Ir-192 HDR Research Articles

A Novel Proton Pencil Beam Scanning Technique for Postmastectomy Chest Wall Irradiation

Purpose/Objective(s): A new intra-uterine tandem design is proposed for creating non-isotropic 192-Ir dose distribution and give unprecedented dose conformality for treatment of cervical cancer. Materials/Methods: A DMBT design was made of non-magnetic high density tungsten alloy (18.0g/cc) cylinder with 6 peripheral grooves running along the length for an 192-Ir HDR afterloader source to travel along. The groove has 1.3mm in diameter and evenly distributed on the tungsten alloy, which has a diameter of 5.4mm. The tungsten alloy is enclosed in a total diameter of 6.4mm polyoxymethylene tube (1.41 g/ cc). For calculating dose distribution, 51 million decay events are simulated on Monte Carlo simulation and resulting non-isotropic dose distributions. 75 plans (5 fraction [fx] of 15 patient cases), clinically treated with conventional tandem & ovoids (T&O) applicator, were replanned on an in-house development HDR brachytherapy planning platform, which is intensity modulated planning capability using Simulated Annealing and Constrained-Gradient Optimization algorithms. All DMBT plan results were compared against the T&O clinically treated plans, which were normalized to match the HRCTV V100 coverage. Also all the plans were optimized with the same ovoids in place as the conventional T&O plans. Results: Generally, the plan qualities were markedly better using DMBT. Among the 75 plans, an average bladder, rectum, and sigmoid dose (D2cc) reduction were 0.59 0.87 Gy (8.5 28.7%), 0.48 0.55 Gy (21.1 27.2%), 0.10 0.38 Gy (40.6 214.9%) among the 75 plans, respectively. The bladder dose, 3.20Gy (40.8%), was the best single plan reduction, due to the horseshoe-like bladder around the CTV. The best sigmoid dose reduction was 1.26Gy (27.5%) for endophytic growth type cervical cancer. Overall, the most dose reduction was caused by the dose flexibility of DMBT. The HRCTV D90 was similar with 6.55 0.96 Gy for T&O and 6.59 1.06 for DMBT. Conclusions: The new tandem designs that advance the conformality of image-guided cervix HDR were created in congruence with the current trend of 3D image based planning to maximize the therapeutic ratio. Author Disclosure: D. Han: None. M.J. Webster: None. D.J. Scanderbeg: None. C. Yashar: None. D. Choi: None. B. Song: None. S. Devic: None. A. Ravi: None. W.Y. Song: None.

Comparative Results of Vaginal Relapses and Toxicity of 2 Protracted 192-Ir HDR Brachytherapy (BT) Schedules in Postoperative Endometrial Carcinoma (EC)

occurred in 17.5% of cases. Median time to progression was 20.7 months (range 2-40.5). By Kaplan-Meier estimate, 3 year vaginal recurrence free survival (RFS) was 95%, pelvis RFS was 94%, distant metastasis free survival was 83.8% and both cancer-specific and overall survival (OS) were 83%. Of variables examined by univariate log-rank analysis, we found only lower stage to be significantly associated with better cancerspecific (90% vs 64%, pZ 0.038) and OS (83% vs 55%, pZ 0.03). Of the two patients observed who progressed, one had distant failure alone and the other presented with vaginal cuff recurrence with progression to omental involvement and positive ascites 10 months later despite salvage radiation and chemotherapy. The five adjuvantly treated patients who progressed had all received radiation alone without chemotherapy. Of these, pattern of failure was comprised of three with simultaneous locoregional (vaginal cuff or pelvic lymphadenopathy; although 3 lacked staging LND) and distant failures, one with vaginal failure with distant metastasis only 3 months later, and one with distant metastasis alone. All patients who progressed had distant failures (either during or after initial recurrence) and eventually died of disease. Distant sites included brain, pleural effusion, bone, positive ascites, inguinal nodes and supraclavicular lymph node. Conclusions: In our report of a large series for a rare cancer we found relatively high rates of distant metastasis despite early stage disease. Recurrences were uniformly fatal and worse outcomes were found in those with stage II disease. Based on patterns of failure, more aggressive systemic therapy may be warranted for those with higher stage disease. Author Disclosure: C.N. Chang-Halpenny: None. S. Natarajan: None. J.M. Hwang-Graziano: None.

