that maximizes TCP while constraining PTV coverage, the lower/upper dose, and dose-volume shape for organs-at-risk and PTV. We contrast the standard HDR plan and PET-pocket escalated plan. Results: TCP ranges from 48–63% for standard plans versus 82–99% for PET-guided escalated plans. The increase is most significant with the smallest PET-identified pockets. Uniformly, urethra and rectum receive 5–8% reduced dose. There is marginal difference in PTV dose between standard and escalated plans. All resulting TCP-driven plans are clinically acceptable. Conclusion: TCP can be a very important objective for treatment plan optimization. With advances in biological/functional imaging, there is an urgent need to incorporate radiobiological parameters and TCP within planning. This study marks the first use of TCP as the driving objective for treatment planning. The optimization problem is very difficult to solve and requires computational breakthroughs. The resulting plans improve significantly the overall local tumor control, and reduce organs-at-risk dose. Rush University began clinical studies in 2011. A long term outcome study must be carried out to gauge the overall impact.

Is there a role for an external beam boost in cervical cancer radiotherapy?†

Objectives: Some patients are medically unfit for or averse to undergoing a brachytherapy boost as part of cervical cancer radiotherapy. In order to be able to definitively treat these patients, we assessed whether we could achieve a boost plan that would mimic our brachytherapy plans using external beam radiotherapy.Methods: High dose rate brachytherapy plans of 20 patients with stage IIB cervical cancer treated with definitive chemoradiotherapy were included in this study. Patients had undergone computer tomography (CT) simulations with tandem and ovoids in place. Point “A” dose was 600–700 cGy. We attempted to replicate the boost dose distribution from brachytherapy plans using intensity-modulated radiotherapy (Varian Medical Systems, Palo Alto, CA, USA), volumetric modulated arc therapy (Rapid Arc, Varian Medical Systems, Palo Alto, CA, USA), or TomoTherapy (Accuray, Inc., Sunnyvale, CA, USA) with the brachytherapy 100% isodose line as our target. Target coverage, normal tissue dose, and brachytherapy point doses were compared with ANOVA. Two-sided p-values ≤0.05 were considered significant.Results: External beam plans had excellent planning target volume (PTV) coverage, with no difference in mean PTV V95% among planning techniques (range 98–100%). External beam plans had lower bladder Dmax, small intestine Dmax, and vaginal mucosal point dose than brachytherapy plans, with no difference in bladder point dose, mean bladder dose, mean small intestine dose, or rectal dose. Femoral head dose, parametria point dose, and pelvic sidewall point dose were higher with external beam techniques than brachytherapy.Conclusions: External beam plans had comparable target coverage and potential for improved sparing of most normal tissues compared to tandem and ovoid brachytherapy.

The dosimetric impact of vaginal balloon-packing on intracavitary high-dose-rate brachytherapy for gynecological cancer

PurposeWe perform a clinical retrospective study to determine whether a vaginal balloon-packing system provides a dosimetric reduction to organs at risk (OARs) versus traditional gauze packing for gynecological high-dose-rate brachytherapy (HDR-BT). We also test various balloon filling materials for optimizing imaging quality.Material and methodsFilling materials for balloon-packing were evaluated based on imaging quality with X-ray, computerized tomography, and magnetic resonance imaging modalities. We then retrospectively reviewed 45 HDR-BT plans of 18 patients performed with gauze packing and 39 plans of 16 patients performed with balloon-packing. Twelve patients received both gauze and balloon-packing. HDR-BT was delivered with an iridium-192 afterloader and a Fletcher-Suit-Declos-style T&O applicator. At each fraction, 3D imaging was obtained. The D2cc values of OARs were calculated, as well as ICRU-defined point doses.ResultsIn the 84 HDR fractions reviewed, vaginal balloon-packing provides statistically equivalent doses to rectum, bladder, and sigmoid compared to gauze packing. On average balloon-packing produced average reductions of 3.3% and 6.9% in the rectal and sigmoid D2cc doses and an increase of 3.2% to the bladder D2cc dose (normalized to prescription dose), although none of these values were statistically significant for the twelve patients who received both gauze and balloon-packing (32 and 40 total fractions, respectively).ConclusionsIn the 84 HDR fractions analyzed, vaginal balloon-packing is as effective as gauze packing for dose sparing to the rectum, bladder, and sigmoid. A 1: 1 solution of saline and contrast for filling material enables easy contouring for image-guided HDR with minimal artefacts.

