Brachytherapy Dose Distributions Research Articles

SU‐E‐T‐150: Brachytherapy QA Employing a High Resolution Liquid Filled Ionisation Chamber Array: Initial Experience and Limitations

Purpose:Verifying a complex 3D brachytherapy dose distribution by measurement is non‐trivial. Ideally a photon detector array should be independent of energy and angle, have high spatial resolution and be robust for routine clinical use.Methods:An iridium‐192 source was used. A PMMA jig was constructed comprising an outer slab and a central insert with eight milled channels for 1.33mm (outer diameter) steel needles, see figure. All calculations were performed using an empty CT study reconstructing eight virtual needles (QA‐CT), using the v2 source model (Elekta AG, Sweden). A high resolution liquid filled ionisation chamber array SRS1000, together with Verisoft software v6.0 (PTW Freiburg, Germany), was used to perform measurements of plans of increasing complexity to evaluate its suitability for device‐ and patient‐specific QA. The dimension of backscatter material was investigated. The patient plan dwell time distribution was entered manually into the QA‐CT and the dose distribution was calculated.Results:Our measurements indicate that the array is independent of energy and angle. The resulting measured dose values are linearly interpolated to 2025 values. Shifts of 1mm of the entire needle are readily detectable. Individual dwell position shifts (2.5mm) are also readily measurable. Moreover a dwell time increase of 1 second both in the edge and central region are detectable.Conclusion:The high resolution SRS1000 array is a powerful instrument for brachytherapy QA enabling 977 simultaneous measurements to be performed. Our measurements suggest 60mm of RW3 backscatter material upstream and downstream are sufficient. Local percentage difference analysis is useful for device based QA, normalized relative percentage difference is arguably better for patient specific QA. Automated transfer of patient plan dwell time distribution to the QA plan is required to enable a comprehensive patient QA study to be performed. Moreover the described measurement technique can be used for commissioning new brachytherapy applicators.

Is Supplemental External Beam Radiation Therapy Necessary for Patients With Higher Risk Disease Treated With Pd-103? Results of Two Prospective Randomized Trials

The necessity and/or the dose of supplemental external beam radiotherapy (XRT) in conjunction with Pd-103 brachytherapy remains controversial for patients with higher risk disease. Herein, we report brachytherapy outcomes for patients randomized to two sequential prospective trials evaluating different supplemental XRT regimens and Pd-103 boost doses. The initial trial randomized 247 patients to 44Gy plus a 90Gy Pd-103 boost vs. 20Gy plus a 115Gy Pd-103 boost (trial 44/20) while 383 patients were randomized to a subsequent prospective randomized trial - 20Gy plus 115Gy Pd-103 vs. 125Gy Pd-103 without any supplemental XRT (20/0 trial). Eligibility criteria included clinically organ confined disease with Gleason scores 7-9 and/or a pre-treatment PSA 10-20ng/mL. The brachytherapy prescription dose was prescribed to the prostate gland with a generous periprostatic margin and the proximal seminal vesicles. Median follow up was 11 years in 44/20 trial and 5.0 years in the 20/0 trial. Biochemical progression free survival (bPFS) was defined as a PSA ≤ 0.40ng/mL after nadir. The median day 0 D90 was 125.7% of prescription dose in the 44/20 trial and 121.0% in the 20/0 trial. Multiple parameters were evaluated for effect on bPFS, prostate cancer specific mortality (PCSM) and overall mortality (OM). In the 44/20 trial, the 13 year biochemical failure (BF), PCSM and OM were 8.2%, 4.0% and 42.8% vs. 8.0%, 1.0% and 40.3% for the 44Gy and 20Gy arms, respectively. In 20/0 trial, 8 year BF, PCSM and OM were 2.1%, 0% and 14.4% vs. 3.6%, 0% and 16.1% in the 20Gy vs. 0Gy arms. When stratified by either a pre-treatment PSA ≤ 10ng/mL vs. > 10ng/mL or by Gleason score ≤ 7 vs. Gleason score 8-9, the dose of supplemental XRT did not impact biochemical outcomes, PCSM or OM. In multivariate analysis, BF was most closely related to pre-implant PSA and Gleason score. In both trials, patients with biochemically controlled disease had a median PSA < 0.02ng/mL. In a brachytherapy population with high quality intra-prostatic brachytherapy dose distributions and generous extra-prostatic treatment margins, supplemental external beam radiation therapy did not influence biochemical progression-free survival or prostate cancer specific mortality.

