Dose Distribution For Treatment Research Articles

Assessment of volume segmentation in radiotherapy of adolescents; a treatment planning study by the Swedish Workgroup for Paediatric Radiotherapy

Background and purpose. The variability in target delineation for similar cases between centres treating paediatric and adolescent patients, and the apparent differences in interpretation of radiotherapy guidelines in the treatment protocols encouraged us to perform a dummy-run study as a part of our quality assurance work. The aim was to identify and quantify differences in the segmentation of target volumes and organs at risk (OARs) and to analyse the treatment plans and dose distributions. Materials and methods. Four patient cases were selected: Wilm's tumour, Hodgkin's disease, rhabdomyosarcoma of the prostate and chordoma of the skull base. The five participating centres received the same patient-related material. They introduced the cases in their treatment planning system, delineated target volumes and OARs and created treatment plans. Dose-volume histograms were retrieved for relevant structures and volumes and dose metrics were derived and compared, e.g. target volumes and their concordance, dose homogeneity index (HI), treated and irradiated volumes, remaining volume at risk and relevant Vx and Dx values. Results. We found significant differences in target segmentation in the majority of the cases. The planning target volumes (PTVs) varied two- to four-fold and conformity indices were in the range of 0.3–0.6. This resulted in large variations in dose distributions to OARs as well as in treated and irradiated volumes even though the treatment plans showed good conformity to the PTVs. Potential reasons for the differences in target delineation were analysed. Conclusion. Considerations of the growing child and difficulties in interpretation of the radiotherapy information in the treatment protocols were identified as reasons for the variation. As a result, clarified translated detailed radiotherapy guidelines for paediatric/adolescent patients have been recognised as a way to reduce this variation.

Study of dental prostheses influence in radiation therapy

Dental prostheses made of high density material contribute to modify dose distribution in head and neck cancer treatment. Our objective is to quantify dose perturbation due to high density inhomogeneity with experimental measurements and Monte Carlo simulations.Firstly, measurements were carried in a phantom representing a human jaw with thermoluminescent detectors (GR200A) and EBT2 Gafchromic films in the vicinity of three samples: a healthy tooth, a tooth with amalgam and a Ni–Cr crown, irradiated in clinical configuration. Secondly, Monte Carlo simulations (BEAMnrc code) were assessed in an identical configuration.Experimental measurements and simulation results confirm the two well-known phenomena: firstly the passage from a low density medium to a high density medium induces backscattered electrons causing a dose increase at the interface, and secondly, the passage from a high density medium to a low density medium creates a dose decrease near the interface. So, the results show a 1.4% and 23.8% backscatter dose rise and attenuation after sample of 26.7% and 10.9% respectively for tooth with amalgam and crown compared to the healthy tooth.Although a tooth with amalgam has a density of about 12–13, the changes generated are not significant. However, the results for crown (density of 8) are very significant and the discordance observed may be due to calculation point size difference 0.8 mm and 0.25 mm respectively for TLD and Monte Carlo. The use of Monte Carlo simulations and experimental measurements provides objective evidence to evaluate treatment planning system results with metal dental prostheses.

Feasibility evaluation of a new irradiation technique: three-dimensional unicursal irradiation with the Vero4DRT (MHI-TM2000)

The Vero4DRT (MHI-TM2000) is a newly designed unique image-guided radiotherapy system consisting of an O-ring gantry. This system can realize a new irradiation technique in which both the gantry head and O-ring continuously and simultaneously rotate around the inner circumference of the O-ring and the vertical axis of the O-ring, respectively, during irradiation. This technique creates three-dimensional (3D) rotational dynamic conformal arc irradiation, which we term ‘3D unicursal irradiation’. The aim of this study was to present the concept and to estimate feasibility and potential advantages of the new irradiation technique. Collision maps were developed for the technique and a 3D unicursal plan was experimentally created in reference to the collision map for a pancreatic cancer case. Thereafter, dosimetric comparisons among the 3D unicursal, a two-dimensionally rotational dynamic conformal arc irradiation (2D–DCART), and an intensity-modulated radiation therapy (IMRT) plan were conducted. Dose volume data of the 3D unicursal plan were comparable or improved compared to those of the 2D–DCART and IMRT plans with respect to both the target and the organs at risk. The expected monitor unit (MU) number for the 3D unicursal plan was only 7% higher and 22.1% lower than the MUs for the 2D–DCART plan and IMRT plan, respectively. It is expected that the 3D unicursal irradiation technique has potential advantages in both treatment time and dose distribution, which should be validated under various conditions with a future version of the Vero4DRT fully implemented the function.