Feasibility of MatriXX 2D detector array for HDR brachytherapy planning system assessment

IBA Dosimetry GmbH participated in the Joint Research Project ‘Increasing cancer treatment efficacy using 3D brachytherapy’ as a non-funded partner in the work package which was mostly dedicated to the determination of dose-to-water distribution from a high-dose-rate (HDR) brachytherapy source.The dose distribution was measured with a MatriXX (MXX) 2D detector array and compared with Dose Cube Data, calculated by treatment planning systems (TPS). All measurements and calculations were performed in cooperation with OUSA, Bratislava and FNB, Prague. The comparison has been carried out for three irradiation geometries: single source position, single line and four line motions of the source, and with the effective point of measurement in a plane at 6 mm, 10 mm and 20 mm distance from the source position.The comparison of the MXX measurements and the TPS calculations was evaluated by the commercial IBA Dosimetry software OmniPro I'mRT (1) as the difference between maximum of measured and calculated values and (2) as the maximum difference between the two-dimensional distributions of measured and calculated values. The dose distribution was evaluated by the gamma method with parameters 3 mm and 3%.All differences of comparison of the MXX measurements and TPS calculations were within the range ±10% and the γ-index was less than 1 for 96% (or 97%, respectively) of the dose distribution in the plane at 10 mm distance from the source position.

SU-E-J-94: Investigating the Use of Deformable Algorithms to Register MR Images Acquired with and Without an Endo-Rectal Coil.

Magnetic resonance spectroscopic imaging (MRSI) can identify the dominant intraprostatic lesion, which can then be boosted using high-dose rate (HDR) brachytherapy. MRSI requires the use of an endo-rectal coil, which deforms the prostate. We investigate the performance of deformable registration algorithms to deform MR images acquired with an endo-rectal coil to images acquired without a coil for HDR brachytherapy planning. This study utilized MR images of five HDR brachytherapy patients acquired with and without an endo-rectal coil during the same MRSI imaging study. The prostate was contoured on each set of 'Coil-In' and 'Coil-Out' MR images by an experienced radiation oncologist. The 'Coil-In' and 'Coil-Out' images were rigidly aligned such that the posterior margins of the prostate were as close as possible without deformation. Variations on a commercially available deformation algorithm using the B-spline method were applied to a volume of interest that surrounded the prostate and excluded as much of the coil and rectum as possible. The Dice similarity index (DSI) was calculated between rigidly registered contours (DSI1), and compared to the DSI calculated between the 'Coil-In' contour and the contour deformed from the 'Coil-Out' image to the 'Coil-In' image (DSI2). The difference was calculated between DSI2 and DSI1, where a positive change in DSI would indicate an improvement in contour agreement over rigid registration of the prostate in the 'Coil-Out' and 'Coil-In' images. The B-spline deformation algorithm designed specifically for MR images was most effective in deforming the 'Coil-Out' to the 'Coil-In' prostate contour with a mean change in DSI of 0.0124 with contrast correction, and 0.0004 without. Out of six algorithms tested, only the MR-specific algorithm produced positive changes in DSI. The accuracy of the deformation algorithm on mapping the internal structures of the prostate between 'Coil-Out' and 'Coil-In' images is under investigation. We acknowledge Velocity for providing the registration software.