Improving anatomical mapping of complexly deformed anatomy for external beam radiotherapy and brachytherapy dose accumulation in cervical cancer.

In the treatment of cervical cancer, large anatomical deformations, caused by, e.g., tumor shrinkage, bladder and rectum filling changes, organ sliding, and the presence of the brachytherapy (BT) applicator, prohibit the accumulation of external beam radiotherapy (EBRT) and BT dose distributions. This work proposes a structure-wise registration with vector field integration (SW+VF) to map the largely deformed anatomies between EBRT and BT, paving the way for 3D dose accumulation between EBRT and BT. T2w-MRIs acquired before EBRT and as a part of the MRI-guided BT procedure for 12 cervical cancer patients, along with the manual delineations of the bladder, cervix-uterus, and rectum-sigmoid, were used for this study. A rigid transformation was used to align the bony anatomy in the MRIs. The proposed SW+VF method starts by automatically segmenting features in the area surrounding the delineated organs. Then, each organ and feature pair is registered independently using a feature-based nonrigid registration algorithm developed in-house. Additionally, a background transformation is calculated to account for areas far from all organs and features. In order to obtain one transformation that can be used for dose accumulation, the organ-based, feature-based, and the background transformations are combined into one vector field using a weighted sum, where the contribution of each transformation can be directly controlled by its extent of influence (scope size). The optimal scope sizes for organ-based and feature-based transformations were found by an exhaustive analysis. The anatomical correctness of the mapping was independently validated by measuring the residual distances after transformation for delineated structures inside the cervix-uterus (inner anatomical correctness), and for anatomical landmarks outside the organs in the surrounding region (outer anatomical correctness). The results of the proposed method were compared with the results of the rigid transformation and nonrigid registration of all structures together (AST). The rigid transformation achieved a good global alignment (mean outer anatomical correctness of 4.3 mm) but failed to align the deformed organs (mean inner anatomical correctness of 22.4 mm). Conversely, the AST registration produced a reasonable alignment for the organs (6.3 mm) but not for the surrounding region (16.9 mm). SW+VF registration achieved the best results for both regions (3.5 and 3.4 mm for the inner and outer anatomical correctness, respectively). All differences were significant (p < 0.02, Wilcoxon rank sum test). Additionally, optimization of the scope sizes determined that the method was robust for a large range of scope size values. The novel SW+VF method improved the mapping of large and complex deformations observed between EBRT and BT for cervical cancer patients. Future studies that quantify the mapping error in terms of dose errors are required to test the clinical applicability of dose accumulation by the SW+VF method.

The use of tetrahedral mesh geometries in Monte Carlo simulation of applicator based brachytherapy dose distributions

Accounting for brachytherapy applicator attenuation is part of the recommendations from the recent report of AAPM Task Group 186. To do so, model based dose calculation algorithms require accurate modelling of the applicator geometry. This can be non-trivial in the case of irregularly shaped applicators such as the Fletcher Williamson gynaecological applicator or balloon applicators with possibly irregular shapes employed in accelerated partial breast irradiation (APBI) performed using electronic brachytherapy sources (EBS). While many of these applicators can be modelled using constructive solid geometry (CSG), the latter may be difficult and time-consuming. Alternatively, these complex geometries can be modelled using tessellated geometries such as tetrahedral meshes (mesh geometries (MG)). Recent versions of Monte Carlo (MC) codes Geant4 and MCNP6 allow for the use of MG. The goal of this work was to model a series of applicators relevant to brachytherapy using MG. Applicators designed for 192Ir sources and 50 kV EBS were studied; a shielded vaginal applicator, a shielded Fletcher Williamson applicator and an APBI balloon applicator.All applicators were modelled in Geant4 and MCNP6 using MG and CSG for dose calculations. CSG derived dose distributions were considered as reference and used to validate MG models by comparing dose distribution ratios.In general agreement within 1% for the dose calculations was observed for all applicators between MG and CSG and between codes when considering volumes inside the 25% isodose surface. When compared to CSG, MG required longer computation times by a factor of at least 2 for MC simulations using the same code. MCNP6 calculation times were more than ten times shorter than Geant4 in some cases.In conclusion we presented methods allowing for high fidelity modelling with results equivalent to CSG. To the best of our knowledge MG offers the most accurate representation of an irregular APBI balloon applicator.

