Brachytherapy Dosimetry Research Articles

On source models for 192Ir HDR brachytherapy dosimetry using model based algorithms

A source model is a prerequisite of all model based dose calculation algorithms. Besides direct simulation, the use of pre-calculated phase space files (phsp source models) and parameterized phsp source models has been proposed for Monte Carlo (MC) to promote efficiency and ease of implementation in obtaining photon energy, position and direction. In this work, a phsp file for a generic 192Ir source design (Ballester et al ) is obtained from MC simulation. This is used to configure a parameterized phsp source model comprising appropriate probability density functions (PDFs) and a sampling procedure. According to phsp data analysis 15.6% of the generated photons are absorbed within the source, and 90.4% of the emergent photons are primary. The PDFs for sampling photon energy and direction relative to the source long axis, depend on the position of photon emergence. Photons emerge mainly from the cylindrical source surface with a constant probability over ±0.1 cm from the center of the 0.35 cm long source core, and only 1.7% and 0.2% emerge from the source tip and drive wire, respectively. Based on these findings, an analytical parameterized source model is prepared for the calculation of the PDFs from data of source geometry and materials, without the need for a phsp file. The PDFs from the analytical parameterized source model are in close agreement with those employed in the parameterized phsp source model. This agreement prompted the proposal of a purely analytical source model based on isotropic emission of photons generated homogeneously within the source core with energy sampled from the 192Ir spectrum, and the assignment of a weight according to attenuation within the source. Comparison of single source dosimetry data obtained from detailed MC simulation and the proposed analytical source model show agreement better than 2% except for points lying close to the source longitudinal axis.

Redesign of the Intracavitary Ring and Tandem Applicator for Improved Dosimetry in High Dose-Rate Brachytherapy

The commercial ring and tandem brachytherapy applicators (RTA) do not consistently match the specific anatomy of the endometrium and cervix of the patient. To conform variability in such situations we propose a redesign of the RTA used for cervical cancer treatments with the goal of improving patient-specific dosimetry. Unlike in the commercially available RTAs, the novel design of the RTA includes flexibility in positioning the ring with respect to the tandem. In particular, beside the possibility to place the tandem in the center of the ring, we propose an RTA capable of positioning the tandem distally and proximally with respect to the center of the ring (Fig.1). Therefore, the novel RTA has three possible variations in its final configuration. This RTA possesses additional flexibility to adapt different patient anatomies including daily variations in position and size of the organs at risk (OAR). For the evaluation of the benefit of the proposed methodology, 2 patients diagnosed with cervical cancer were treated with high dose-rate (HDR) brachytherapy using standard RTA and RTA in which the ring was placed distally towards the patient’s anterior surface during the same HDR brachytherapy course. A total of 10 brachytherapy treatment plans (TP) were analyzed; 5 per each patient. The HDR brachytherapy TPs were delivered as a boost after the patients had previously received 45 Gy in the initial external beam (EB) course. The prescription dose for the HDR treatments was 27.5 Gy in 5 fractions. The first patient was treated with 2 TPs generated using the standard RTA and 3 plans with the proposed RTA. The second patient was treated with 3 TPs generated using the standard RTA and 2 plans with the proposed RTA. The relevant dosimetric parameters such as D90 for the clinical target volumes (CTV) and D2cm3 for the OARs were compared to evaluate the patient dosimetry. An equivalent dose (EQD2) calculation was performed so that the total dose from 2 courses (EB and HDR) could be evaluated. In addition, we generated another 5 plans on the same computer tomography image sets to evaluate additional specific differences in dosimetry. When the modified RTA was used, the first patient received an average lower daily dose to the D2cm3 of the rectum, bladder and sigmoid of 54, 78 and 18 cGy, respectively, while the same CTV coverage (D90) was maintained. The average increase of the dose to the D2cm3 of the bowel was 29 cGy. This treatment resulted in a decrease of the total EQD2 of the rectum, bladder and sigmoid of 223, 488 and 105 cGy. The increase of the total EQD2 of the bowel was 182 cGy; however, the total dose to the bowel was significantly below the limits. The second patient received an average lower daily dose to the D2cm3 of the rectum and bladder of 52 and 44 cGy, which resulted in a decrease of the total EQD2 of 320 and 455 cGy, respectively. In this case, the increase of the total EQD2 for the CTV was 167 cGy. The total increase of the dose to the D2cm3 of the sigmoid was 156 cGy. The additional TPs generated for these patients revealed a possible decrease of the daily dose to the bladder, bowel, rectum and sigmoid of 33 (SD = 136) cGy, 1 (SD = 48) cGy, 34 (SD = 65) cGy and 67 (SD = 63) cGy, respectively. The novel design of the RTA has a strong potential for the complementary adjustment of the dose distributions. This RTA is capable of succeeding the daily variations of the patient’s anatomy so the daily dose to the critical OAR can be reduced. The increase of the total dose to the OARs (bowel in the first and sigmoid in the second case) can be regulated using the different position of the ring, so a greater decrease of the dose to the more affected OARs can be achieved. Therefore, the total equivalent dose can be modulated while the maximum dose constraints to the OARs are respected. Extended studies are required for additional conclusions.