Target volume changes through high-dose-rate brachytherapy for cervical cancer when evaluated on high resolution (3.0 Tesla) magnetic resonance imaging

PurposeTo evaluate tumor volume changes that occurred during courses of high-dose-rate brachytherapy (HDR) using high resolution (3.0 Tesla) magnetic resonance imaging (MRI), along with the regression after external beam radiation therapy (EBRT). Methods and MaterialsFourteen patients with International Federation of Gynecology and Obstetrics stage IB1-IV cervical cancer were studied retrospectively. All patients underwent EBRT with concurrent chemotherapy followed by HDR brachytherapy. Gross tumor volume (GTV) and high-risk clinical target volume (HR-CTV) were contoured on a 3.0 Tesla MRI on the day of the HDR and on diagnostic MRI (1.5 Tesla) prior to EBRT. Two physicians independently contoured the GTV and HR-CTV on a total of 46 MRI data sets for the HDR plans. The percent volume changes of GTV and HR-CTV were quantified after EBRT and again after each HDR. The conformity indices (CIs) of the 2 contours were assessed. ResultsGTV and HR-CTV considerably regressed after the first ( --31.7% ± 19.3% and --26.4% ± 6.9%, respectively) and the second (--26.8% ± 14.3% and --23.8% ± 11.0%) fraction of HDR while relatively small regressions were observed after the third (--16.3% ± 14.2% and --10.6% ± 13.4%) and the fourth (--8.0% ± 3.4% and --9.0% ± 8.0%) fractions. The lymph node-positive on positron emission tomography (PET) and stage III or IV group showed, on average, more than 200% larger GTV and HR-CTV before EBRT than those of the other patients. The GTV and HR-CTV for the group were larger on average more than 150% after EBRT and before the first HDR fraction than the other group. Interobserver CI did not vary significantly (0.75 ± 0.11) for HR-CTV, although a smaller CI (0.56 ± 0.21) was found for GTV. ConclusionsLarger tumor regressions were observed after the first and second fractions of HDR than after all subsequent fractions. The PET-identified lymph node-positive patient group and stage III or higher tumors showed larger tumor volumes before and after EBRT than other cases.

Comparison of intracavitary brachytherapy and stereotactic body radiotherapy dose distribution for cervical cancer

PurposeTo compare the dose distribution characteristics of stereotactic body radiotherapy (SBRT) with intracavitary high-dose-rate (HDR) brachytherapy in patients with cervical carcinoma. Methods and MaterialsHDR intracavitary brachytherapy treatment plans for 11 women with cervical carcinoma were evaluated in this analysis. The total HDR brachytherapy dose was 28Gy given in four fractions. HDR brachytherapy was delivered with the microSelectron HDR therapy unit (Nucletron B. V., Veenendaal, The Netherlands). SBRT plans for each patient were generated with MultiPlan for CyberKnife Robotic Radiosurgery System (Accuray Inc., Sunnyvale, CA). The dose distributions, dose–volume histograms, and maximum dose points of the target and critical organs were recorded for both plans. ResultsSBRT yielded significantly better target coverage; the median target coverage for the 100% isodose line was 50.7% for HDR brachytherapy plans, whereas it was 99.1% for SBRT plans. The dose distributions for critical organs were similar in both types of plans. The exceptions were the 25% isodose being significantly better in brachytherapy plans for rectum, and the 100% isodose exposure being higher in brachytherapy plans for rectum, bladder, and sigmoid colon. Some significant differences were also found in maximum doses received by a 2-cc volume of bladder in favor of SBRT plans. In addition, maximum bone marrow doses were significantly higher in SBRT plans. ConclusionSBRT plans achieved better target coverage and better dose distributions to critical organs except bone marrow compared with HDR brachytherapy plans in patients with locally advanced cervical cancer.