SU‐E‐T‐244: Motion Control Challenges in High Dose Rate Brachytherapy

Purpose:High dose rate (HDR) brachytherapy dose distribution is highly localized and has a very sharp fall‐off. Thus the one of the most important part of the treatment is the localization and immobilization of the applicator from the implantation to the setup verification to the treatment delivery. The smallest motions of the patient can induce a small rotation, tilt, or translational movement of the applicator that can convert into miss of a significant part of the tumor or to over irradiating a nearby critical organ. The purpose of this study is to revise most of the HDR types of treatments with their applicators and their localization challenges. Since every millimeter of misplacement counts the study will look into the necessity of increasing the immobilization for several types of applicators.Methods:The study took over 136 plans generated by the treatment planning system (TPS) looking into the applicator placement in regard to the organs at risk (OR) and simulated the three possible displacements at the hottest dose point on the critical organ for several accessories to evaluate the variation of the delivered dose at the point due to the displacement.Results:Many of the present immobilization devices produced for external radiotherapy can be used to improve the localization of HDR applicators during transportation of the patient and during treatment.Conclusion:This study data indicates that an improvement of the immobilization devices for HDR is absolutely necessary. Better applicator fixation devices are required too. Developing new immobilization devices for all the applicators is recommended.

SU‐C‐16A‐04: Dosimetric Validation of a Partially‐Shielded Gd‐153 Brachytherapy Concept

Purpose:To demonstrate by measurement that using partially shielded Gd‐153 sources for rotating‐shield brachytherapy (RSBT) is feasible. RSBT is a potentially superior alternative to conventional high‐dose‐rate brachytherapy and provides the opportunity to dramatically improve tumor dose conformity for the treatment of, for example, prostate cancer.Methods:A custom‐built, stainless steel encapsulated 150 mCi Gd‐153 capsule with an outer length of 12.8 mm, outer diameter of 2.10 mm, active length of 9.98 mm, and active diameter of 1.53 mm was used. A partially shielded catheter was constructed with a 500 μm platinum shield and a 500 μm aluminum emission window, both with 180° azimuthal coverage. An acrylic phantom was constructed to measure the dose distributions from the shielded catheter in the transverse plane using Gafchromic EBT3 films. Film calibration curves were generated from 50, 70, and 100 kVp x‐ray beams with NIST‐traceable air kerma values to account for energy variation.Results:The transmission ratios of platinum to aluminum shielding at 1 cm off‐axis are 7.5% and 7.6% for Monte Carlo (MCNP5) predicted and experimental results, respectively. The predicted/measured relative dose rates at 1 cm, 2 cm and 3 cm off‐axis through the Al window were 100%/92.9%, 28.6%/27.0% and 13.8%/12.7%, respectively. Through the Pt shield, the predicted/measured relative dose rates were 7.5%/7.1%, 3.8%/3.0% and 2.4%/1.7%, respectively.Conclusion:Using partially‐shielded Gd‐153 sources for RSBT is a promising approach to improving brachytherapy dose distributions. The next step in making Gd‐153 based RSBT a reality is developing a Gd‐153 source that is small enough such that the source, shield, and catheter all fit within a 16 gauge needle, which has a 1.65 mm diameter.University of Iowa Research Foundation

Proof of principle: Applicator-guided stereotactic IMRT boost in combination with 3D MRI-based brachytherapy in locally advanced cervical cancer