In Vivo Rectal Wall Dosimetry in Gynecological HDR Brachytherapy Using a Semi-Flexible Rectal Probe Provided with MOSkin Dosimeters

Comparison between the BrachyVision calculated dose and dosimeter measurement was performed using CERR radiotherapy analysis software. Results: The dosimeter was reconstructed with 1 mm voxel spacing and a 3pixel median filter. Aside from the needle track itself, no fractures formed in the dosimeter from the insertion, resulting in a pristine readout free of major image artifacts. Line profiles show strong agreement between the calculated and measured doses across the entire distribution, with exceptionally high matching at the sharp gradients surrounding the site of insertion (Fig. 1). Gamma index calculation shows 99.9% of voxels passing at 3%/2mm criteria. Conclusions: This work introduces a new formulation of low-durometer, deformable Presage dosimeters, and demonstrates strong viability for their use in complete 3D validation of interstitial brachytherapy treatments. These dosimeters are able to address a growing need in brachytherapy dose planning, quality control, and likely treatment commissioning. Further work will test this dosimeter formulation with multiple needle insertions, anthropomorphically-correct anatomy, and varying deformation to closer represent real-life treatment scenarios.

Addendum to brachytherapy dose-volume histogram commissioning with multiple planning systems.

The process for validating dose‐volume histogram data in brachytherapy software is presented as a supplement to a previously published article. Included is the DVH accuracy evaluation of the Best NOMOS treatment planning system called “Best TPS VohvmePlan” As done previously in other software, a rectangular cuboid was contoured in the treatment planning system. A single radioactive 125I source was positioned coplanar and concentric with one end. Calculations were performed to estimate dose deposition in partial volumes of the cuboid structure, using the brachytherapy dosimetry formalism defined in AAPM Task Group 43. Hand‐calculated, dose‐volume results were compared to TPS‐generated, point‐source‐approximated dose‐volume histogram data to establish acceptance. The required QA for commissioning was satisfied for the DVH as conducted previously for other software, using the criterion that the DVH %VolTPS “actual variance” calculations should differ by no more than 5% at any specific radial distance with respect to %VolTG‐43, and the “average variance” DVH %VolTPS calculations should differ by no more than 2% over all radial distances with respect to %VolTG‐43. The average disagreement observed between hand calculations and treatment planning system DVH was less than 0.5% on average for this treatment planning system and less than 1.1% maximally for 1≤r≤5 cm.PACS number(s): 87.10.+e, 87.55.‐x, 87.53.Jw, 07.05.Tp

Ocular brachytherapy dosimetry for 103Pd and 125I in the presence of gold nanoparticles: a Monte Carlo study.