IPIP: A new approach to inverse planning for HDR brachytherapy by directly optimizing dosimetric indices

Many planning methods for high dose rate (HDR) brachytherapy require an iterative approach. A set of computational parameters are hypothesized that will give a dose plan that meets dosimetric criteria. A dose plan is computed using these parameters, and if any dosimetric criteria are not met, the process is iterated until a suitable dose plan is found. In this way, the dose distribution is controlled by abstract parameters. The purpose of this study is to develop a new approach for HDR brachytherapy by directly optimizing the dose distribution based on dosimetric criteria. The authors developed inverse planning by integer program (IPIP), an optimization model for computing HDR brachytherapy dose plans and a fast heuristic for it. They used their heuristic to compute dose plans for 20 anonymized prostate cancer image data sets from patients previously treated at their clinic database. Dosimetry was evaluated and compared to dosimetric criteria. Dose plans computed from IPIP satisfied all given dosimetric criteria for the target and healthy tissue after a single iteration. The average target coverage was 95%. The average computation time for IPIP was 30.1 s on an Intel(R) Core 2 Duo CPU 1.67 GHz processor with 3 Gib RAM. IPIP is an HDR brachytherapy planning system that directly incorporates dosimetric criteria. The authors have demonstrated that IPIP has clinically acceptable performance for the prostate cases and dosimetric criteria used in this study, in both dosimetry and runtime. Further study is required to determine if IPIP performs well for a more general group of patients and dosimetric criteria, including other cancer sites such as GYN.

SU-E-T-642: PET-Image TCP-Driven Biological Planning for Cervical Cancer High- Dose Rate Brachytherapy

Purpose: Tumor metabolic activities obtained from PET studies may facilitate targeted dose delivery to improve local tumor control. We perform feasibility tests on TCP‐driven PET‐image‐guided dose escalation in high‐ dose‐rate‐brachytherapy(HDR). Quality and robustness of plans, TCP, and potential outcome significance are evaluated. Methods: CT and 18F‐ fluorodeoxyglucosebased‐PET images are obtained for cervical cancer patients. Planning‐target‐volumes(PTV) and critical structures are delineated. Enhanced PET signal defines boost‐target‐volume(BTV). HDR plans are optimized to deliver 35Gy Ir‐192 to PTV and 37–40Gy to BTV following 45Gy external‐beam‐radiotherapy. Each dwell location is modeled via two variables: a binary variable to indicate whether a radioactive seed will be deposited and a continuous variable to denote dwell time. The treatment planning model ensures 95% PTV‐coverage. The objective seeks rapid dose fall‐off from the prescribed dose while maximizing TCP. For each patient‐case, we compare 3 plans: 1)standard HDR plan, 2)escalation with same PTV prescription dose, and 3)escalation with reduced PTV prescription dose. Results: Standard plans have TCP values 48% to 73%. In all patients(15), escalated plans of >=37Gy to BTV result in slight reduction in rectum and bladder dose, and 40Gy for over 99% of BTV. TCP values range from 81% to 96%. When BTV is less than 15% of PTV, dose escalation and standard plans have virtually identical PTV dose. When BTV occupies over 20% of PTV, dose escalation to PET intrinsically increases PTV dose(1–5%). When PTV is prescribed 33Gy, independent of BTV, escalation can be achieved while dose to PTV, bladder and rectum are reduced. Conclusions: The presented algorithm allows for PET‐enhanced treatment, which facilitates targeted delivery of escalated dose and potential improvements in clinical outcome. Our study reveals significant improvement in tumor control and organs‐at‐risk dose reduction. Clinical studies for PET hot‐HDR dose re‐steering are in progress to test its feasibility, validate its importance, and measure potential gain in clinical outcome. The work of the first author is partially supported by funds from the National Science Foundation.