To describe a new technique involving high-precision stereotactic intensity-modulated radiation therapy (IMRT) boost in combination with intracavitary-interstitial (IC-IS) brachytherapy (BT) in cervical tumors that cannot be sufficiently covered by IC-IS-BT due to extensive residual disease and/or difficult topography at the time of BT. Three patients with stage IIIB-IVA cervical cancer had significant residual disease at the time of BT. MRI-guided IC-IS-BT (pulsed-dose rate) was combined with a stereotactic IMRT boost guided according to the BT applicator in situ, using cone beam CT. The planning aim dose (total external beam radiotherapy and BT) for the high-risk clinical target volume (HR-CTV) was D90 >70-85Gy, whereas constraints for organs at risk were D2cm3 <70Gy for rectum, sigmoid, and bowel and <90Gy for bladder in terms of equivalent total dose in 2Gy fractions. An IMRT boost adapted to the BT dose distribution was optimized to target the regions poorly covered by BT. HR-CTV doses of D90 >81Gy were obtained in the central HR-CTV and D90 >69Gy in the distal regions of HR-CTV. Image-guided set up of the IMRT boost with the applicator in situ was feasible. The dose plans were robust to intra-fraction uncertainties of 3mm. Local control with acceptable morbidity was obtained at a followup of 3, 2.5, and 1 year, respectively. The combination of MRI-guided BT with an applicator-guided stereotactic IMRT boost is feasible. This technique seems to be useful in the few cases where HR-CTV coverage cannot be obtained even with IS-IC-BT.

Importance on the Development of a Clinic-Specific Source Path Position Library while Commissioning Ring Applicators when Compared to the Position of Applicator Markers and to the Ring Source Path Position Library Model Currently in Use by Oncentra Treatment Planning

It is well known in the literature that the position of the markers does not correspond to those of the source for ring applicators. To overcome this limitation applicator models are provided in Oncentra Brachy (Nucletron). Each model is supplied with standard source paths. Therefore it is extremely important that the applicator model being used reflects as accurately as possible the positions of the source inside the specific ring applicators for the specifc afterloader used in the clinic. The purpose of the present study is to quantitatively measure the differences in position between not only the markers and the position of the source; but more importantly between the source path in the currently used Oncentra applicator library and our in-house measured source path. Oncentra Brachy v. 4.3 and its Applicator Library module v.2.0 were used for this study. Measurements of the source path for six different CT/MR ring applicators were performed. CT-Scouts were taken for the position of the source inside the ring every 2mm. A Flexitron (Nucletron) afterloader was used and CT-Scouts were taken at 0° and 90°. An in-house digitizing tool was created using Matlab to precisely obtain the position of the source path in X, Y and Z. The maximum distance between the position of the markers inside the ring and the position of the source was measured. The difference in distance between the X, Y, and Z coordinates of the source obtained using the in-house digitizing tool were also compared to those currently in use by the Oncentra applicator models. A maximum difference in distance of 3mm was found between the center of the source and the center of the marker with the position of the source being always before that of the marker. This discrepancy was more pronounced closer to the ring end and the difference gradually decreased as the source got closer to the connector end (0.33mm). It is well known in the literature that these results are caused by friction between the source and the different segments in the ring tube. An average difference of 45% was observed between the values of the Oncentra source path library model in the Z coordinate and those using our in-house model. These results emphasize the importance of creating an in-house model with measurements of the source path not only in X and Y but also in the Z coordinate. These measurements should be done during commissioning of ring applicators. Finally, a 2 mm difference in distance was also found between the Oncentra and our in-house measured source paths. Our results demonstrate that a complete examination of the position of the source path in relation to the position of the markers and to the position of the source path in the Oncentra applicator library is necessary. We conclude that a blind use of either the markers or the Oncentra applicator library should be avoided. Further investigation should be done to measure the impact of these results in brachytherapy dose distribution.