Direction Modulated Brachytherapy (DMBT) for Cervical Cancer

Purpose/Objective(s): A new intra-uterine tandem design is proposed for creating non-isotropic 192-Ir dose distribution and give unprecedented dose conformality for treatment of cervical cancer. Materials/Methods: A DMBT design was made of non-magnetic high density tungsten alloy (18.0g/cc) cylinder with 6 peripheral grooves running along the length for an 192-Ir HDR afterloader source to travel along. The groove has 1.3mm in diameter and evenly distributed on the tungsten alloy, which has a diameter of 5.4mm. The tungsten alloy is enclosed in a total diameter of 6.4mm polyoxymethylene tube (1.41 g/ cc). For calculating dose distribution, 51 million decay events are simulated on Monte Carlo simulation and resulting non-isotropic dose distributions. 75 plans (5 fraction [fx] of 15 patient cases), clinically treated with conventional tandem & ovoids (T&O) applicator, were replanned on an in-house development HDR brachytherapy planning platform, which is intensity modulated planning capability using Simulated Annealing and Constrained-Gradient Optimization algorithms. All DMBT plan results were compared against the T&O clinically treated plans, which were normalized to match the HRCTV V100 coverage. Also all the plans were optimized with the same ovoids in place as the conventional T&O plans. Results: Generally, the plan qualities were markedly better using DMBT. Among the 75 plans, an average bladder, rectum, and sigmoid dose (D2cc) reduction were 0.59 0.87 Gy (8.5 28.7%), 0.48 0.55 Gy (21.1 27.2%), 0.10 0.38 Gy (40.6 214.9%) among the 75 plans, respectively. The bladder dose, 3.20Gy (40.8%), was the best single plan reduction, due to the horseshoe-like bladder around the CTV. The best sigmoid dose reduction was 1.26Gy (27.5%) for endophytic growth type cervical cancer. Overall, the most dose reduction was caused by the dose flexibility of DMBT. The HRCTV D90 was similar with 6.55 0.96 Gy for T&O and 6.59 1.06 for DMBT. Conclusions: The new tandem designs that advance the conformality of image-guided cervix HDR were created in congruence with the current trend of 3D image based planning to maximize the therapeutic ratio. Author Disclosure: D. Han: None. M.J. Webster: None. D.J. Scanderbeg: None. C. Yashar: None. D. Choi: None. B. Song: None. S. Devic: None. A. Ravi: None. W.Y. Song: None.

Surface dose characterisation of the Varian Ir-192 HDR conical surface applicator set with a vertically orientated source.

Conical surface applicators with an Ir-192 high-dose-rate brachytherapy source are a common modality for the treatment of non-melanomatous skin cancer with high tumour control rates. Surface dose characterisation of the Varian Varisource GammaMed+ IX afterloader vertical type surface applicators is performed two dimensionally using high-resolution film dosimetry. The focus of this study was to determine if Varian surface applicators with a vertical source suffer from the dose distribution irregularities reported for comparable applicators. Our goal was to evaluate if the irregularities found affected treatment and dose output verification procedures. Ionisation chamber-based verification of applicator output was established according to guidelines provided by the manufacturer. For additional measurement of surface dose Gafchromic EBT3 film dosimetry was used. The term "therapeutic dose" was defined as 85% of the prescribed dose level. For the 10 different applicator inserts evaluated, cold spots were observed. Mean cold spot size was 2.0 mm × 3.6 mm (± 0.6 mm). The cold spots were dosimetrically well below 85% of the prescribed dose. The cold spot was situated 2.2 mm (1.4-2.7 mm) unilaterally from the central axis and caused general asymmetry in the dose profiles intersecting the cold spot area. A source tilt of approximately 8° (± 1°) was determined for the source used for irradiation. A central underdosed area exceeding 15 % of the prescribed dose has not been previously reported. Source tilt was observed and found to affect clinical use and possibly treatment outcome in applicators using a vertically arranged source. Surface applicators with a vertically orientated source were subject to dose irregularities that could impact on chamber-based applicator output verification procedures. We recommend film dosimetry-backed applicator commissioning to avoid systematic errors.