SU-GG-T-67: Objective Approach to Report Planner-Independent Skin and Rib Distance and Maximal Dose in Contura® HDR Breast Brachytherapy Planning

Purpose: To demonstrate an objective approach to determine planner‐independent skin and rib distance and maximal dose in Contura® high dose rate brachytherapy planning. Method and Materials: A virtual skin volume was produced by expanding the skin surface in three dimensions (3D) external to breast with a certain thickness. Therefore, the maximum dose to this volume occurs on the skin surface as with manual selection method. A 3D rib volume was reconstructed from rib contours on axial CTimages. The maximal dose to skin and rib was extracted from their dose volume histograms. Minimum skin and rib distance from the balloon surface was objectively calculated using the inverse square law assuming that the balloon was a sphere and the prescribed dose was located on the surface of a 1‐cm expansion from the balloon. For the calculation, three different plans were ubli single dwell position with single lumen (MDSL)and multiple dwell position with single lumen (MDSL) and multiple dwell position with multi‐lumen (MDML) plans. Those estimated minimum distance of skin and rib were compared for 24 patients with 48 manual measurements taken by a comprehensive review of 3D planning CTimages.Results: The average ± standard deviation (maximum) of estimated skin and rib distance in comparison with manual measurement was 0.4 mm ± 0.4 (1.8) for SDSL, 0.6 mm ± 0.5 (2.5) for MDSL and, 1.3 mm ± 1.2 (5.5) for MDML plans. Agreement within ± 1 mm difference was observed in 46/48 cases for SDSL, 40/48 cases for MDSL, and 25/48 cases for MDML plans. Conclusion: Volumetric information of skin and rib was used to objectively determine their maximal dose. The SDSL plan is better for skin and rib distance measurement because of its spherically symmetric dose distribution. However, asymmetric dose distribution in the MDML plan should be used for treatment.

Dosimetric Analysis of 3D Image-Guided HDR Brachytherapy Planning for the Treatment of Cervical Cancer: Is Point A-Based Dose Prescription Still Valid in Image-Guided Brachytherapy?

The purpose of this study was to analyze the dosimetric outcome of 3D image-guided high-dose-rate (HDR) brachytherapy planning for cervical cancer treatment and compare dose coverage of high-risk clinical target volume (HRCTV) to traditional Point A dose. Thirty-two patients with stage IA2–IIIB cervical cancer were treated using computed tomography/magnetic resonance imaging–based image-guided HDR brachytherapy (IGBT). Brachytherapy dose prescription was 5.0–6.0 Gy per fraction for a total 5 fractions. The HRCTV and organs at risk (OARs) were delineated following the GYN GEC/ESTRO guidelines. Total doses for HRCTV, OARs, Point A, and Point T from external beam radiotherapy and brachytherapy were summated and normalized to a biologically equivalent dose of 2 Gy per fraction (EQD2). The total planned D90 for HRCTV was 80–85 Gy, whereas the dose to 2 mL of bladder, rectum, and sigmoid was limited to 85 Gy, 75 Gy, and 75 Gy, respectively. The mean D90 and its standard deviation for HRCTV was 83.2 ± 4.3 Gy. This is significantly higher ( p < 0.0001) than the mean value of the dose to Point A (78.6 ± 4.4 Gy). The dose levels of the OARs were within acceptable limits for most patients. The mean dose to 2 mL of bladder was 78.0 ± 6.2 Gy, whereas the mean dose to rectum and sigmoid were 57.2 ± 4.4 Gy and 66.9 ± 6.1 Gy, respectively. Image-based 3D brachytherapy provides adequate dose coverage to HRCTV, with acceptable dose to OARs in most patients. Dose to Point A was found to be significantly lower than the D90 for HRCTV calculated using the image-based technique. Paradigm shift from 2D point dose dosimetry to IGBT in HDR cervical cancer treatment needs advanced concept of evaluation in dosimetry with clinical outcome data about whether this approach improves local control and/or decreases toxicities.