A modelled comparison of prostate cancer control rates after high-dose-rate brachytherapy (3145 multicentre patients) combined with, or in contrast to, external-beam radiotherapy

To analyse biochemical relapse-free-survival results for prostate cancer patients receiving combined external beam and high-dose-rate brachytherapy, in comparison with expected results using projections based on dose/fractionation/response parameter values deduced from a previous external-beam-alone 5969-patient multicentre dataset. Results on a total of 3145 prostate cancer patients receiving brachytherapy (BT) as part or all of their treatment were collected from 10 institutions, and subjected to linear-quadratic (LQ) modelling of dose response and fractionation parameters. Treatments with BT components of less than 25Gy, 3-4 BT fractions, doses per BT fraction up to 6Gy, and treatment times of 3-7weeks, all gave outcomes expected from LQ projections of the external-beam-alone data (α/β=1.42Gy). However, BT doses higher than 30Gy, 1-2 fractions, 9 fractions (BT alone), doses per fraction of 9-15Gy, and treatment in only 1week (one example), gave local control levels lower than the expected levels by up to ∼35%. There are various potential causes of the lower-than-projected control levels for some schedules of brachytherapy: it seems plausible that cold spots in the brachytherapy dose distribution may be contributory, and the applicability of the LQ model at high doses per fraction remains somewhat uncertain. The results of further trials may help elucidate the true benefit of hypofractionated high-dose-rate brachytherapy.

Model‐based dose calculations for COMS eye plaque brachytherapy using an anatomically realistic eye phantom

To investigate the effects of the composition and geometry of ocular media and tissues surrounding the eye on dose distributions for COMS eye plaque brachytherapy with(125)I, (103)Pd, or (131)Cs seeds, and to investigate doses to ocular structures. An anatomically and compositionally realistic voxelized eye model with a medial tumor is developed based on a literature review. Mass energy absorption and attenuation coefficients for ocular media are calculated. Radiation transport and dose deposition are simulated using the EGSnrc Monte Carlo user-code BrachyDose for a fully loaded COMS eye plaque within a water phantom and our full eye model for the three radionuclides. A TG-43 simulation with the same seed configuration in a water phantom neglecting the plaque and interseed effects is also performed. The impact on dose distributions of varying tumor position, as well as tumor and surrounding tissue media is investigated. Each simulation and radionuclide is compared using isodose contours, dose volume histograms for the lens and tumor, maximum, minimum, and average doses to structures of interest, and doses to voxels of interest within the eye. Mass energy absorption and attenuation coefficients of the ocular media differ from those of water by as much as 12% within the 20-30 keV photon energy range. For all radionuclides studied, average doses to the tumor and lens regions in the full eye model differ from those for the plaque in water by 8%-10% and 13%-14%, respectively; the average doses to the tumor and lens regions differ between the full eye model and the TG-43 simulation by 2%-17% and 29%-34%, respectively. Replacing the surrounding tissues in the eye model with water increases the maximum and average doses to the lens by 2% and 3%, respectively. Substituting the tumor medium in the eye model for water, soft tissue, or an alternate melanoma composition affects tumor dose compared to the default eye model simulation by up to 16%. In the full eye model simulations, the average dose to the lens is larger by 7%-9% than the dose to the center of the lens, and the maximum dose to the optic nerve is 17%-22% higher than the dose to the optic disk for all radionuclides. In general, when normalized to the same prescription dose at the tumor apex, doses delivered to all structures of interest in the full eye model are lowest for(103)Pd and highest for (131)Cs, except for the tumor where the average dose is highest for (103)Pd and lowest for (131)Cs. The eye is not radiologically water-equivalent, as doses from simulations of the plaque in the full eye model differ considerably from doses for the plaque in a water phantom and from simulated TG-43 calculated doses. This demonstrates the importance of model-based dose calculations for eye plaque brachytherapy, for which accurate elemental compositions of ocular media are necessary.

Effect of tissue composition on dose distribution in brachytherapy with various photon emitting sources

