Brachytherapy treatment verification using gamma radiation from the internal treatment source combined with an imaging panel-a phantom study.

Brachytherapy has an excellent clinical outcome for different treatment sites. However,in vivotreatment verification is not performed in the majority of hospitals due to the lack of proper monitoring systems. This study investigates the use of an imaging panel (IP) and the photons emitted by a high dose rate (HDR)192Ir source to track source motion and obtain some information related to the patient anatomy. The feasibility of this approach was studied by monitoring the treatment delivery to a 3D printed phantom that mimicks a prostate patient. A 3D printed phantom was designed with a template for needle insertion, a cavity ('rectum') to insert an ultrasound probe, and lateral cavities used to place tissue-equivalent materials. CT images were acquired to create HDR192Ir treatment plans with a range of dwell times, interdwell distances and needle arrangements. Treatment delivery was verified with an IP placed at several positions around the phantom using radiopaque markers on the outer surface to register acquired IP images with the planning CT. All dwell positions were identified using acquisition times ≤0.11 s (frame rates ≥ 9 fps). Interdwell distances and dwell positions (in relation to the IP) were verified with accuracy better than 0.1 cm. Radiopaque markers were visible in the acquired images and could be used for registration with CT images. Uncertainties for image registration (IP and planning CT) between 0.1 and 0.4 cm. The IP is sensitive to tissue-mimicking insert composition and showed phantom boundaries that could be used to improve treatment verification. The IP provided sufficient time and spatial resolution for real-time source tracking and allows for the registration of the planning CT and IP images. The results obtained in this study indicate that several treatment errors could be detected including swapped catheters, incorrect dwell times and dwell positions.

Characterization of passive dosimeters in proton pencil beam scanning - A EURADOS intercomparison for mailed dosimetry audits in proton therapy centres.

The lack of mailed dosimetry audits of proton therapy centres in Europe has encouraged researchers of EURADOS Working Group 9 (WG9) to compare response of several existing passive detector systems in therapeutic pencil beam scanning. Alanine Electron Paramagnetic Resonance dosimetry systems from 3 different institutes (ISS, Italy; UH, Belgium and IFJ PAN, Poland), natLiF:Mg, Ti (MTS-N) and natLiF:Mg, Cu, P (MCP-N) thermoluminescent dosimeters (TLDs), GD-352M radiophotoluminescent glass dosimeters (RPLGDs) and Al2O3:C optically stimulated dosimeters (OSLDs) were evaluate. Dosimeter repeatability, batch reproducibility and response in therapeutic Pencil Beam Scanning were verified for implementation as mail auditing system. Alanine detectors demonstrated the lowest linear energy transfer (LET) dependence with an agreement between measured and treatment planning system (TPS) dose below 1%. The OSLDs measured on average a 6.3% lower dose compared to TPS calculation, with no significant difference between varying modulations and ranges. Both GD-352M and MCP-N measured a lower dose than the TPS and luminescent response was dependent on the LET of the therapeutic proton beam. Thermoluminescent response of MTS-N was also found to be dependent on the LET and a higher dose than TPS was measured with the most pronounced increase of 11%. As alanine detectors are characterized by the lowest energy dependence for different parameters of therapeutic pencil beam scanning they are suitable candidates for mail auditing in proton therapy. The response of luminescence detector systems have shown promises even though more careful calibration and corrections are needed for its implementation as part of a mailed dosimetry audit system.

Dose distribution for gynecological brachytherapy with dose accumulation between insertions: Feasibility study.