Cone Beam CT-Based Three-Dimensional Planning in High-Dose-Rate Brachytherapy for Cervical Cancer

To evaluate dose-volume histograms (DVHs) of bladder and rectum from the use of cone beam CT (CBCT)-based three-dimensional (3D) treatment planning in intracavitary high-dose-rate brachytherapy (HDRB) for cervical cancer patients and to compare these parameters with International Commission on Radiation Units and Measurements (ICRU) of rectal and bladder reference point dose measurements. Thirteen patients with cervical cancer underwent HDRB insertions. CT-compatible tandem and ovoid applicators were used to obtain intraoperative CBCT images. The use of a rectal tube and injection of bladder contrast before scanning facilitated contouring the rectum and bladder. All patients underwent intraoperative orthogonal x-ray filming, and treatments were prescribed using standard two-dimensional planning and dosimetry. DVHs for the bladder and rectum were constructed for each treatment. The minimum dose in the most irradiated 2.0-cm(3) volume of bladder (B(D2V)) and rectum (R(D2V)) were determined from DVHs and compared to ICRU reference point estimates of bladder (B(ICRU)) and rectum (R(ICRU)) doses. Twenty-six CBCT-based plans were evaluated. The median B(ICRU) dose (347 cGy; range, 164-601 cGy) was significantly lower (p < 0.001) than the median B(D2V) (594 cGy; range, 260-969 cGy). The median R(ICRU) dose (405 cGy; range, 189-700 cGy) was also significantly lower (p = 0.037) than the median R(D2V) (488 cGy; range, 227-786 cGy). CBCT-based 3D planning can be used in HDRB for cervical cancer and is a convenient alternative to CT-based planning, with the advantage of minimizing applicator motion. Correlation with late effects will further define the role of CBCT-based 3D dosimetry in HDRB planning.

SU‐FF‐T‐35: A Monte Carlo Study for Evaluation of Tissue Heterogeneity Effect of Inversely Optimized Intracavitary High Dose Rate Brachytherapy Plan

Purpose: To investigate tissue heterogeneity effect of an intracavitary high dose rate brachytherapy plan which is optimized by the inverse planning option. Method and Materials: A CT and VOI based brachytherapy plan for an intra‐uterine and cervix cancer treatment was made using inverse planning simulated‐annealing (IPSA) module provided by Plato TPS. The microSelectron‐HDR Ir‐192 afterloader and 3 catheters (2 ovoids and 1 tandem) were used for the treatment. For a verification of the plan result, EGSnrc based dosxyznrc Monte Carlo code was employed. Patient CT data was converted to egsphant data by the ctcreate code and each source position and weight from the plan data was used for a Monte Carlo simulation as well. Ir‐192 source was approximately modeled by 3.6 mm length and 0.6 mm diameter and it was assumed to be located at the center of the catheters, while parallel to the z‐axis at each position because of restricted freedom of source modeling. The microSelectron‐HDR Ir‐192 spectrum data could be obtained from the EGSnrc code, however, number of history was determined empirically based on known air‐kerma and dose rate value. Conventional point A and B doses and DVHs of PTV, bladder and rectum were evaluated for a quantitative comparison. Results: Significant decrease of dose at point A and B more than 10% was observed in the Monte Carlo simulation. PTV coverage by 100% isodose surface was degraded while rectum and bladder mean dose was reduced up to 15%. Resultant dose rate decrease was also observed. Conclusion: Conventional brachytherapy dose calculation method could not appropriately consider tissue heterogeneity effect and inverse planning optimization could invoke serious erroneous result. Therefore inverse planning optimization should be carefully implemented for a HDR brachytherapy planning.

SU-FF-T-63: Comparison of Optimized Interstitial HDR Brachytherapy Plans Using Adaptive Simulated Annealing Algorithm and Physical, Biological and Hybrid Cost Functions