Sci-Fri AM: Mountain - 03: Current status of the NRC primary standard for 192 Ir HDR brachytherapy sources

The Canadian primary standard for 192Ir HDR brachytherapy sources has been recently revised in a more accurate manner allowing for more realistic uncertainty estimation. Air-kerma strength Sk is derived from measurements of the source's output using a graphite-walled spherical ionization chamber (2S) at several distances. Traceability to NRC primary standards for the 192Ir calibration coefficient Nk is insured by estimating it as the inverse arithmetic mean of the inverse of the calibration coefficients for a 137Cs beam and the medium energy x-ray beam quality N250, both of which are traceable to NRC primary standards. The multiple-distance method is combined with a non-linear least squares fit to determine St, while at the same time removing the effects of room scatter and position offset. The previously used shadow-cone method for directly measuring the room scatter is found to be inadequate due to the increased scatter contribution from the lead cone itself, especially at short source-detector distances. Rather than including the reported 1% difference in source strength for 192Ir HDR sources of different construction into the total uncertainty, users are cautioned that the calibration coefficient provided by NRC is only valid for a microSelectron V2 model. A comprehensive uncertainty budged shows that the total one sigma uncertainty of the standard is actually 0.6% rather than the previously assumed 1.2%. NRC measured Sk agrees within 0.03% of the manufacturer's value.

An investigation of a PRESAGE® in vivo dosimeter for brachytherapy

Determining accurate in vivo dosimetry in brachytherapy treatment with high dose gradients is challenging. Here we introduce, investigate, and characterize a novel in vivo dosimeter and readout technique with the potential to address this problem. A cylindrical (4 mm × 20 mm) tissue equivalent radiochromic dosimeter PRESAGE® in vivo (PRESAGE®-IV) is investigated. Two readout methods of the radiation induced change in optical density (OD) were investigated: (i) volume-averaged readout by spectrophotometer, and (ii) a line profile readout by 2D projection imaging utilizing a high-resolution (50 micron) telecentric optical system. Method (i) is considered the gold standard when applied to PRESAGE® in optical cuvettes. The feasibility of both methods was evaluated by comparison to standard measurements on PRESAGE® in optical cuvettes via spectrophotometer. An end-to-end feasibility study was performed by a side-by-side comparison with TLDs in an 192Ir HDR delivery. 7 and 8 Gy was delivered to PRESAGE®-IV and TLDs attached to the surface of a vaginal cylinder. Known geometry enabled direct comparison of measured dose with a commissioned treatment planning system. A high-resolution readout study under a steep dose gradient region showed 98.9% (5%/1 mm) agreement between PRESAGE®-IV and Gafchromic® EBT2 Film. Spectrometer measurements exhibited a linear dose response between 0–15 Gy with sensitivity of 0.0133 ± 0.0007 ΔOD/(Gy ⋅ cm) at the 95% confidence interval. Method (ii) yielded a linear response with sensitivity of 0.0132 ± 0.0006 (ΔOD/Gy), within 2% of method (i). Method (i) has poor spatial resolution due to volume averaging. Method (ii) has higher resolution (∼1 mm) without loss of sensitivity or increased noise. Both readout methods are shown to be feasible. The end-to-end comparison revealed a 2.5% agreement between PRESAGE®-IV and treatment plan in regions of uniform high dose. PRESAGE®-IV shows promise for in vivo dose verification, although improved sensitivity would be desirable. Advantages include high-resolution, convenience and fast, low-cost readout.

SU-E-T-169: Evaluation of Oncentra TPS for Nasopharynx Brachy Using Patient Specific Voxel Phantom and EGSnrc

Purpose: Nasopharnx carcinoma (NPC) treatment is being carried out using Ir-192 HDR seeds in Mehdieh Hospital in Hamadan, Iran. The Oncentra™ TPS is based on optimized TG-43 formalism which disregards heterogeneity in the treatment area. Due to abundant heterogeneity in head and neck, comparison of the Oncentra™ TPS dose evaluation and an accurate dose calculation method in NPC brachytherapy is the objective of this study. Methods: CT DICOMs of a patient with NPC obtained from Mehdieh Hospital used to create 3D voxel phantom with CTCREATE utility of EGSnrc code package. The voxel phantom together with Ir-192 HDR brachytherapy source were the input to DOSXYZnrc to calculate the 3D dose distribution. The sources were incorporate with type 6 source in DOSXYZnrc and their dwell times were taken into account in final dose calculations. Results: The direct comparison between isodoses as well as DVHs for the GTV, PTV and CTV obtained by Oncentra™ and EGSnrc Monte Carlo code are made. EGSnrc results are obtained using 5×109 histories to reduce the statistical error below 1% in GTV and 5% in 5% dose areas. The standard ICRP700 cross section library is employed in DOSXYZnrc dose calculation. Conclusion: A direct relationship between increased dose differences and increased material density (hence heterogeneity) is observed when isodoses contours of the TPS and DOSXYZnrc are compared. Regarding the point dose calculations, the differences range from 1.2% in PTV to 5.6% for cavity region and 7.8% for bone regions. While Oncentra™ TPS overestimates the dose in cavities, it tends to underestimate dose depositions within bones.