PURPOSE: For gynecological treatments, it is standard to acquire CT images and preferably also MR images before each treatment to calculate the dose of the day. The dose of the complete treatment is calculated by adding the dose metrics of each fraction. It makes the conservative assumption that the same part of the organs at risk always receives the highest dose. The dose calculated this way often limits the prescription dose or the target coverage. We investigated the use of deformable image registration (DIR) as an alternative method to assess the cumulative dose for a treatment course. METHODS AND MATERIALS: Rigid registration is preformed on CT images, followed by DIR. DIR can be based either solely on the three-dimensional images or combined with organ contours. To improve DIR in the pelvic region with low CT contrast, we propose (1) using contours drawn on CT or (2) modifying artificially the contrast in certain volumes. The dose matrix from fraction_n (n > 1) is deformed using a calculated deformation field. RESULTS: The use of the contrast-enhanced images or of contour information helps to guide the DIR. However, because of the very high dose gradients involved in brachytherapy, the uncertainty on the accumulated dose remains of the order of 5-10%. Even for good contour matching, a small local error in the deformation can have significant consequences for the dose distribution. CONCLUSIONS: Using DIR, based on image features and contours, allows to accumulate the dose from different brachytherapy fractions. A robust validation procedure should be developed.

The contribution from transit dose for (192)Ir 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 (192)Ir 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.

ALGEBRA: ALgorithm for the heterogeneous dosimetry based on GEANT4 for BRAchytherapy.

Task group 43 (TG43)-based dosimetry algorithms are efficient for brachytherapy dose calculation in water. However, human tissues have chemical compositions and densities different than water. Moreover, the mutual shielding effect of seeds on each other (interseed attenuation) is neglected in the TG43-based dosimetry platforms. The scientific community has expressed the need for an accurate dosimetry platform in brachytherapy. The purpose of this paper is to present ALGEBRA, a Monte Carlo platform for dosimetry in brachytherapy which is sufficiently fast and accurate for clinical and research purposes. ALGEBRA is based on the GEANT4 Monte Carlo code and is capable of handling the DICOM RT standard to recreate a virtual model of the treated site. Here, the performance of ALGEBRA is presented for the special case of LDR brachytherapy in permanent prostate and breast seed implants. However, the algorithm is also capable of handling other treatments such as HDR brachytherapy.

In vivo dosimetry for gynaecological brachytherapy using a novel position sensitive radiation detector: feasibility study.

PURPOSE: In gynecological radiotherapy with high dose rate (HDR)(192)Ir brachytherapy, the treatment complexity has increased due to improved optimization techniques and dose constraints. As a consequence, it has become more important to verify the dose delivery to the target and also to the organs at risk (e.g., the bladder). In vivo dosimetry, where dosimeters are placed in or on the patient, is one way of verifying the dose but until recently this was hampered by motion of the radiation detectors with respect to the source. The authors present a novel dosimetry method using a position sensitive radiation detector. METHODS: The prototype RADPOS system (Best Medical Canada) consists of a metal oxide field effect transistor (MOSFET) dosimeter coupled to a position-sensor, which deduces its 3D position in a magnetic field. To assess the feasibility of in vivo dosimetry based on the RADPOS system, different characteristics of the detector need to be investigated. Using a PMMA phantom, the positioning accuracy of the RADPOS system was quantified by comparing position readouts with the known position of the detector along the x and y-axes. RADPOS dose measurements were performed at various distances from a Nucletron(192)Ir source in a PMMA phantom to evaluate the energy dependence of the MOSFET. A sensitivity analysis was performed by calculating the dose after varying (1) the position of the RADPOS detector to simulate organ motion and (2) the position of the first dwell position to simulate errors in delivery. The authors also performed an uncertainty analysis to determine the action level (AL) that should be used during in vivo dosimetry. RESULTS: Positioning accuracy is found to be within 1 mm in the 1-10 cm range from the origin along the x-axis (away from the transmitter), meeting the requirements for in vivo dosimetry. Similar results are obtained for the other axes. The ALs are chosen to take into account the total uncertainty on the measurements. As a consequence for in vivo dosimetry, it is determined that the RADPOS sensor, if placed, for example, in the bladder Foley balloon, would detect a 2 mm motion of the bladder, at a 5% chance of a false positive, with an AL limit of 9% of the dose delivered. The authors found that source position errors, caused by, e.g., a wrong first dwell position, are more difficult to detect; indeed, with our single RADPOS detector, positioned in the bladder, dwell position errors below 5 mm and resulting in a dose error within 10%, could be detected in the tandem but not in the colpostats. A possible solution to improve error detection is to use multiple MOSFETs to obtain multiple dose values. CONCLUSIONS: In this study, the authors proposed a dosimetry procedure, based on the novel RADPOS system, to accurately determine the position of the radiation dosimeter with respect to the applicator. The authors found that it is possible to monitor the delivered dose in a point and compare it to the predetermined dose. This allows in principle the detection of problems such as bladder motion/filling or source mispositioning. Further clinical investigation is warranted.