The aim of the present Monte Carlo study is to evaluate the variation of energy deposition in healthy tissues in the human eye which is irradiated by brachytherapy sources in comparison with the resultant dose increase in the gold nanoparticle (GNP)‐loaded choroidal melanoma. The effects of these nanoparticles on normal tissues are compared between 103Pd and 125I as two ophthalmic brachytherapy sources. Dose distribution in the tumor and healthy tissues has been taken into account for both brachytherapy sources. Also, in certain points of the eye, the ratio of the absorbed dose by the normal tissue in the presence of GNPs to the absorbed dose by the same point in the absence of GNPs has been calculated. In addition, differences of the absorbed dose in the tumor observed in the comparison of simple water phantom and actual simulated human eye in presence of GNPs are also a matter of interest that have been considered in the present work. The difference between the eye globe and the water phantom is more obvious for 125I than that of the 103Pd when the ophthalmic dosimetry is done in the presence of GNPs. Whenever these nanoparticles are utilized in enhancing the absorbed dose by the tumor, the use of 125I brachytherapy source will greatly amplify the amount of dose enhancement factor (DEF) in the tumor site without inflicting much damage to healthy organs, when compared to the 103Pd source. For instance, in the concentration of 30 mg GNPs, the difference amongst the calculated DEF for 125I between these phantoms is 5.3%, while it is 2.45% for 103Pd. Furthermore, in Monte Carlo studies of eye brachytherapy, more precise definition of the eye phantom instead of a water phantom will become increasingly important when we use 125I as opposed to 103Pd.PACS number(s): 87.53.Jw, 87.85.Rs, 87.10.Rt

Are We Ready to Replace Post-Implant CT Dosimetry? Permanent Prostate Brachytherapy Post-Implant MRI Dosimetry Using Positive Contrast MRI Markers

Are We Ready to Replace Post-Implant CT Dosimetry? Permanent Prostate Brachytherapy Post-Implant MRI Dosimetry Using Positive Contrast MRI Markers

Comment on "Comparison and uncertainty evaluation of different calibration protocols and ionization chambers for low-energy surface brachytherapy dosimetry" [Med. Phys. 42, 4954-4964 (2015)

Medical PhysicsVolume 43, Issue 4 p. 2005-2006 Correspondence Comment on “Comparison and uncertainty evaluation of different calibration protocols and ionization chambers for low-energy surface brachytherapy dosimetry” [Med. Phys. 42, 4954–4964 (2015)] Regina Fulkerson, Regina Fulkerson Author to whom correspondence should be addressed. Electronic mail: rmkenned@gmail.comSearch for more papers by this author Regina Fulkerson, Regina Fulkerson Author to whom correspondence should be addressed. Electronic mail: rmkenned@gmail.comSearch for more papers by this author First published: 31 March 2016 https://doi.org/10.1118/1.4942806Citations: 1 0094-2405, Med. Phys., 42, 4954, 4964, 2015 Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume43, Issue4April 2016Pages 2005-2006 RelatedInformation

Response to "Comment on 'Comparison and uncertainty evaluation of different calibration protocols and ionization chambers for low-energy surface brachytherapy dosimetry' " [Med. Phys. 42, 4954-4964 (2015)

Response to "Comment on 'Comparison and uncertainty evaluation of different calibration protocols and ionization chambers for low-energy surface brachytherapy dosimetry' " [Med. Phys. 42, 4954-4964 (2015)

Deformable registration of X-ray to MRI for post-implant dosimetry in prostate brachytherapy.

Post-implant dosimetric assessment in prostate brachytherapy is typically performed using CT as the standard imaging modality. However, poor soft tissue contrast in CT causes significant variability in target contouring, resulting in incorrect dose calculations for organs of interest. CT-MR fusion-based approach has been advocated taking advantage of the complementary capabilities of CT (seed identification) and MRI (soft tissue visibility), and has proved to provide more accurate dosimetry calculations. However, seed segmentation in CT requires manual review, and the accuracy is limited by the reconstructed voxel resolution. In addition, CT deposits considerable amount of radiation to the patient. In this paper, we propose an X-ray and MRI based post-implant dosimetry approach. Implanted seeds are localized using three X-ray images by solving a combinatorial optimization problem, and the identified seeds are registered to MR images by an intensity-based points-to-volume registration. We pre-process the MR images using geometric and Gaussian filtering. To accommodate potential soft tissue deformation, our registration is performed in two steps, an initial affine transformation and local deformable registration. An evolutionary optimizer in conjunction with a points-to-volume similarity metric is used for the affine registration. Local prostate deformation and seed migration are then adjusted by the deformable registration step with external and internal force constraints. We tested our algorithm on six patient data sets, achieving registration error of (1.2±0.8) mm in < 30 sec. Our proposed approach has the potential to be a fast and cost-effective solution for post-implant dosimetry with equivalent accuracy as the CT-MR fusion-based approach.