SU‐E‐T‐46: Application of a Twin‐Detector Method for the Determination of the Mean Photon Energy Em at Points of Measurement in a Water Phantom Surrounding a GammaMed HDR 192Ir Brachytherapy Source

Purpose:In a brachytherapy photon field in water the fluence‐averaged mean photon energy Em at the point of measurement correlates with the radiation quality correction factor kQ of a non water‐equivalent detector. To support the experimental assessment of Em, we show that the normalized signal ratio NSR of a pair of radiation detectors, an unshielded silicon diode and a diamond detector can serve to measure quantity Em in a water phantom at a Ir‐192 unit.Methods:Photon fluence spectra were computed in EGSnrc based on a detailed model of the GammaMed source. Factor kQ was calculated as the ratio of the detector's spectrum‐weighted responses under calibration conditions at a 60Co unit and under brachytherapy conditions at various radial distances from the source. The NSR was investigated for a pair of a p‐type unshielded silicon diode 60012 and a synthetic single crystal diamond detector 60019 (both PTW Freiburg). Each detector was positioned according to its effective point of measurement, with its axis facing the source. Lateral signal profiles were scanned under complete scatter conditions, and the NSR was determined as the quotient of the signal ratio under application conditions x and that at position r_ref = 1 cm.Results:The radiation quality correction factor kQ shows a close correlation with the mean photon energy Em. The NSR of the diode/diamond pair changes by a factor of two from 0–18 cm from the source, while Em drops from 350 to 150 keV. Theoretical and measured NSR profiles agree by ± 2 % for points within 5 cm from the source.Conclusion:In the presence of the close correlation between radiation quality correction factor kQ and photon mean energy Em, the NSR provides a practical means of assessing Em under clinical conditions. Precise detector positioning is the major challenge.

SU‐F‐19A‐10: Recalculation and Reporting Clinical HDR 192‐Ir Head and Neck Dose Distributions Using Model Based Dose Calculation

Purpose:To retrospectively re‐calculate dose distributions for selected head and neck cancer patients, earlier treated with HDR 192Ir brachytherapy, using Monte Carlo (MC) simulations and compare results to distributions from the planning system derived using TG43 formalism. To study differences between dose to medium (as obtained with the MC code) and dose to water in medium as obtained through (1) ratios of stopping powers and (2) ratios of mass energy absorption coefficients between water and medium.Methods:The MC code Algebra was used to calculate dose distributions according to earlier actual treatment plans using anonymized plan data and CT images in DICOM format. Ratios of stopping power and mass energy absorption coefficients for water with various media obtained from 192‐Ir spectra were used in toggling between dose to water and dose to media.Results:Differences between initial planned TG43 dose distributions and the doses to media calculated by MC are insignificant in the target volume. Differences are moderate (within 4–5 % at distances of 3–4 cm) but increase with distance and are most notable in bone and at the patient surface. Differences between dose to water and dose to medium are within 1‐2% when using mass energy absorption coefficients to toggle between the two quantities but increase to above 10% for bone using stopping power ratios.Conclusion:MC predicts target doses for head and neck cancer patients in close agreement with TG43. MC yields improved dose estimations outside the target where a larger fraction of dose is from scattered photons. It is important with awareness and a clear reporting of absorbed dose values in using model based algorithms. Differences in bone media can exceed 10% depending on how dose to water in medium is defined.