Monte Carlo study of LDR seed dosimetry with an application in a clinical brachytherapy breast implant.

A Monte Carlo (MC) study was carried out to evaluate the effects of the interseed attenuation and the tissue composition for two models of 125I low dose rate (LDR) brachytherapy seeds (Medi-Physics 6711, IBt InterSource) in a permanent breast implant. The effect of the tissue composition was investigated because the breast localization presents heterogeneities such as glandular and adipose tissue surrounded by air, lungs, and ribs. The absolute MC dose calculations were benchmarked by comparison to the absolute dose obtained from experimental results. Before modeling a clinical case of an implant in heterogeneous breast, the effects of the tissue composition and the interseed attenuation were studied in homogeneous phantoms. To investigate the tissue composition effect, the dose along the transverse axis of the two seed models were calculated and compared in different materials. For each seed model, three seeds sharing the same transverse axis were simulated to evaluate the interseed effect in water as a function of the distance from the seed. A clinical study of a permanent breast 125I implant for a single patient was carried out using four dose calculation techniques: (1) A TG-43 based calculation, (2) a full MC simulation with realistic tissues and seed models, (3) a MC simulation in water and modeled seeds, and (4) a MC simulation without modeling the seed geometry but with realistic tissues. In the latter, a phase space file corresponding to the particles emitted from the external surface of the seed is used at each seed location. The results were compared by calculating the relevant clinical metrics V85, V100, and V200 for this kind of treatment in the target. D90 and D50 were also determined to evaluate the differences in dose and compare the results to the studies published for permanent prostate seed implants in literature. The experimental results are in agreement with the MC absolute doses (within 5% for EBT Gafchromic film and within 7% for TLD-100). Important differences between the dose along the transverse axis of the seed in water and in adipose tissue are obtained (10% at 3.5 cm). The comparisons between the full MC and the TG-43 calculations show that there are no significant differences for V85 and V100. For V200, 8.4% difference is found coming mainly from the tissue composition effect. Larger differences (about 10.5% for the model 6711 seed and about 13% for the InterSource125) are determined for D90 and D50. These differences depend on the composition of the breast tissue modeled in the simulation. A variation in percentage by mass of the mammary gland and adipose tissue can cause important differences in the clinical dose metrics V200, D90, and D50. Even if the authors can conclude that clinically, the differences in V85, V100, and V200 are acceptable in comparison to the large variation in dose in the treated volume, this work demonstrates that the development of a MC treatment planning system for LDR brachytherapy will improve the dose determination in the treated region and consequently the dose-outcome relationship, especially for the skin toxicity.

Poster - Thurs Eve-05: An assessment of PDDs and outputs predicted by a Monte Carlo-based treatment planning system for electron beams.

Monte Carlo simulation is currently considered to be the most accurate method of calculating dose distributions for electron beam therapy, and commercial treatment planning software using simplified macro Monte Carlo is available for electron treatment planning. In this work, Eclipse V8.1.18 is being investigated in preparation for the clinical use of CT-based electron treatment planning. Water tank measurements of percentage depth doses (PDDs) and absolute outputs at depth of maximum dose (Zmax ) under different geometric conditions are compared to the results calculated by Eclipse. The measurements are carried out for a range of electron energies (6, 9, 12, and 16 MeV) for the standard open field (10×10 cm2 ) and for circular cutouts (2, 3, and 6 cm diameters) at SSD of 100 cm. In addition, extended SSDs (105 and 110 cm) and oblique beam incident (gantry 345 degree) for the open field and 3 cm diameter cutout are measured and compared to Eclipse. For PDDs, the results predicted by Eclipse are generally acceptable, falling mostly within 5% of those measured in water. For output, the results predicted by Eclipse are similar, falling mostly within 3% of those measured in water. We observed the greatest differences between Eclipse and measurements near the water surface and in high dose gradients for PDDs. A similar observation is noted for a small field in the case of outputs.