Verification of Oncentra brachytherapy planning using independent calculation

This study was done to investigate the verification technique of treatment plan quality assurance for brachytherapy. It is aimed to verify the point doses in 192Ir high dose rate (HDR) brachytherapy between Oncentra Masterplan brachytherapy treatment planning system and independent calculation software at a region of rectum, bladder and prescription points for both pair ovoids and full catheter set ups. The Oncentra TPS output text files were automatically loaded into the verification programme that has been developed based on spreadsheets. The output consists of source coordinates, desired calculation point coordinates and the dwell time of a patient plan. The source strength and reference dates were entered into the programme and then dose point calculations were independently performed. The programme shows its results in a comparison of its calculated point doses with the corresponding Oncentra TPS outcome. From the total of 40 clinical cases that consisted of two fractions for 20 patients, the results that were given in term of percentage difference, it shows an agreement between TPS and independent calculation are in the range of 2%. This programme only takes a few minutes to be used is preferably recommended to be implemented as the verification technique in clinical brachytherapy dosimetry.

Simulation evaluation of NIST air-kerma rate calibration standard for electronic brachytherapy.

Dosimetry for the model S700 50 kV electronic brachytherapy (eBT) source (Xoft, Inc., a subsidiary of iCAD, San Jose, CA) was simulated using Monte Carlo (MC) methods by Rivard et al. ["Calculated and measured brachytherapy dosimetry parameters in water for the Xoft Axxent x-ray source: An electronic brachytherapy source," Med. Phys. 33, 4020-4032 (2006)] and recently by Hiatt et al. ["A revised dosimetric characterization of the model S700 electronic brachytherapy source containing an anode-centering plastic insert and other components not included in the 2006 model," Med. Phys. 42, 2764-2776 (2015)] with improved geometric characterization. While these studies examined the dose distribution in water, there have not previously been reports of the eBT source calibration methods beyond that recently reported by Seltzer et al. ["New national air-kerma standard for low-energy electronic brachytherapy sources," J. Res. Natl. Inst. Stand. Technol. 119, 554-574 (2014)]. Therefore, the motivation for the current study was to provide an independent determination of air-kerma rate at 50 cm in air K̇air(d=50 cm) using MC methods for the model S700 eBT source. Using CAD information provided by the vendor and disassembled sources, an MC model was created for the S700 eBT source. Simulations were run using the mcnp6 radiation transport code for the NIST Lamperti air ionization chamber according to specifications by Boutillon et al. ["Comparison of exposure standards in the 10-50 kV x-ray region," Metrologia 5, 1-11 (1969)], in air without the Lamperti chamber, and in vacuum without the Lamperti chamber. K̇air(d=50 cm) was determined using the *F4 tally with NIST values for the mass energy-absorption coefficients for air. Photon spectra were evaluated over 2 π azimuthal sampling for polar angles of 0° ≤ θ ≤ 180° every 1°. Volume averaging was averted through tight radial binning. Photon energy spectra were determined over all polar angles in both air and vacuum using the F4 tally with 0.1 keV resolution. A total of 10(11) simulated histories were run for the Lamperti chamber geometry (statistical uncertainty of 0.14%), with 10(10) histories for the in-air and in-vacuum simulations (statistical uncertainty of 0.04%). The total standard uncertainty in the calculated air-kerma rate determination amounted to 6.8%. MC simulations determined the air-kerma rate at 50 cm from the source with the modeled Lamperti chamber to be (1.850 ± 0.126) × 10(-4) Gy/s, which was within the range of K̇air(d=50 cm) values (1.67-2.11) × 10(-4) Gy/s measured by NIST. The ratio of the photon spectra in air and in vacuum were in good agreement above 13 keV, and for θ < 150° where the influence of the Kovar sleeve and the Ag epoxy components caused increased scatter in air. Below 13 keV, the ratio of the photon spectra in air to vacuum exhibited a decrease that was attributed to increased attenuation of the photons in air. Across most of the energy range on the source transverse plane, there was good agreement between the authors' simulated spectra and that measured by NIST. Discrepancies were observed above 40 keV where the NIST spectrum had a steeper fall-off towards 50 keV. Through MC simulations of radiation transport, this study provided an independent validation of the measured air-kerma rate at 50 cm in air at NIST for the model S700 eBT source, with mean results in agreement within 3.3%. This difference was smaller than the range (i.e., 23%) of the measured values.