SU‐F‐19A‐04: Dosimetric Evaluation of a Novel CT/MR Compatible Fletcher Applicator for Intracavitary Brachytherapy of the Cervix Uteri

Purpose:To validate a Monte Carlo model and evaluate the dosimetric capabilities of a novel commercial CT/MR compatible Fletcher applicator for cervical cancer brachytherapy.Methods:MCNPX 2.7.0 was used to model the Fletcher CT/MR shielded applicator (FA) and 192Ir HDR source. Energy deposition was calculated with a track length estimator modified by an energy‐dependent heating function. A high density polystyrene phantom was constructed with three film pockets for validation of the MCNPX model. Three planes of data were calculated with the MCNPX model corresponding to the three film planes in phantom. The planes were located 1 cm from the most anterior, posterior, and medial extents of the FA right ovoid. Unshielded distributions were calculated by modeling the shielded cells as air instead of the tungsten alloy. A third order polynomial fit to the OD to dose curve was used to convert OD of the three film planes to dose. Each film and MCNPX plane dose distribution was normalized to a point 2 cm from the center of the film plane and in a region of low dose gradient. MCNPX and film were overlaid and compared with a distance‐to‐agreement criterion of (±2%/±2mm). Shielded and unshielded distributions were overlaid and a percent shielded plot was created.Results:85.2%, 97.1%, and 96.6% of the MCNPX points passed the (±2%/±2mm) criterion respectively for the anterior, lateral, and posterior film comparison planes. A majority of the points in the anterior plane that exceeded the DTA criterion were either along edges of where the film was cut or near the terminal edges of the film. The percent shielded matrices indicated that the maximum % shielding was 50%.Conclusion:These data confirm the validity of the FA Monte Carlo model. The FA ovoid can shield up to 50% of the dose in the anteroposterior direction.

An investigation into the accuracy of Acuros(TM) BV in heterogeneous phantoms for a (192)Ir HDR source using LiF TLDs.

The aim of this study was to evaluate the accuracy of the new Acuros(TM) BV algorithm using well characterized LiF:Mg,Ti TLD 100 in heterogeneous phantoms. TLDs were calibrated using an (192)Ir source and the AAPM TG-43 calculated dose. The Tölli and Johansson Large Cavity principle and Modified Bragg Gray principle methods confirm the dose calculated by TG-43 at a distance of 5cm from the source to within 4%. These calibrated TLDs were used to measure the dose in heterogeneous phantoms containing air, stainless steel, bone and titanium. The TLD results were compared with the AAPM TG-43 calculated dose and the Acuros calculated dose. Previous studies by other authors have shown a change in TLD response with depth when irradiated with an (192)Ir source. This TLD depth dependence was assessed by performing measurements at different depths in a water phantom with an (192)Ir source. The variation in the TLD response with depth in a water phantom was not found to be statistically significant for the distances investigated. The TLDs agreed with Acuros(TM) BV within 1.4% in the air phantom, 3.2% in the stainless steel phantom, 3% in the bone phantom and 5.1% in the titanium phantom. The TLDs showed a larger discrepancy when compared to TG-43 with a maximum deviation of 9.3% in the air phantom, -11.1% in the stainless steel phantom, -14.6% in the bone phantom and -24.6% in the titanium phantom. The results have shown that Acuros accounts for the heterogeneities investigated with a maximum deviation of -5.1%. The uncertainty associated with the TLDs calibrated in the PMMA phantom is±8.2 % (2SD).

Enhancement and validation of Geant4 Brachytherapy application on clinical HDR 192Ir source

The Geant4 Monte Carlo MC associated Brachytherapy example was adapted, enhanced and several analysis techniques have been developed. The simulation studies the isodose distribution of the total, primary and scattered doses around a Nucletron microSelectron 192Ir source. Different phantom materials were used (water, tissue and bone) and the calculation was conducted at various depths and planes. The work provides an early estimate of the required number of primary events to ultimately achieve a given uncertainty at a given distance, in the otherwise CPU and time consuming clinical MC calculation. The adaptation of the Geant4 toolkit and the enhancements introduced to the code are all validated including the comprehensive decay of the 192Ir source, the materials used to build the geometry, the geometry itself and the calculated scatter to primary dose ratio. The simulation quantitatively illustrates that the scattered dose in the bone medium is larger than its value in water and tissue. As the distance away from the source increases, scatter contribution to dose becomes more significant as the primary dose decreases. The developed code could be viewed as a platform that contains detailed dose calculation model for clinical application of HDR 192Ir in Brachytherapy.