Sci-Sat AM(2): Brachy-07: Tomosynthesis-based seed reconstruction in LDR prostate brachytherapy: A clinical study.

To develop a tomosynthesis-based dose assessment procedure that can be performed after an I-125 prostate seed implantation, while the patient is still under anaesthesia on the treatment table. Our seed detection procedure involves the reconstruction of a volume of interest based on the backprojection of 7 seed-only binary images acquired over an angle of 60° with an isocentric imaging system. A binary seed-only volume is generated by a simple thresholding of the volume of interest. Seeds positions are extracted from this volume with a 3D connected component analysis and a statistical classifier that determines the number of seeds in each cluster of connected voxels. A graphical user interface (GUI) allows to visualize the result and to introduce corrections, if needed. A phantom and a clinical study (24 patients) were carried out to validate the technique. A phantom study demonstrated a very good localization accuracy of (0.4+/-0.4) mm when compared to CT-based reconstruction. This leads to dosimetric error on D90 and V100 of respectively 0.5% and 0.1%. In a patient study with an average of 56 seeds per implant, the automatic tomosynthesis-based reconstruction yields a detection rate of 96% of the seeds and less than 1.5% of false-positives. With the help of the GUI, the user can achieve a 100% detection rate in an average of 3 minutes. This technique would allow to identify possible underdosage and to correct it by potentially reimplanting additional seeds. A more uniform dose coverage could then be achieved in LDR prostate brachytherapy.

Sci-Sat AM(2): Brachy-05: Dosimetry effects of the TG-43 approximations for two iodine seeds in LDR brachytherapy.

PURPOSES: This work consists of studying the interseed and tissue composition effects for two model iodine seeds: the IBt Interseed-125 and the 6711 model seed. MATERIALS & METHODS: Three seeds were modeled with the MCNP MC code in a water sphere to evaluate the interseed effect. The dose calculated at different distances from the centre was compared to the dose summed when the seeds were simulated separately. The tissue composition effect was studied calculating the radial dose function for different tissues. Before carrying out post-implant studies, the absolute dose calculated by MC was compared to experiment results: with LiF TLDs in an acrylic breast phantom and with an EBT Gafchromic film placed in a water tank. Afterwards, the TG-43 approximation effects were studied for a prostate and breast post-implant. RESULTS AND DISCUSSION: The interseed effect study shows that this effect is more important for model 6711 (15%) than for IBt (10%) due to the silver rod in 6711. For both seed models the variations of the radial dose function as a function of the tissue composition are quasi similar. The absolute dose comparisons between MC calculations and experiments give good agreement (inferior to 3% in general). For the prostate and breast post-implant studies, a 10% difference between MC calculations and the TG-43 is found for both models of seeds. CONCLUSION: This study shows that the differences in dose distributions between TG43 and MC are quite similar for the two models of seeds and are about 10% for the studied post-implant treatments.

Sci-Sat AM(2): Brachy-04: Spectral and dosimetric study of the Xoft electronic brachytherapy system.

The Axxent developed by Xoft Inc. is a miniature x-ray tube capable of generating a 50 kVp x-ray spectrum with dose-rates suitable for HDR applications. Results of spectral measurements compared with Geant4 Monte Carlo simulations have been published. This study is a continuation of previous work with shifting emphasis towards dosimetric characterization of the miniature x-ray tube. Dose distributions using EBT Gafchromic films agree to within 10 % of Geant4 results. In addition, TG-43 parameters can be calculated. However, consideration should be given to the biological effectiveness of the spectrum at different depths. Spectral measurements show significant beam hardening with 1st HVL increasing from 0.55 to 1.20 mm Al after 11.50 mm of water filtration. This effect may be attributed to the significant loss of low energy characteristic photons. Furthermore, the degree of beam hardening is dependent of the material, with 1st HVLs of 1.20 and 1.03 mm Al after 11.50 mm of water and Lucite respectively. The biological effect is quantified by calculating the number of single and double strand breaks. The number of strand breaks for the 50 kVp x-ray spectrum is similar to that of I-125 radiation.