TH‐A‐BRA‐04: Evaluation of Three IGRT Approaches for Prostate Cancer Treatment

Purpose: To evaluate two online IGRT and one hybrid adaptive modification for prostate cancer treatment with onboard CBCT imager. Methods: Two online IGRT and one hybrid adaptive modification were simulated and evaluated retrospectively using daily CBCT images obtained from 5 prostate cancer patients treated with total dose 64Gy in 20 fractions. For each daily treatment, two CBCTs obtained at the pre‐ and post‐treatment delivery were used in the study. Online IGRT techniques include (1) Online‐ correction: a pre‐treatment IMRT plan with 3mm CTV‐to‐PTV margin was delivered following the CBCT imaging and online target position correction; and (2) Online‐planning: an online inverse plan designed on the pre‐ treatment CBCT image with 3mm target margin was accomplished and delivered. The Hybrid‐adaptation consists of the online prostate position correction and delivery of a pre‐treatment IMRT plan with no target margin for the first week of the treatment, and an offline adaptive inverse planning with using the planning CT and the first week of post‐treatment CBCT images for the remaining treatment. Treatment dose for all 3 approaches were constructed using all the post‐treatment CBCT images. Evaluations were all performed using the treatment dose distribution in organs of interest for all patients. Results: The minimal delivered dose (D99) in CTV (prostate + seminal vesicle) is in the range of [51.6, 65.6]Gy for Online‐ correction; [63.2, 66.1]Gy for Online‐planning; and [62.8, 66.1]Gy for the Hybrid‐adaptation. The rectal volume with 62.6Gy, V62.6, for Online‐ correction, Online‐planning, Hybrid‐adaptation are 0.7+/−1.0%, 0.4+/−0.2%, 0.5+/−1.0%; the rectal V59.4 are 2.1+/−1.8%, 1.5+/‐1.1%, 1.6+/−1.9%; and V56.5 are 4.0+/−3.2%, 3.0+/−2.2%, 3.5+/−3.2%. The bladder volume dose, V64.1, are 2.3+/−2.2%, 0.5+/−0.6%, 0.4+/−0.3%, and V59.4 are 5.2+/−3.6%, 2.0+/−1.1%, 1.6+/−0.8% respectively for the 3 approaches. Conclusions: Both Online‐planning and Hybrid‐adaptation achieved comparable target coverage and normal tissue sparing, and were superior to Online‐correction. However, Hybrid‐adaptation is more efficiency in clinical practice. This research was partially supported by Elekta.

SU-E-J-117: Computer-Assisted Verification of Accumulated Dose Distribution during the Treatment Time Based on Estimation of Four-Dimensional Dose Distribution Using an Electronic Portal Imaging Device.

The accumulated dose distributions during the course of radiation treatment are substantially important for verifying whether treatment dose distributions are produced according to planned dose distributions. The purpose of this study was to develop a computer-assisted verification method of accumulated dose distribution during the irradiation of a tumor based on estimation of four-dimensional (4D) dose distribution using an electronic portal imaging device (EPID). The 4D 'treatment' computed tomography (CT) images during the irradiation were estimated based on affine transformations including respiratory motions, which were derived by registration between a planning portal dose image and treatment portal dose dynamic image. Planning portal dose images were calculated from planning CT images and an algorithm for calculation of dose spatial distribution. Treatment portal dose images were estimated from EPID dynamic images obtained during a treatment time. The planning portal dose images were registered to the treatment portal dose images to obtain the affine transformation, which could include respiratory motion in a patient body. The CT images at a treatment time were determined by deforming the planning CT images using the affine transformation matrix. 4D dose distributions during a treatment delivery were obtained by applying a dose calculation algorithm to the 4D treatment CT images. Finally, accumulated dose distributions during the course of radiation treatment were verified with planned dose distributions. We applied the proposed method to EPID dynamic images of 2 lung cancer patients, and evaluated the difference in accumulated dose distribution between the plan and treatment using a gamma evaluation (3mm/3%). The average pass rate for 2 cases was 78.2%. The proposed method can be used for adaptively modifying the plan based on the dose discrepancy between the plan and treatment. This work was partially supported by Grant-in-Aid for Scientific Research (C) (22611011) and Okawa Foundation for Information and Telecommunications.