Film based verification of calculation algorithms used for brachytherapy planning-getting ready for upcoming challenges of MBDCA.

PurposeWell-known defect of TG-43 based algorithms used in brachytherapy is a lack of information about interaction cross-sections, which are determined not only by electron density but also by atomic number. TG-186 recommendations with using of MBDCA (model-based dose calculation algorithm), accurate tissues segmentation, and the structure's elemental composition continue to create difficulties in brachytherapy dosimetry. For the clinical use of new algorithms, it is necessary to introduce reliable and repeatable methods of treatment planning systems (TPS) verification. The aim of this study is the verification of calculation algorithm used in TPS for shielded vaginal applicators as well as developing verification procedures for current and further use, based on the film dosimetry method.Material and methodsCalibration data was collected by separately irradiating 14 sheets of Gafchromic® EBT films with the doses from 0.25 Gy to 8.0 Gy using HDR 192Ir source. Standard vaginal cylinders of three diameters were used in the water phantom. Measurements were performed without any shields and with three shields combination. Gamma analyses were performed using the VeriSoft® package.ResultsCalibration curve was determined as third-degree polynomial type. For all used diameters of unshielded cylinder and for all shields combinations, Gamma analysis were performed and showed that over 90% of analyzed points meets Gamma criteria (3%, 3 mm).ConclusionsGamma analysis showed good agreement between dose distributions calculated using TPS and measured by Gafchromic films, thus showing the viability of using film dosimetry in brachytherapy.

Dosimetry of permanent interstitial prostate brachytherapy for an interoperative procedure, using O-arm based CT and TRUS

PurposeThe aim of this report is dosimetric evaluation for an intraoperative fusion computed tomography (CT) as a superior predictor of 1-month CT based dosimetry in comparison to transrectal ultrasound (TRUS) in permanent interstitial prostate brachytherapy.Material and methodsData of 65 patients treated with seed implantation were analyzed. All procedures has been performed with patients in the lithotomy position inside the O-arm system. An end-fine probe is used as a landmark to fuse TRUS and O-arm-based CT images. There was no difference in the patient's position, probe position, and timing of image acquisition between the two imaging modalities. Dose-volume histogram (DVH) parameters such as the dose to 90% of prostate volume (D90) has been analyzed.ResultsThe area under the curve of the receiver operating characteristic tended to be larger on fusion CT than on TRUS for most DVH parameters (71.85% vs. 59.59% for D90; p = 0.07). Significant relationships between fusion CT and 1-month CT were confirmed using Pearson's correlation coefficients for most DVH parameters (R = 0.48, p < 0.01 for D90), although the relationship between TRUS and 1-month CT was poor. Large dose reduction (35 Gy for D90) was seen from TRUS to fusion CT, especially in patients with high body weight and small prostate volume.ConclusionsIntraoperative fusion CT appears to have higher predictive power for 1-month CT-based dosimetry than TRUS. A prospective trial using fusion CT-based planning is warranted.

Focal partial salvage low-dose-rate brachytherapy for local recurrent prostate cancer after permanent prostate brachytherapy with a review of the literature.

PurposeTo investigate the treatment results for focal partial salvage re-implantation against local recurrence after permanent prostate brachytherapy.Material and methodsBetween January 2010 and September 2015, 12 patients were treated with focal partial salvage re-implantation for local recurrence after low-dose-rate brachytherapy using 125I seeds. The focal clinical target volume (F-CTV) was delineated on positive biopsy areas in a mapping biopsy, combining the cold spots on the post-implant dosimetry for initial brachytherapy. The F-CTV was expanded by 3 mm to create the planning target volume (PTV) as a margin to compensate for uncertainties in image registration and treatment delivery. The prescribed dose to the PTV was 145 Gy. The characteristics and biochemical disease-free survival (BdFS) rates were analyzed. Genitourinary (GU) and gastrointestinal (GI) toxicities were evaluated using the Common Terminology Criteria for Adverse Events version 4.ResultsThe median prostate-specific antigen (PSA) level at re-implantation was 4.09 ng/ml (range: 2.91-8.24 ng/ml). The median follow-up time was 56 months (range: 6-74 months). The median RD2cc and UD10 were 63 Gy and 159 Gy, respectively. The 4-year BdFS rate was 78%, which included non-responders. Biochemical recurrence occurred in two patients after 7 and 31 months, respectively. The former was treated with hormonal therapy after biochemical failure, and the latter underwent watchful waiting (PSA at the last follow-up of 53 months: 7.3 ng/ml) at the patient's request. No patients had grade 3 GU/GI toxicities or died after salvage re-implantation.ConclusionsThe partial salvage low-dose-rate brachytherapy used to treat local recurrence after permanent prostate brachytherapy is well-tolerated, with high biochemical response rates. This treatment can be not only a method to delay chemical castration but also a curative treatment option in cases of local recurrence of prostate carcinoma after seed implantation.