The contribution from transit dose for 192Ir HDR brachytherapy treatments

Brachytherapy treatment planning systems that use model-based dose calculation algorithms employ a more accurate approach that replaces the TG43-U1 water dose formalism and adopt the TG-186 recommendations regarding composition and geometry of patients and other relevant effects. However, no recommendations were provided on the transit dose due to the source traveling inside the patient. This study describes a methodology to calculate the transit dose using information from the treatment planning system (TPS) and considering the source's instantaneous and average speed for two prostate and two gynecological cases. The trajectory of the 192Ir HDR source was defined by importing applicator contour points and dwell positions from the TPS. The transit dose distribution was calculated using the maximum speed, the average speed and uniform accelerations obtained from the literature to obtain an approximate continuous source distribution simulated with a Monte Carlo code. The transit component can be negligible or significant depending on the speed profile adopted, which is not clearly reported in the literature. The significance of the transit dose can also be due to the treatment modality; in our study interstitial treatments exhibited the largest effects. Considering the worst case scenario the transit dose can reach 3% of the prescribed dose in a gynecological case with four catheters and up to 11.1% when comparing the average prostate dose for a case with 16 catheters. The transit dose component increases by increasing the number of catheters used for HDR brachytherapy, reducing the total dwell time per catheter or increasing the number of dwell positions with low dwell times. This contribution may become significant (>5%) if it is not corrected appropriately. The transit dose cannot be completely compensated using simple dwell time corrections since it may have a non-uniform distribution. An accurate measurement of the source acceleration and maximum speed should be incorporated in clinical practice or provided by the manufacturer to determine the transit dose component with high accuracy.

On the Influence of HDR Ir-192 Source Position Uncertainty on Dose Variation within the Varian Ring Applicator

The tandem and ring (T&R) applicator is used as an alternate to tandem and ovoid (T&O) applicators to deliver HDR Ir-192 brachytherapy for cervical cancer. Since the ring interior lumen is larger than the HDR Ir-192 source capsule outer diameter, the source path within the ring is hard to predict. Some studies have shown that the actual source positions could be several millimeters different from planned positions, with potential for inducing significant dose deviation. However, other studies have demonstrated minimal dosimetric impact (Craciunescu et al 2012). Though most centers choose to implement a global source shift during commissioning (McMahon et al 2011) to limit the effect of inherent offsets, potential dose variations due to source position uncertainty are not yet well quantified. The purpose of this study was to investigate these dose variations for the worst geometry condition, and also to assess the validity of implementing a global offset using Monte Carlo (MC) simulations for dose calculation. The MCNP5 radiation transport code was used for all MC simulations. The GammaMedPlus HDR Ir-192 source core was simulated as a 0.09 cm diameter and 0.35 cm long right cylinder. Dose distributions were simulated in water (12 x 20 cm3 right cylinder) using the *F4 FMESH tally in both transverse (X-Y) and coronal (X-Z) plane. With a (0.1 cm)3 grid size, 109 photon histories gave a typical statistical error (k=1) of <1%. The Varian ring was simulated as a torus (0.29 cm and 0.35 cm inner and outer diameters) with a 0.03 cm thick titanium wall. The black buildup cap (0.5 cm) was simulated with polyoxymethylene (1.425 g/cm3) and dwell positions started from 129.5 cm with 0.5 cm step size in the ring lumen center. Dose variations for eccentric source positions were investigated by simulating dwell positions ± 0.007 cm (largest distance) from the lumen center along the Z-direction as shown in Figure 1a-c. To mimic a T&O geometry, dwell positions were simulated only in 135° to 225° and 315° to 45° as shown in Figure 1.e. To assess the influence of a global shift, ± 0.1, ± 0.3, ± 0.5 cm (Figure 1.d and 1.f), shifts were simulated distal and proximal from the ring end. Dose was normalized to 100% at 0.5 cm from the cap on the X-axis. For dose comparison, Point A was defined 2 cm up from the buildup cap and 2 cm laterally from the ring CAX on right (A_RT) and left side (A_LT). A typical dose distribution (X-Z plane) for the source in the ring lumen center was shown in Figure 1.g. Compared to sources in the ring lumen center, dose to points A_LT and A_RT changed ˗3.2% and ˗3.8%, respectively, for sources 0.007 cm away from Point A, and by 2.4% and 4.0% toward Point A, respectively. Compared to no shift, average dose to Point A was 1.6%, 2.3%, and 3.8% lower for the 0.1, 0.3, and 0.5 cm distal global source shifts, respectively. Doses were 1.2%, 1.4%, and 2.6% lower for the proximal shifts. Neither source position uncertainty within ring lumen nor global source shift caused significant dose variation to either point A_RT or point A_LT. Dose variation due to source position uncertainty was small (< 4%), and can be attributed mostly to inverse square law. Dose variations due to global source shifts also were not substantial. Applying a global shift will improve the accuracy of the dose distribution, but not applying the shift or applying a shift in wrong direction may not cause substantial dose calculation errors.