Design and development of a new micro-beam treatment planning system: effectiveness of algorithms of optimization and dose calculations and potential of micro-beam treatment

A new treatment planning system (TPS) was designed and developed for a new treatment system, which consisted of a micro-beam-enabled linac with robotics and a real-time tracking system. We also evaluated the effectiveness of the implemented algorithms of optimization and dose calculations in the TPS for the new treatment system. In the TPS, the optimization procedure consisted of the pseudo Beam's-Eye-View method for finding the optimized beam directions and the steepest-descent method for determination of beam intensities. We used the superposition-/convolution-based (SC-based) algorithm and Monte Carlo-based (MC-based) algorithm to calculate dose distributions using CT image data sets. In the SC-based algorithm, dose density scaling was applied for the calculation of inhomogeneous corrections. The MC-based algorithm was implemented with Geant4 toolkit and a phase-based approach using a network-parallel computing. From the evaluation of the TPS, the system can optimize the direction and intensity of individual beams. The accuracy of the dose calculated by the SC-based algorithm was less than 1% on average with the calculation time of 15 s for one beam. However, the MC-based algorithm needed 72 min for one beam using the phase-based approach, even though the MC-based algorithm with the parallel computing could decrease multiple beam calculations and had 18.4 times faster calculation speed using the parallel computing. The SC-based algorithm could be practically acceptable for the dose calculation in terms of the accuracy and computation time. Additionally, we have found a dosimetric advantage of proton Bragg peak-like dose distribution in micro-beam treatment.

Efficacy Analysis of Simplified Intensity-modulated Radiotherapy with High or Conventional Dose and Concurrent Chemotherapy for Patients with Neck and Upper Thoracic Esophageal Carcinoma

For patients with neck and upper thoracic esophageal carcinoma, it is difficult to control lymph node metastases with conventional dose therapy. In this study, we assessed the feasibility of simplified intensity-modulated radiotherapy (sIMRT) and concurrent chemotherapy for 44 patients and boosted high-dose to metastatic lymph nodes.Three radiation treatment volumes were defined: PGTVnd, with which 68.1 Gy was delivered in high dose group (hsIMRT group), and 60 Gy in the conventional dose group (csIMRT group); PTV1, featuring 63.9 Gy in the hsIMRT group and 60Gy in the csIMRT group; PTV2, with 54 Gy given to both groups. The sIMRT plan included 5 equi-angular coplanar beams. All patients received the cisplatin and 5-FU regimen concurrently with radiotherapy. The treatment was completed within six weeks and one case with grade three acute bronchitis was observed in hsIMRT group. For esophageal lesions, 80% complete response (CR) and 20% partial response (PR) rates were found in the hsIMRT group, and 79.2% CR, with 20.8% PR, in the csIMRT group; for lymph node lesions, 75% CR and 25% PR rates were observed in the hsIMRT group, with 45.8% and 37.5% respectively in the csIMRT group (P <0.05). The differences in 1-, 2- and 3-year relapse-free survival rates were all statistically significant (P <0.05). The major toxicity observed in both groups was Grade I~II leucopenia. sIMRT can generate a desirable dose distribution in treatment of neck and upper thoracic esophageal carcinoma with a better short-term efficacy. Boosted high dosing to metastatic lymph nodes can increase the relapse-free survival rate.