PurposeThe aim of this study is to compare the dose in various soft tissues in brachytherapy with photon emitting sources.Material and methods103Pd, 125I, 169Yb, 192Ir brachytherapy sources were simulated with MCNPX Monte Carlo code, and their dose rate constant and radial dose function were compared with the published data. A spherical phantom with 50 cm radius was simulated and the dose at various radial distances in adipose tissue, breast tissue, 4-component soft tissue, brain (grey/white matter), muscle (skeletal), lung tissue, blood (whole), 9-component soft tissue, and water were calculated. The absolute dose and relative dose difference with respect to 9-component soft tissue was obtained for various materials, sources, and distances.ResultsThere was good agreement between the dosimetric parameters of the sources and the published data. Adipose tissue, breast tissue, 4-component soft tissue, and water showed the greatest difference in dose relative to the dose to the 9-component soft tissue. The other soft tissues showed lower dose differences. The dose difference was also higher for 103Pd source than for 125I, 169Yb, and 192Ir sources. Furthermore, greater distances from the source had higher relative dose differences and the effect can be justified due to the change in photon spectrum (softening or hardening) as photons traverse the phantom material.ConclusionsThe ignorance of soft tissue characteristics (density, composition, etc.) by treatment planning systems incorporates a significant error in dose delivery to the patient in brachytherapy with photon sources. The error depends on the type of soft tissue, brachytherapy source, as well as the distance from the source.

The evolution of computerized treatment planning for brachytherapy: American contributions.

PurposeTo outline the evolution of computerized brachytherapy treatment planning in the United States through a review of technological developments and clinical practice refinements.Material and methodsA literature review was performed and interviews were conducted with six participants in the development of computerized treatment planning for brachytherapy.ResultsComputerized brachytherapy treatment planning software was initially developed in the Physics Departments of New York's Memorial Hospital (by Nelson, Meurk and Balter), and Houston's M. D. Anderson Hospital (by Stovall and Shalek). These public-domain programs could be used by institutions with adequate computational resources; other clinics had access to them via Memorial's and Anderson's teletype-based computational services. Commercial brachytherapy treatment planning programs designed to run on smaller computers (Prowess, ROCS, MMS), were developed in the late 1980s and early 1990s. These systems brought interactive dosimetry into the clinic and surgical theatre.ConclusionsBrachytherapy treatment planning has evolved from systems of rigid implant rules to individualized pre- and intra-operative treatment plans, and post-operative dosimetric assessments. Brachytherapy dose distributions were initially calculated on public domain programs on large regionally located computers. With the progression of computer miniaturization and increase in processor speeds, proprietary software was commercially developed for microcomputers that offered increased functionality and integration with clinical practice.

Effects of bladder distension on dose distribution of vaginal vault brachytherapy in patients with endometrial cancer.

PurposeTo investigate dosimetric effects of bladder distention on organs at risk (OARs) during treatment of endometrial cancer using 3D image-based planning of postoperative vaginal vault brachytherapy (BRT).Material and methodsFifteen patients with early-stage endometrial cancer were studied, each undergoing adjuvant BRT of vaginal vault via 3.5 cm diameter cylinder. As treatment, 25 Gy in 5 fractions were delivered to 5 mm depth of the vaginal mucosa. Dose-volume histograms of OARs were generated individually with bladder empty and with bladder inflated by sterile saline (180 ml), to compare doses received.ResultsBladder distention appreciably impacted dosimetry of bladder, sigmoid colon, and small bowel, but dosimetry of rectum was unaffected. With bladder inflated, mean cylinder-to-bowel distance increased significantly (1.69 cm vs. 1.20 cm; p = 0.006). Mean minimum dose to most exposed 2 cc (D2cc) volume also rose significantly at bladder (5.40 Gy vs. 4.55 Gy [18.7%]; p < 0.001), as opposed to near-significant reductions in D2cc at sigmoid colon (15.1%; p = 0.11) and at small bowel (10.5%; p = 0.14). A full bladder had no effect on dose to 50% volume (D50%) of bladder or rectum, and declines seen in mean D50% values of sigmoid colon (22.7%; p = 0.12) and small bowel (19.0%; p = 0.13) again fell short of statistical significance.ConclusionsThe combination of a full bladder and an empty rectum may cause significant unwanted increases in BRT dosing of bladder, without significantly impacting sigmoid colon and small bowel exposures. These findings should be validated through further clinical studies.