Prostate deformation from inflatable rectal probe cover and dosimetric effects in prostate seed implant brachytherapy: Deformation on prostate permanent brachytherapy dosimetry

Prostate deformation from inflatable rectal probe cover and dosimetric effects in prostate seed implant brachytherapy: Deformation on prostate permanent brachytherapy dosimetry

Commissioning of a well type chamber for HDR and LDR brachytherapy applications: a review of methodology and outcomes.

For safe and accurate dose delivery in brachytherapy, associated equipment is subject to commissioning and ongoing quality assurance (QA). Many centres depend on the use of a well-type chamber ('well chamber') for performing brachytherapy dosimetry. Documentation of well chamber commissioning is scarce despite the important role the chamber plays in the whole brachytherapy QA process. An extensive and structured commissioning of the HDR 1000 plus well chamber (Standard Imaging Inc, Middleton WI) for HDR and LDR dosimetry was undertaken at Sir Charles Gairdner Hospital. The methodology and outcomes of this commissioning is documented and presented as a guideline to others involved in brachytherapy. The commissioning tests described include mechanical integrity, leakage current, directional dependence, response, length of uniform response, the influence of insert holders, ion collection efficiency, polarity effect, accuracy of measured air kerma strength (S(K)) or reference air kerma rate (K(R)) and baseline setting (for ongoing constancy checks). For the HDR 1000 plus well chamber, some of the insert holders modify the response curve. The measured sweet length was 2.5 cm which is within 0.5% of that specified by the manufacturer. Correction for polarity was negligible (0.9999) and ion recombination was small (0.9994). Directional dependence was small (less than 0.2%) and leakage current was negligible. The measured K(R) for (192)Ir agreed within 0.11% compared with a second well chamber of similar model and was within 0.5% of that determined via a free-in-air measurement method. Routine constancy checks over a year agreed with the baseline within 0.4%.

Real Time Dosimetry for Gynecologic Brachytherapy: Initial Results of a Prospective Clinical Trial

Real Time Dosimetry for Gynecologic Brachytherapy: Initial Results of a Prospective Clinical Trial

MRI-guided single fraction ablative radiotherapy for early-stage breast cancer: a brachytherapy versus volumetric modulated arc therapy dosimetry study

Background and purposeA radiosurgical treatment approach for early-stage breast cancer has the potential to minimize the patient’s treatment burden. The dosimetric feasibility for single fraction ablative radiotherapy was evaluated by comparing volumetric modulated arc therapy (VMAT) with an interstitial multicatheter brachytherapy (IMB) approach. Methods and materialsThe tumors of 20 patients with early-stage breast cancer were delineated on a preoperative contrast-enhanced planning CT-scan, co-registered with a contrast-enhanced magnetic resonance imaging (MRI), both in radiotherapy supine position. A dose of 15Gy was prescribed to the planned target volume of the clinical target volume (PTVCTV), and 20Gy integrated boost to the PTV of the gross tumor volume (PTVGTV). Treatment plans for IMB and VMAT were optimized for adequate target volume coverage and minimal organs at risk (OAR) dose. ResultsThe median PTVGTV/CTV receiving at least 95% of the prescribed dose was ⩾99% with both techniques. The median PTVCTV unintentionally receiving 95% of the prescribed PTVGTV dose was 65.4% and 4.3% with IMB and VMAT, respectively. OAR doses were comparable with both techniques. ConclusionMRI-guided single fraction radiotherapy with an integrated ablative boost to the GTV is dosimetrically feasible with both techniques. We perceive IMB less suitable for clinical implementation due to PTVCTV overdosage. Future studies have to confirm the clinical feasibility of the single fraction ablative approach.