Monte Carlo Simulation of HDR Ir-192 Brachytherapy Cancer Treatments

Modern treatment planning systems (TPS) for brachytherapy are now available that are based on model-based dose calculation algorithms (MBDCA). They enable heterogeneity corrections which are needed to replace the TG-43U1 water dose formalism with a more accurate approach. With the aim of evaluating the impact of the transit dose, applicators, body boundaries and tissue composition in clinical cases, several HDR Ir-192 brachytherapy treatment cases including prostate, gynecological (Gyn), arm and head and neck cases were evaluated.

Kerma and Photons Absorbed-Dose from Ir-192 HDR Seeds

Precise calculations of absorbed dose (AD) are a difficult task involving physical phenomena such as emission, transport, and absorption of radiation. We used the Monte Carlo method to calculate Kerma as well as AD in water imparted by two different Ir-192 HDR brachytherapy seeds (Flexisource and microSelectron-v2) taking into account the AAPM TG-43 formalism and AAPM & ESTRO most recent reports recommendations. The aim of this work is to evaluate when can Kerma be used as a measurement of AD for this type of seeds. Thus, we analyse the behaviour of both quantities in whole space, putting special emphasis near the source surface. We carried out calculations using microvoxels to obtain high spatial resolution of data close to the source. We observed differences of up to 6 % between AD and Kerma within 1 mm around the seed, and less than 1 % in any other region of the phantom. This allows us to analyse build-up region for Ir-192 HDR brachytherapy seeds. As it will be further discussed in this paper, our results can be explained in terms of partial electronic equilibrium reached on different regions of the phantom. Both seeds showed common overall behaviour, providing generality to the conclusions drawn. The complete bearing of the radial dose function (defined in the TG43 formalism) as it traverses the surface of the seed is reported. Whenever comparisons are possible, our results are in agreement with those reported by other authors. Tables of radial dose function, including new data computed from AD rate (instead of Kerma rate), are presented.

EP-1919: Characterization of the cone beam CT in a hybrid surgery room for Ir-192 HDR 3D brachytherapy

EP-1919: Characterization of the cone beam CT in a hybrid surgery room for Ir-192 HDR 3D brachytherapy

PO-1016: Parameterization of Ir-192 HDR source phase space file for Monte Carlo based dose calculation

PO-1016: Parameterization of Ir-192 HDR source phase space file for Monte Carlo based dose calculation

Experimental dosimetry for model-based 192Ir HDR brachytherapy treatment planning

Experimental dosimetry for model-based 192Ir HDR brachytherapy treatment planning

Stem effect of a Ce3+ doped SiO2 optical dosimeter irradiated with a 192Ir HDR brachytherapy source

Fiber-optic-coupled scintillation dosimeters are characterized by their small active volume if compared to other existing systems. However, they potentially show a greater stem effect, especially in external beam radiotherapy where the Cerenkov effect is not negligible. In brachytherapy, due to the lower energies and the shorter high dose range of the employed sources, the impact of the stem effect to the detector accuracy might be low. In this work, the stem effect of a Ce3+ doped SiO2 scintillation detector coupled to a SiO2 optical fiber was studied for high dose rate brachytherapy applications. Measurements were performed in a water phantom at changing source-detector mutual positions. The same irradiations were performed with a passive optical fiber, which doesn't have the dosimeter at its end. The relative contribution of the passive fiber with respect to the uncorrected readings of the detector in each one of the investigated source dwell positions was evaluated. Furthermore, the dosimeter was calibrated both neglecting and correcting its response for the passive fiber readings. The obtained absolute dose measurements were then compared to the dose calculations resulting from the treatment planning system. Dosimeter uncertainties with and without taking into account the passive fiber readings were generally below 2.8% and 4.3%, respectively. However, a particular exception results when the source is positioned near to the optical fiber, where the detector underestimates the dose (−8%) or at source-detector longitudinal distances higher than 3cm.The obtained results show that the proposed dosimeter might be adopted in high dose rate prostate brachytherapy with satisfactory accuracy, without the need for any stem effect correction. However, accuracy further improves by subtraction of the noise signal produced by the passive optical fiber.