The microdosimetry of low-energy photons in radiotherapy.

Low energy photons are more and more in use in clinical practice, for treatment in radiotherapy as well as for imaging purposes. Their relative biological effectiveness is however still debated. In this paper, some microdosimetric parameters have been calculated for different sources: (125)I, (103)Pd, (131)Cs, an electronic brachytherapy source and various clinical mammography X-ray qualities. These parameters have been used to deduce the quality factors as defined in ICRU 40.

Microdosimetric analysis of various mammography spectra: lineal energy distributions and ionization cluster analysis.

In view of recent recommendations on the frequency and the starting age of mammography screening in healthy women, it is desirable to quantify the enhanced relative biological effectiveness (RBE) of mammography X rays compared to hard X rays. While there is little doubt that the former are more potent in inducing biological damage than the latter, the magnitude of the effect is still hotly debated in the literature. We used Monte Carlo simulations and track structure analysis in micrometer and nanometer volumes to investigate differences in distributions of lineal energy and ionization clusters for a range of mammography X-ray qualities. Dose-averaged lineal energies, (yD), in breast tissue for various mammography qualities were found to result in quality factors about 40% higher than unity. Among the various mammography qualities studied, the popular molybdenum/molybdenum target/filter combination was found to have the highest (yD) in 1-microm spheres (about 5.0 keV/microm near the entrance surface of breast tissue). In 10-nm radius spheres, the mean ionization cluster order was found to be about 35% higher in mammography X rays compared to 300 keV electrons (roughly representing 60Co or 192Ir photon radiation). In even smaller spheres (2 nm radius), no significant differences were observed for the mean ionization cluster order between mammography X rays and 300 keV electrons. We conclude that the potential of mammography X rays to induce biological damage is probably not much higher than a factor of two compared to hard X rays.

Theoretical analysis of microdosimetric spectra and cluster formation for 103Pd and 125I photon emitters.

In this work we have compared 125I or 103Pd from a microdosimetric point of view. The photon spectra at different positions around the seeds have first been calculated using EGSnrc Monte Carlo (MC) code. These photon spectra are used as input for the event-by-event MC code TRION to calculate the microdosimetric lineal energy (y) distribution for each isotope. The microdosimetric dose average lineal energy, yD, calculated in a sphere of 1 microm is 3.5 keV microm(-1) for 125I and 4.0 keV microm(-1) for 103Pd, agreeing well with values reported in the literature. yD in a 1 microm sphere diminishes slightly with the distance from the seed for 103Pd. This is due to the spectral hardening caused by the presence of a gamma-ray of 357.5 keV in the initial spectrum of 103Pd. In parallel with the calculation of the microdosimetric spectra, we have analysed the distribution of the size of the energy deposition clusters generated by these low energy photons in structures of 2 and 10 nm of radius. Due to Compton interactions, the fraction of very low energy electrons (<5 keV) generated by 125I photons is 51%, whereas it is only 27% for 103Pd. As these electrons deposit their energy very locally, the pattern of energy depositions contains more clusters of a few nm of radius for 125I than for 103Pd; the mean cluster orders are respectively 3.3 and 3.0 for 10 nm clusters. This is in opposition with the prediction based on the microdosimetric spectrum and the parameter yD and could be of importance for the damage to the cells.

The radial dose function of low-energy brachytherapy seeds in different solid phantoms: comparison between calculations with the EGSnrc and MCNP4C Monte Carlo codes and measurements.