An experimental investigation into the effect of periodic motion on proton dosimetry using polymer gel dosimeters and a programmable motion platform

Organ motion in proton therapy affects treatment dose distribution during both double-scattering (DS) and uniform-scanning (US) deliveries. We investigated the dosimetric impact of target motion using three-dimensional polymer gel dosimeters and a programmable motion platform. A simple one-beam treatment plan with 16 cm range and 6 cm modulation was generated from the treatment planning system (TPS) in both the DS and US modes. One gel dosimeter was irradiated with a stationary DS beam. Two other gel dosimeters were irradiated with the DS and US beams while they moved in the same sinusoidal motion profile using a programmable motion platform. The dose distribution of the stationary DS delivery agreed with the TPS plan. Dosimetric comparisons between DS motion delivery and the MATLAB-based motion model showed insignificant differences. Dose–volume histograms of a cylindrical target volume inside the gel dosimeters showed target coverage degradation caused by motion. A three-dimensional gamma index calculation (3% and 3 mm) confirmed different dosimetric impacts from DS and US with the same target motion. This polymer-gel-dosimeter-based study confirmed the dosimetric impact of intrafraction target motion and its interplay with temporal delivery of different energy layers in US proton treatments.

TU-A-BRC-07: Toward Predicting the Location of GBM Recurrence after Radiotherapy Using Patient-Specic DTI and Numerical Modelling

Purpose: Patients diagnosed with glioblastoma multiforme (GBM), the most aggressive form of braintumour, have a median survival of 15 months when receiving standard treatment. We have developed a three‐dimensional reaction‐diffusion model which uses patient‐specific diffusiontensorimaging (DTI) and real dose distributions to simulate the growth and radiotherapy treatment of a GBM. We highlight paths for tumour recurrence and predict a gain in survival when treatment margins are adjusted according to model inputs. Methods: A DTI sequence is added to pre‐ and post‐treatment MRI for ten GBM patients receiving standard treatment in our department. Our numerical model uses clinically available images to initialize the tumour cell density and to measure invasion velocity. Patient‐ specific DTI is used to modeltumour cell motility and to prioritize migration along white matter fibres. The treatment dose distribution is used to simulate the radiotherapy treatment actually received by the patient. Finally, we simulate an alternative treatment plan that increases the dose in the region where the model predicts the formation of a recurrent tumour. Results: The general behaviour of the model was evaluated and found to be adequate for a patient with no DTI but whose post‐treatment images were available. For another patient, model initialization with pre‐ treatment DTI shows that a second tumour focus forms outside the original radiation field in a region where the tumour migration is high due to white matter fibres. The application of an alternative virtual plan integrating the location of the recurrence suggests a 2‐month gain of survival time, based on tumour cell density. Conclusions: Our results indicate that a reaction‐ diffusionmodel using patient‐specific DTI information could potentially be used to modify GBM treatment margins, leading to an increased survival time. Integrating a long‐term outcome study will allow us to verify the predictive efficiency of the model. This work is funded by the Fonds quebecois de la recherche sur la nature et les technologies (FQRNT) and by the Natural Sciences and Engineering Research Council of Canada (NSERC).

SU‐E‐T‐212: Measurement Comparison and Monte Carlo Analysis for Rapid Arc Delivery Verification Using ArcCheck Dosimetry System

Purpose: The RapidArc technique requires sophisticated treatment plan optimization and beam delivery technology. This work investigates the capabilities of the ArcCheck dosimetry system for RapidArc treatment planning verification and Linac dose delivery validation. Methods: A total of 10 (6 prostate, H&amp;N, pelvis, liver and breast) clinical RapidArc treatment cases were investigated using a delivery QA procedure with the use of the ArcCheck dosimetry system. A custom acrylic plug with the ability to host an ion chamber was used for CAX phantom dose measurements. In addition to the Eclipse TPS, the treatment dose distributions were simulated using a Monte Carlo code in order to compare and evaluate the dose delivery based on the treatment plan. In both ways the distributions were evaluated in terms of the absolute dose and distance to agreement on a cylindrical plane dose distribution that overlaps with the ArcCheck detector plane. The system sensitivity to setup misalignment errors, and field size dependency, were also investigated. Results: Excluding the H&amp;N case, the difference between calculated to measure absolute CAX dose was less than 3%. The H&amp;N CAX fell on a dose distribution gradient, and hence showed a 6.6% and 7.1% dose difference, respectively, between the measurement dose and that calculated by the TPS and Monte Carlo. Gamma analysis passing rate varied from 92.8% for the pelvis case, and 98.8% for a prostate case, when using the 3% dose difference and 3mm distance to agreement criteria for the TPS dose distributions. A field‐size dependence was found for large field sizes (above 21 × 21 cm2). Conclusions: The ArcCheck dosimetry system showed good capabilities for RapidArc plan verification and dose delivery validation, with respect to the CAX dose and the overall dose distribution. The system sensitivity to setup misalignment and dose variation is demonstrated for the clinical cases investigated.