Design and implementation of a film dosimetry audit tool for comparison of planned and delivered dose distributions in high dose rate (HDR) brachytherapy

A novel phantom is presented for ‘full system’ dosimetric audit comparing planned and delivered dose distributions in HDR gynaecological brachytherapy, using clinical treatment applicators. The brachytherapy applicator dosimetry test object consists of a near full-scatter water tank with applicator and film supports constructed of Solid Water, accommodating any typical cervix applicator. Film dosimeters are precisely held in four orthogonal planes bisecting the intrauterine tube, sampling dose distributions in the high risk clinical target volume, points A and B, bladder, rectum and sigmoid. The applicator position is fixed prior to CT scanning and through treatment planning and irradiation. The CT data is acquired with the applicator in a near clinical orientation to include applicator reconstruction in the system test. Gamma analysis is used to compare treatment planning system exported RTDose grid with measured multi-channel film dose maps. Results from two pilot audits are presented, using Ir-192 and Co-60 HDR sources, with a mean gamma passing rate of 98.6% using criteria of 3% local normalization and 3 mm distance to agreement (DTA). The mean DTA between prescribed dose and measured film dose at point A was 1.2 mm. The phantom was funded by IPEM and will be used for a UK national brachytherapy dosimetry audit.

MRI-guided adaptive radiotherapy in locally advanced cervical cancer from a Nordic perspective

Background. The first Nordic protocol for three-dimensional (3D) planned radiotherapy in locally advanced cervical cancer was the prospective NOCECA study (1994–2000). NOCECA consisted of computed tomography (CT)-based 3D conformal external beam radiotherapy (EBRT) with a simultaneous integrated boost (SIB) to the primary tumour combined with brachytherapy (BT) based on x-ray imaging. In NOCECA the planning aim was to achieve 80 Gy at point A from EBRT and BT combined. However, the balance of dose between EBRT and BT was determined by tumour size at diagnosis with more EBRT dose given to point A and less by BT in more advanced stages. In 2005 image-guided adaptive brachytherapy (IGABT) based on magnetic resonance imaging (MRI) and optimisation of the BT dose distribution to the remaining tumour and cervix at time of BT (HR CTV) was introduced in Aarhus. EBRT remained like in NOCECA until 2008 when the SIB to the primary tumour was abandoned and IMRT was introduced as routine technique. In this study, we report outcome of our first five-year experience with IGABT using our NOCECA cohort as reference. Material and methods. The NOCECA cohort comprising 99 patients was compared with 140 consecutive patients treated by IGABT. Patients with para-aortic nodes were excluded in NOCECA but were present in 9% of the patients treated with IGABT. No patient in NOCECA received chemotherapy whereas concomitant cisplatin was given to 79% of the IGABT patients. Results. With IGABT actuarial local control was 91% at three years. When comparing NOCECA with IGABT overall survival was significantly improved from 63% to 79% (p = 0.005). In parallel, both moderate and severe late morbidity were reduced by about 50% (p = 0.02). Conclusion. Introduction of IGABT reduced morbidity and generated a very high rate of local control, which likely has improved survival by at least as much as concomitant chemotherapy.

Dose distribution response in HDRB measured with EBT2 and compared with PLATO SYSTEM

Dose distribution of a High Dose Rate Brachytherapy (BHDR) oncological treatment with 192Ir was measured using a Gafchromic EBT2 film. The film calibration was performed with a 60Co unit and a LINAC of 6mV and 18mV. Gafchromic behavior of a dosimeter varies in respect of energy. Experimental results of dose distribution match with those planned in the PLATO commercial system, they also show that there is a difference of 2.11% between the planning system and isodoses measured.

A linear programming model for optimizing HDR brachytherapy dose distributions with respect to mean dose in the DVH-tail

Recent research has shown that the optimization model hitherto used in high-dose-rate (HDR) brachytherapy corresponds weakly to the dosimetric indices used to evaluate the quality of a dose distribution. Although alternative models that explicitly include such dosimetric indices have been presented, the inclusion of the dosimetric indices explicitly yields intractable models. The purpose of this paper is to develop a model for optimizing dosimetric indices that is easier to solve than those proposed earlier. In this paper, the authors present an alternative approach for optimizing dose distributions for HDR brachytherapy where dosimetric indices are taken into account through surrogates based on the conditional value-at-risk concept. This yields a linear optimization model that is easy to solve, and has the advantage that the constraints are easy to interpret and modify to obtain satisfactory dose distributions. The authors show by experimental comparisons, carried out retrospectively for a set of prostate cancer patients, that their proposed model corresponds well with constraining dosimetric indices. All modifications of the parameters in the authors' model yield the expected result. The dose distributions generated are also comparable to those generated by the standard model with respect to the dosimetric indices that are used for evaluating quality. The authors' new model is a viable surrogate to optimizing dosimetric indices and quickly and easily yields high quality dose distributions.