The use of low-energy photon emitters for brachytherapy applications, as in the treatment of the prostate or of eye tumours, has drastically increased in the last few years. New seed models for 103Pd and 125I have recently been introduced. The American Association of Physicists in Medicine recommends that measurements are made to obtain the dose rate constant, the radial dose function and the anisotropy function. These results must then be compared with Monte Carlo calculations to finally obtain the dosimetric parameters in liquid water. We have used the results obtained during the characterization of the new InterSource (furnished by IBt, Seneffe, Belgium) palladium and iodine sources to compare two Monte Carlo codes against experiment for these low energies. The measurements have been performed in three different media: two solid water plastics, WT1 and RW1, and polymethylmetacrylate. The Monte Carlo calculations were made using two different codes: MCNP4C and EGSnrc. These codes use photon cross-section data of a different origin. Differences were observed between both sets of input data below 100 keV, especially for the photoelectric effect. We obtained differences in the radial dose functions calculated with each code, which can be explained by the difference between the input data. New cross-section data were then tested for both codes. The agreement between the calculations using these new libraries is excellent. The differences are within the statistical uncertainties of the calculations. These results were compared with the experimental data. A good agreement is reached for both isotopes and in the three phantoms when the measured values are corrected for the presence of the TLDs in the phantom.

Dosimetric study of a new palladium seed.

In this paper, the dosimetric parameters for a new palladium seed design: the InterSource(103,)(1) are presented as recommended by the AAPM in the TG-43 formalism. Measurements are made with LiF thermoluminescent dosimeters (size of 1mm(3)) in solid water phantoms WT1 to obtain the dose constant, the radial dose function and the anisotropy function. The TLD were calibrated at 6 MV and an energy correction factor of 1.40 has been applied. The same dose parameters are also obtained by Monte Carlo calculations (MCNP4B) in solid water and in liquid water. The calculated and the measured TG-43 functions for solid water are in excellent agreement. In WT1, the calculated dose rate constant is 0.657+/-1% and the measured value is 0.672+/-7%. The calculated value for water is 0.692+/-1%. The comparison with the previous study (Med. Phys. 27(5) (2000)) shows a very good agreement for the dose rate constant. The agreement for the radial function is poorer. For the measurements, it can be due to the difference of TLD settings. For the calculations the discrepancy could come from the different cross-section data utilized in the different Monte Carlo codes. In conclusion, the dosimetric functions for the new iodine seed InterSource(103) have been determined using the MCNP4B Monte Carlo code and TLD measurement in solid water WT1.

Dosimetric study of the new Intersource125 iodine seed.

The use of low energy photon emitters for brachytherapy applications, as in the treatment of the prostate or of eye tumors, has significantly increased these last few years. New seed models for 125I have been recently introduced. The aim of this study is to determine the dosimetric parameters as recommended by the AAPM in the TG43 formalism for a new iodine seed design: the InterSource125 (Furnished by IBt, Seneffe, Belgium). Measurements are made with LiF thermoluminescent dosimeters (size of 1 mm3) in solid water phantoms to obtain the dose constant, the radial dose function, and the anisotropy function. The TLDs were calibrated at 6 MV and an energy correction factor of 1.41 has been applied. The same dose parameters are also obtained by Monte Carlo calculations (MCNP4B) in solid water and in liquid water. The radial function was measured at 1, 1.5, 2, 3, 4, 5, 6, and 7 cm and calculated between 0.3 and 7 cm. The anisotropy functions were measured at 2, 3, and 5 cm and calculated between 0.3 and 7 cm. The calculated and the measured TG43 functions for solid water are in excellent agreement. We have then calculated these functions in liquid water to obtain the dosimetric information for clinical applications as per TG43 recommendations. In WTI, the calculated dose rate constant is 0.98+/-1% and the measured value is 1.03 +/- 7 %. The calculated value for water is 1.02+/- 1 %. In conclusion, the dosimetric functions for the new iodine seed InterSource125 have been determined. They are quite different from the data of the well-known model 6711 from Amersham due to the absence of silver in the new seed. The characteristics are very similar to those of model 6702.