SU‐E‐T‐772: Volume Matching for Brachytherapy Planning

Purpose: To perform brachytherapy treatment plan by comparing it with existing data base from earlier treatments of similar type. Method: When brachytherapy target to be treated is closely related in shape and volume to the target treated in the past for which we had well developed plan based on sources energy, activity and spatial distribution we can assume that plan of essentially the same quality will be achieved when the same distribution of sources that was used in the past will be imposed over the new, identical to one in the past treatment volume. The planning strategy under these circumstances relies on matching the volume under consideration with the volume of the target from earlier treatment. Characteristic scalar parameters of volumes will be compared first to filter the database (these will include volume, orientation of principal axes and moments of the characteristic set of the compared volumetric domains). When these parameters match closely between considered volume and one of stored in library volumes they are suspect of close similarity. The next stage of comparisons matches volumes by translating them in 3D space so that the maximal intersection between them is achieved. When closeness criterion is satisfied for this matching the distribution of sources is moved from the old plan to the new target completing the planning process. Results: The program for the algorithm has been written and sequence of plans have been computed for dose distributions obtained by sources placement based on domain comparison planning described above. Conclusion: The excellent agreement between set of plans based on geometrical planning approach and traditional planning optimization approach has been discovered. The domain comparison planning leads to enviable dose distribution in brachytherapy treatment and it is fast enough to be applicable to real time treatments.

WE-G-220-06: Using MR-Guided Focused Ultrasound to Study Heating Pattern Variations Due to Reflections from Interfaces in Both Thermal and Tissue Phantoms

Purpose:To investigate the impact of ultrasoundwave reflections from distal surfaces on aimed treatment center shifts and dose distributions during MR‐ guided‐FUS (focused ultrasound) to provide important information for treatment planning. Methods: An integrated Philips Sonalleve system on a 1.5T was used for this study. FUS waves reflected from four different reflectors (air, acrylic (modeling bone), rubber, and a gel pad) after propagation through a thermal phantom and an ex vivo tissue phantom were investigated. FUS was aimed at positions 1 cm, 2 cm and 4 cm below the distal interface surface. Volumetric sonications were 20 seconds, at 50 watts for all cases. Proton resonance frequency shift imaging was used to measuretemperature during and after FUS. Target location shifts toward the distal interface are reported, along with the maximum temperature at both the center of the treatment area and at the interface. Results: Surface heating was greatest at the acrylic‐thermal phantom interface. The greatest shifts from planned treatment position were beneath acrylic and rubber interfaces at an aimed depth of 2cm in the thermal phantom. Shifts in position observed in the tissue phantom were less extreme. Conclusions: In both the thermal and tissue phantoms, the gel pad interface causes the least increase in temperature at the surface, and minimal displacement from the target. For translational research utilizing animal cancer models, this work demonstrates the benefit of placing a gel pad on the distal surface of small volumes to reduce the shift that would occur if that surface was left open to the air. Future work will compare experimental data with simulations and investigate the effect of reflections from distal surfaces using in vivo animal studies. This work was supported by grants from the Focused Ultrasound Surgery Foundation and Philips Healthcare.