The mean photon energy ĒF at the point of measurement determines the detector-specific radiation quality correction factor kQ,M in 192Ir brachytherapy dosimetry

The application of various radiation detectors for brachytherapy dosimetry has motivated this study of the energy dependence of radiation quality correction factor kQ,M, the quotient of the detector responses under calibration conditions at a 60Co unit and under the given non-reference conditions at the point of measurement, M, occurring in photon brachytherapy. The investigated detectors comprise TLD, radiochromic film, ESR, Si diode, plastic scintillator and diamond crystal detectors as well as ionization chambers of various sizes, whose measured response-energy relationships, taken from the literature, served as input data. Brachytherapy photon fields were Monte-Carlo simulated for an ideal isotropic 192Ir point source, a model spherical 192Ir source with steel encapsulation and a commercial HDR GammaMed Plus source. The radial source distance was varied within cylindrical water phantoms with outer radii ranging from 10 to 30cm and heights from 20 to 60cm. By application of this semiempirical method - originally developed for teletherapy dosimetry - it has been shown that factor kQ,M is closely correlated with a single variable, the fluence-weighted mean photon energy ĒF at the point of measurement. The radial profiles of ĒF obtained with either the commercial 192Ir source or the two simplified source variants show little variation. The observed correlations between parameters kQ,M and ĒF are represented by fitting formulae for all investigated detectors, and further variation of the detector type is foreseen. The herewith established close correlation of radiation quality correction factor kQ,M with local mean photon energy ĒF can be regarded as a simple regularity, facilitating the practical application of correction factor kQ,M for in-phantom dosimetry around 192Ir brachytherapy sources. ĒF values can be assessed by Monte Carlo simulation or measurement. A technique describing the local measurement of ĒF will be published separately.

Urethral and bladder dosimetry of total and focal salvage Iodine-125 prostate brachytherapy: Late toxicity and dose constraints

IntroductionSalvage Iodine-125 brachytherapy (I-125-BT) constitutes a curative treatment approach for patients with organ-confined recurrent prostate cancer after primary radiotherapy. Currently, focal salvage (FS) instead of whole-gland or total salvage (TS) is being investigated, to reduce severe toxicity associated with cumulative radiation dose. Differences in urethral and bladder dosimetry and constraints to reduce late (>90days) genitourinary (GU) toxicity are presented here. Materials and methodsDosimetry on intraoperative ultrasound (US) of 20 FS and 28 TS patients was compared. The prostate, bladder, urethra and bulbomembranous (BM) urethra were delineated. Toxicity was assessed using the CTCAE version 4.0. Dose constraints to reduce toxicity in TS patients were evaluated with receiver operating characteristic (ROC) analysis. ResultsFS I-125 BT significantly reduces bladder and urethral dose compared to TS. Grade 3 GU toxicity occurred once in the FS group. For TS patients late severe (⩾grade 3) GU toxicity was frequent (38% in the total 61 patients and 56% in the 27 analyzed patients). TS patients with ⩾grade 3 GU toxicity showed higher bladder D2cc than TS patients without toxicity (median 43Gy) (p=0.02). The urethral V100 was significantly higher in TS patients with several toxicity profiles: ⩾grade 3 urethral strictures, ⩾grade 2 urinary retention and multiple ⩾grade 2 GU toxicity events. Dose to the BM urethra did not show a relation with stricture formation. ROC-analysis indicated a bladder D2cc <70Gy to prevent ⩾grade 3 GU toxicity (AUC 0.76, 95%CI: 0.56–0.96, p=0.02). A urethral V100 <0.40cc (AUC from 0.73–0.91, p=0.003–0.05) could prevent other late GU toxicity. ConclusionFS I-125 BT reduces urethral and bladder dose significantly compared to TS. With TS, there is an increased risk of cumulative dose and severe GU toxicity. Based on these findings, bladder D2cc should be below 70Gy and urethral V100 below 0.40cc.