Purpose: Comparing the optimized interstitial High Dose Rate (HDR) Brachytherapy plans based on physical, biological and hybrid cost functions. Method and Materials: Using an Adaptive Simulated Annealing (ASA) algorithm we have developed a method to optimize dwell times for HDR brachytherapytreatment planning. Three cost functions are used: (1) a function based on doses to PTV; (2) a logistic function based on generalized Equivalent Uniform Dose (gEUD) of PTV and organs‐at‐risk (OAR), and (3) a hybrid function, which includes physical dose‐based objectives of PTV and gEUD‐based objectives of PTV and OAR. We applied these functions to three gynecological cancer cases. Optimizations are run up to 75000 iterations. The prescription dose is normalized to the isodose volume covering 95% of the PTV. Plans are evaluated by DVH, isodose distributions, gEUD, tumor control probability (TCP), normal tissue complication probability (NTCP), homogeneity index (HI), and overdose index (OI) of the target and OARs. Results: The optimized results of three cases based on the three cost functions are summarized. The physical model produces best target dose uniformity and lowest target maximum dose. However, the doses to OARs are also the highest. The gEUD model is able to increase gEUD to the target, reduce gEUD to the OARs, but produce less uniform dose and larger hot spots in the target. By choosing proper penalty factors, the hybrid model can produce balanced plans to achieve high gEUD to the target, low gEUD to the OARs and smaller hot spots on target. Conclusion: We are able to optimize the dwell times of interstitial HDR brachytherapy using an ASA algorithm. The optimization results can be controlled by choosing different cost functions and penalty factors depending on clinical needs of patients.

SU-FF-T-27: Dosimetric Evaluation of a Redesigned High-Dose Rate (HDR) Vaginal Applicator for Treatment of Cervical Cancer with Extension Into Lower Vagina

Purpose: The vaginal applicator was redesigned to reduce the rectal and bladder dose to optimal level for patient having cervical cancer with extension into the lower vagina without compromising the dose to the point A. Compared with the Miami applicator; it can be implanted easily without affecting by necrotic nature of the tumour around cervix. Method and Materials: The original set of the vaginal applicator consists of different lengths of intrauterine tube with different angles and different diameters of cylinder. Six cylindrical holes were drilled at 30°, 90°, 150°, 210°, 270° and 330° positions where are located 5 mm away from the surface of the cylinder. Six OncoSmart ProGuide Needles (Nucletron) were placed into the holes to form a multi‐channel vaginal applicator which allows dose optimization to the area of interest. The applicator was anchored by a Perspex lock and affixed to a base plate to prevent displacement. A set of CTimages were taken to reconstruct the applicator. Using HDR brachytherapy planning system, the standard dose distribution was adjusted to achieve the prescribed dose which is 7 Gy at point A and 5 mm away from the cylinder surface. After insertion in the patient, one set of orthogonal films was taken to localize the ICRU points with reference to the applicator. Doses to the bladder and rectum are optimized to approximately 80% and 75% respectively of the prescribed dose. The biologically effective dose (BED) to all ICRU points was compared. Results: The redesigned multi‐channel cylinder with intrauterine tube allows much better control than the original vaginal applicator. The BED of the rectum and target decreased by 20% and increased by 24% respectively during brachytherapy.Conclusions: The redesigned multi‐channel vaginal applicator enables conformal isodose shaping and dose control to organs at risk without compromising the dose to the target.

SU-GG-T-24: Dose Difference of Nucletron HDR Planning with Three Source Localization Methods

Purpose: High dose rate brachytherapy (HDR) is a very effective cancer treatment method. Localization of source positions is essential for dosimetric accuracy. Nucletron's PLATO TPS provides multiple means to reconstruct and localize source positions. Three commonly used methods are: (1) catheter describing using film, (2) catheter tracking using film, and (3) seed identifying on CT slices. Although all of these methods have been used in the clinic, the dose variance among them is unknown. The purpose of this project is to study quantitively the dose discrepancy of these different source reconstruction methods. Method and Materials: A brachy phantom was constructed with CT compatible dummy sources and a tandem. Fiducials of 5mm in diameter were attached to the phantom surface. The phantom was CT scanned with 1mm slice thickness, and then was moved to a treatment couch under the reconstruction bridge. Orthogonal films were taken, and three HDR plans were created. Plan 1 and 2 used film images. Plan 1 reconstructed the source with the catheter describing method while Plan 2 employed the tracking method. Plan 3 was based on the CTimages.Doses at the four fiducial points were calculated and compared. In order to minimize the uncertainty caused by human operators, each plan was repeated by three experienced physicists; one physicist repeated planning three times on every plan. Results:Dose points from CT based plan was used as a reference. When comparing plans by the source reconstruction method, we found significant dose difference on certain dose point between film based planning and CT based planning. The biggest dose difference was 10.3% on fiducial A. Conclusion: Through a series of planning on a phantom, we found the doses can differ significantly between the film and CT based‐ method. At certain locations, the film based‐ plan may underestimate the dose.