Analysis of high–dose rate brachytherapy dose distribution resemblance in CyberKnife hypofractionated treatment plans of localized prostate cancer

The present study is to analyze the CyberKnife hypofractionated dose distribution of localized prostate cancer in terms of high–dose rate (HDR) brachytherapy equivalent doses to assess the degree of HDR brachytherapy resemblance of CyberKnife dose distribution. Thirteen randomly selected localized prostate cancer cases treated using CyberKnife with a dose regimen of 36.25Gy in 5 fractions were considered. HDR equivalent doses were calculated for 30Gy in 3 fractions of HDR brachytherapy regimen. The D5% of the target in the CyberKnife hypofractionation was 41.57 ± 2.41Gy. The corresponding HDR fractionation (3 fractions) equivalent dose was 32.81 ± 1.86Gy. The mean HDR fractionation equivalent dose, D98%, was 27.93 ± 0.84Gy. The V100% of the prostate target was 95.57% ± 3.47%. The V100% of the bladder and the rectum were 717.16 and 79.6mm3, respectively. Analysis of the HDR equivalent dose of CyberKnife dose distribution indicates a comparable resemblance to HDR dose distribution in the peripheral target doses (D98% to D80%) reported in the literature. However, there is a substantial difference observed in the core high-dose regions especially in D10% and D5%. The dose fall-off within the OAR is also superior in reported HDR dose distribution than the HDR equivalent doses of CyberKnife.

In vivo dosimetry in brachytherapy

In vivo dosimetry (IVD) has been used in brachytherapy (BT) for decades with a number of different detectors and measurement technologies. However, IVD in BT has been subject to certain difficulties and complexities, in particular due to challenges of the high-gradient BT dose distribution and the large range of dose and dose rate. Due to these challenges, the sensitivity and specificity toward error detection has been limited, and IVD has mainly been restricted to detection of gross errors. Given these factors, routine use of IVD is currently limited in many departments. Although the impact of potential errors may be detrimental since treatments are typically administered in large fractions and with high-gradient-dose-distributions, BT is usually delivered without independent verification of the treatment delivery. This Vision 20/20 paper encourages improvements within BT safety by developments of IVD into an effective method of independent treatment verification.

Thermoluminescent and Monte Carlo dosimetry of a new 170Tm brachytherapy source

In this Study characteristics of a new 170Tm brachytherapy seed using thermoluminescent dosimeter and also the Monte Carlo simulations to evaluate between calculated and measured values was determined. Titanium tube contained Tm(NO3)3 powders bombardment at the Tehran Research Reactor (TRR) for a period of 7 days at a flux of 2–3 × 1013 neutrons/cm2 s. To obtain the radial dose function, g(r), and the anisotropy function, F(r, θ), according to the AAPM TG-43U1 recommendations, 30 cm × 30 cm × 15 cm phantoms of Perspex slabs were used. Brachytherapy dose distributions were simulated with the MCNP5 Monte Carlo (MC) radiation transport code. The MCPLIB04 photon cross-section library was applied using data from ENDF/B-VI. Cell-heating tally, F6 was employed to calculate absorbed dose in two separate runs for both beta and gamma particles. The calculated dose rate constant for the HDR source was found to be 1.113 ± 0.021 cGyU−1 h−1. Nominal uncertainty in the measured and calculated radial dose functions, g(r), for the IR-170Tm source in Perspex is tabulated is approximately 6% (ranging from 2% to 9%). The anisotropy function, F(r, θ), of the IR-170Tm source was measured at radial distances of r = 1.5, 2, 3, 5 cm relative to the seed center, and polar angles θ ranging from 0° to 330° in 30° increments.