SU‐E‐J‐67: Dual Resolution CT‐Based Phantoms for Monte Carlo Simulation Against Small‐Field Irradiations in Tiny/heterogeneous Tissues

Purpose: Dose over/underestimation is a common problem for highly irregular fields or around tiny and heterogeneous tissues such as the mucosa/epidermis near teeth/false teeth, skin, bone marrow, vestibulocochlear nerve, and pituitary glands, leading to unexpected normal tissue complications or tumour recurrence. Owing to the limitation of computer efficiency and RAM capacity, it is impossible at present to construct a very high‐resolution phantom for calculating tumour/organ dose together with realistic and accurate dose estimation for tiny and heterogeneous tissues in one simulation. This study aimed to develop dull resolution CT‐based phantoms for Monte Carlo simulation against small‐field irradiation in tiny/heterogeneous tissues. Methods: A homemade VC++ 6.0 code was developed to translate DICOM CT images to dual resolution CT‐based phantoms in the format of an egs‐phantom used in DOSXYZnrc. This code uses regular spatial resolution to define the tumour/organ, but enhances the resolution around the region of interest. This dual resolution phantom can be used for patient treatment evaluation and dose distribution around tiny and heterogeneous tissues in one simulation. Results: A dual resolution pure water phantom will not impact the outcome of MC simulation under large uniform field irradiation. However, the results of this pure water phantom under small field irradiation showed dose underestimation of more than 30% owing to the partial volume effect of not using enhanced resolution. Another dual resolution heterogeneous phantom was used to investigate the interface dose distribution near the boundary of different materials. Finally, a dual‐resolution CT‐based phantom, BigMouse, was created to study the dose distribution around teeth. Isodose curves and PDD/profile comparisons showed that enhanced resolution doses were higher when irradiation fields or phantoms were not uniform. Conclusions: Dose underestimation due to over‐sized voxels can cause severe damage to mucosa in the oral cavity.This work was supported by NSC99‐2314‐B‐182‐040‐MY2 from the National Science Council, Taiwan and CMRPD180162 from the Chang Gung Memorial Hospital

SU-E-T-826: Dosimetric Comparison of Intensity-Modulated Proton Therapy and 3D Proton Radiotherapy with Passively Scattered Beams for Primary Prostate Cancer

Purpose: To evaluate the dose distributions for treatment of primary prostate tumor using plans with intensity modulated proton therapy (IMPT) and plans with 3D conformal proton beam using passive scattering beams. Methods: We generated treatment plans for five patients with two different proton treatment planning technique (IMPT and 3D conformal) using opposed lateral beams. The 3D conformal plans included passively scattered beams with appropriate beam compensators to conform the dose along the distal region of each beam. IMPT planning applies the spot scanning beam and uses the dose optimization process to reduce the dose to normal tissue and provide adequate prescription dose to prostate. The planning target volume (PTV) included prostate and margins to account for dosimetric and setup uncertainty. The total prescription dose (PD) for each plan is 79.2 Gy. We require that 99% of the clinical target volume (prostate) is covered by 98% of PD (D99%=78.4 Gy) and the PTV dose criteria is D98%=75.2 Gy. The dose for rectum and bladder for volumes of 20% and 40% (D20%, D40%) was optimized such that D20% and D40% are less than or equal to 60 Gy and 40 Gy respectively. The index of conformality (CI) was evaluated using the ratio of PTV volume to that of volume covered by 95% of PD. Results: The PTV dose coverage is achieved for both techniques but IMPT plans showing better dose homogeneity and lower maximum dose. Overall, the D20% and D40% slightly decreased for rectum for all plans. However, for bladder, there is higher dose reduction for IMPT plans as compared to 3D proton plans. The CI was improved for IMPT plans. Conclusions: The IMPT technique results in plans with better PTV coverage and lower dose to organ at risk as compared to plans with passively scattered beams.

AN INVESTIGATION OF THE DOSE DISTRIBUTION FROM LDR IR-192 WIRES IN THE TRIANGULAR IMPLANTS OF THE PARIS SYSTEM USING POLYMER GEL DOSIMETRY