Rectum dose reduction and individual treatment plan optimization for high-dose-rate prostate brachytherapy

Purpose Different doses and techniques used in high-dose-rate (HDR) prostate brachytherapy make it difficult to define universal quality parameters. The aim of this study was to develop individual, objective parameters for the evaluation of an HDR brachytherapy plan for prostate radiation. Methods Fifty-three patients who received an HDR brachytherapy boost after external radiation were analyzed in this study. Brachytherapy was performed with a 192Ir source after ultrasound-guided, transperineal metal needle application followed by removal of the ultrasound probe to reduce organ dose levels at the anterior rectum wall. The rectum and prostate locations as well as the dose at the anterior rectum wall were estimated under the anatomical conditions of HDR prostate brachytherapy. The doses at the organs at risk (rectum and urethra) were analyzed for several parameters, which were compared to values of former patients before the start of treatment. In cases of major deviations, modifications of the treatment plan were performed before starting the treatment. Results Deflating of the water balloon led to an increase of the space between the anterior rectal wall and the dorsal margin of the prostate (mean, 6 mm; 1–10 mm). The dose of the introduced “virtual rectum,” represented by the ventral surface of the ultrasound probe, in the treatment plan correlated to dose measurements in the rectum. Pretreatment evaluation and comparison of the established individual quality parameters led in two cases to a treatment plan modification. Conclusions This method allows a fast and objective individual brachytherapy treatment plan evaluation and improvement.

The use and advantages of a multichannel vaginal cylinder in high-dose-rate brachytherapy

Purpose: This paper describes California Endocurietherapy’s (CET) high-dose-rate (HDR) multichannel cylinder, the rationale for its design, procedure for its insertion, and the dosimetry involved in its use. A study was done that compared the doses achieved using the CET multichannel cylinder to the same cylinder if it only had a central channel. Methods and Materials: The CET multichannel vaginal cylinder was inexpensively constructed, using parts from various suppliers. After insertion in the patient, the cylinder is affixed to a base plate to prevent displacement. Two sets of orthogonal films (without and with rectal barium) are taken in preparation for digitization of the catheters, bladder, rectal, and pelvic sidewall points. Using HDR brachytherapy planning software, the dose distribution is adjusted to achieve the prescribed dose (5 Gy HDR) 5 mm lateral to the cylinder surface, 5 mm lateral at the proximal parametrial tissue, and 4 mm superior to the vaginal apex. Doses to the bladder and rectum are limited to approximately 85% and 75%, respectively, of the prescribed dose. The plan is optimized on geometric parameters. For dose comparison to treatment using a central channel cylinder, the lateral channels are de-activated, leaving only the central channel activated. Dose points are placed 5 mm laterally and superiorly from the cylinder surface, and the plan is optimized to deliver a uniform dose to the defined dose points. The doses and treatment volumes are statistically compared. Results: The CET multichannel cylinder allows much better dose control than the central channel cylinder. The multichannel cylinder achieves lower bladder and rectal doses by 14% and 15%, respectively, when compared to the central channel cylinder. By increasing the dwell times of certain dwell positions, the prescription dose is achieved in the vaginal apex and proximal parametrial tissues and along the length of the cylinder. Conclusion: The multichannel cylinder enables more flexibility in isodose shaping and dose control to various points and structures when compared to the conventional central channel cylinder.