Introduction: Polymer gels are modern dosimeters providing three dimensional dose distributions. These dosimeters can be used in brachytherapy in which the tumor dimension is relatively small and the dose gradient is high. In this study, the ability of the MAGICA polymer gel was investigated for assessing the absolute dose values as well as the dose distribution of low dose rate (LDR) Ir-192 wires in interstitial brachytherapy based in triangular implants of the Paris system. Material and Methods: A suitable phantom was made from Perspex. Glass tubes were used as the external tubes for holding the Ir-192 wires in the phantom. The MAGICA polymer gel was made and placed in the phantom. The phantom and the calibration tubes were irradiated using LDR Ir-192 wires and a Co-60 teletherapy unit respectively. They were subsequently imaged using an MRI scanner. The R2 (=1/T2) maps were extracted from several sequential T2-weighted MRI images. The dose values resulting from the polymer gel measurements at the reference points were compared with those from the common calculation method at the same points. In addition, the isodose curves resulting from gel dosimetry were compared with those from a brachytherapy treatment planning system (Flexiplan). Results: The average of the dose values measured with the gel at the reference points was 62.75% higher than those calculated at the same points. Investigating the isodose curves revealed that the maximum distance to agreement (DTAmax) between the isodoses resulting from the gel and those obtained from the treatment planning system was less than 3 mm at different dose levels. Discussion and Conclusion: Although the MAGICA gel indicates a higher absolute dose value than those calculated commonly, it can give the relative dose values accurately. Therefore, it can be recommended to be used for the assessment of dose distributions for the treatment of tissues as well as quality control of the treatment planning systems.

Verification of patient‐specific dose distributions in proton therapy using a commercial two‐dimensional ion chamber array

The purpose of this study was to determine whether a two-dimensional (2D) ion chamber array detector quickly and accurately measures patient-specific dose distributions in treatment with passively scattered and spot scanning proton beams. The 2D ion chamber array detector MatriXX was used to measure the dose distributions in plastic water phantom from passively scattered and spot scanning proton beam fields planned for patient treatment. Planar dose distributions were measured using MatriXX, and the distributions were compared to those calculated using a treatment-planning system. The dose distributions generated by the treatment-planning system and a film dosimetry system were similarly compared. For passively scattered proton beams, the gamma index for the dose-distribution comparison for treatment fields for three patients with prostate cancer and for one patient with lung cancer was less than 1.0 for 99% and 100% of pixels for a 3% dose tolerance and 3 mm distance-to-dose agreement, respectively. For spot scanning beams, the mean (+/- standard deviation) percentages of pixels with gamma indices meeting the passing criteria were 97.1% +/- 1.4% and 98.8% +/- 1.4% for MatriXX and film dosimetry, respectively, for 20 fields used to treat patients with prostate cancer. Unlike film dosimetry, MatriXX provides not only 2D dose-distribution information but also absolute dosimetry in fractions of minutes with acceptable accuracy. The results of this study indicate that MatriXX can be used to verify patient-field specific dose distributions in proton therapy.

SU-GG-T-101: Dosimetric Analysis of Matching 6 MV Photon and Electron Fields in Chestwall Treatment

Purpose: To investigate dose distribution for matching 6MV photon and electron fields in chestwall treatment. Method and Materials: A rectangular water equivalent phantom was used to investigate dose distribution at junction of adjacent photon and electron fields. Both 3D and simple IMRT plans have been created for photon fields. Electron plans with different electron energy were created and normalized to 90% iso‐dose line. Photon and electron fields were matched based on skin markings. Monte Carlo simulation has been applied for photon plans. We compared plans with a single photon‐electron match line to “mixed treatment” plans using 2 photon‐electron match lines (ie day1/day2) Results: Hot spots are observed at junction of photon field for all energy electron fields. The highest dose are 129%, 134%, 137%, 139% and 139% of dose for 6, 9, 12, 16 and 20 MeV electrons, respectively, with 6MV photon fields. The depths of these hot spots from skin surface are 1.5, 2.5, 3.4, 4.6 and 5.5cm, as compared with dmax for the electron fields individually (1.4, 2.2, 3.0, 3.0 and 2.0 cm, respectively). The hot spots for “mixed treatment” arrangement (day 1 and day 2) are 120%, 123%, 126%, 129% and 130% of dose, much lower than for the single match‐line fields. The depth for hot spot for day 1 and day 2 arrangement stays similar with day1 treatment. Results from Monte Carlo simulation agree with pencil beam dose calculation algorithm in skin region. Conclusion: Hot spots are reduced to clinically acceptable level by mixed treatment arrangement with two or even more matching lines between photon and electron fields. Further study is warranted to compare these results with intensity and/or energy modulated electron treatment, which may provide a more homogeneous dose distribution for chestwall treatment.

SU‐GG‐T‐547: New Measures for Evaluating SBRT Dose Distribution Quality

Purpose. To define a measure that indicates how well the dose distribution in SBRT treatment satisfies standards of dose symmetry and compactness required by SBRT RTOG protocols. Method. Any SBRT dose has to provide the adequate coverage of the PTV and away from PTV it has to decrease fast enough to minimize the volume of tissue in the organ in which the tumor is contained (e.g. lung, liver) (see RTOG 0236). To measure the tightness of dose around PTV one may create a sequence of regions that increase their detachment from PTV quantifying the dose in subsequent regions away from the target. The convenient approach is to build histograms based on a sequence of shells that wrap the target one after the other. This can be achieved by creating sequence of shell structures in the TPS system (script in case of Pinnacle) for which values of minimal, maximal and average dose will be automatically assigned and displayed in the form of appropriate histogram. Results. The script has been written for Pinnacle 8.0m and examples of dose distributions for sequence of SBRT plans have been analyzed by means of computed for these plans histograms. Histograms provide natural means for verification of SBRT RTOG parameters denoted as R50 and D2cm. These histograms provide, however, also additional information on symmetry and steepness of dose gradient in SBRT dose distributions. Conclusion. As in SBRT the quality of treatment depends on the location relative to the target of volumes of tissue exposed to given dose levels measures that go beyond DVH and quantify spatial properties of dose distribution in tissue are needed for the assessment of SBRT treatment quality.

Rectal dose variation during the course of image-guided radiation therapy of prostate cancer

Background and purpose To investigate the change in rectal dose during the treatment course for intensity-modulated radiotherapy (IMRT) of prostate cancer with image-guidance. Materials and methods Twenty prostate cancer patients were recruited for this retrospective study. All patients have been treated with IMRT. For each patient, MR and CT images were fused for target and critical structure delineation. IMRT treatment planning was performed on the simulation CT images. Inter-fractional motion during the course of treatment was corrected using a CT-on-rails system. The rectum was outlined on both the original treatment plan and the subsequent daily CT images from the CT-on-rails by the same investigator. Dose distributions on these daily CT images were recalculated with the isocenter shifts relative to the simulation CT images using the leaf sequences/MUs based on the original treatment plan. The rectal doses from the subsequent daily CTs were compared with the original doses planned on the simulation CT using our clinical acceptance criteria. Results Based on 20 patients with 139 daily CT sets, 28% of the subsequent treatment dose distributions did not meet our criterion of V 40 < 35%, and 27% did not meet our criterion of V 65 < 17%. The inter-fractional rectal volume variation is significant for some patients. Conclusions Due to the large inter-fractional variation of the rectal volume, it is more favorable to plan prostate IMRT based on an empty rectum and deliver treatment to patients with an empty rectum. Over 70% of actual treatments showed better rectal doses than our clinical acceptance criteria. A significant fraction (27%) of the actual treatments would benefit from adaptive image-guided radiotherapy based on daily CT images.

Practical considerations for prostate HDR brachytherapy

Purpose A process for prostate high-dose-rate (HDR) brachytherapy was developed and implemented successfully in the community hospital setting. The practical aspects of the program are reviewed and may serve as a foundation for clinics interested in offering this clinical service. Methods and Materials A generic needle distribution geometry was established to accommodate target volumes of variable size. A system to identify and assign treatment channels to each implant needle was devised. The computerized tomography (CT)–based treatment planning was used with dose constraints defined for sensitive structures and target uniformity. Implant needle stability was promoted by supporting the patient on a CT compatible padded sliding board. A process that aligns dwell position to CT imaging without the use of radiographic markers was followed. Graphical optimization of dwell times was used to generate the treatment dose distributions. Results Prostate HDR brachytherapy as a boost or as monotherapy has been offered in a program that has evolved over the past 8 years. Practical aspects of the program promote its feasibility and precision. Collaboration with commercial entities has also led to the development of products that support the technique. Conclusions Prostate HDR brachytherapy offers a relatively high degree of dose distribution control in comparison with other prostate radiotherapy modalities. The practical aspects described offer assurance to achieve that goal.