Three-dimensional Dose Calculations Research Articles

Pencil beam kernel-based dose calculations on CT data for a mixed neutron-gamma fission field applying tissue correction factors.

Objective.For fast neutron therapy with mixed neutron and gamma radiation at the fission neutron therapy facility MEDAPP at the research reactor FRM II in Garching, no clinical dose calculation software was available in the past. Here, we present a customized solution for research purposes to overcome this lack of three-dimensional dose calculation.Approach.The applied dose calculation method is based on two sets of decomposed pencil beam kernels for neutron and gamma radiation. The decomposition was performed using measured output factors and simulated depth dose curves and beam profiles in water as reference medium. While measurements were performed by applying the two-chamber dosimetry method, simulated data was generated using the Monte Carlo code MCNP. For the calculation of neutron dose deposition on CT data, tissue-specific correction factors were generated for soft tissue, bone, and lung tissue for the MEDAPP neutron spectrum. The pencil beam calculations were evaluated with reference to Monte Carlo calculations regarding accuracy and time efficiency.Main results.In water, dose distributions calculated using the pencil beam approach reproduced the input from Monte Carlo simulations. For heterogeneous media, an assessment of the tissue-specific correction factors with reference to Monte Carlo simulations for different tissue configurations showed promising results. Especially for scenarios where no lung tissue is present, the dose calculation could be highly improved by the applied correction method.Significance.With the presented approach, time-efficient dose calculations on CT data and treatment plan evaluations for research purposes are now available for MEDAPP.

Monte Carlo Modeling of Dynamic Tumor Tracking on a Gimbaled Linear Accelerator.

The Vero4DRT (Brainlab AG) linear accelerator is capable of dynamic tumor tracking (DTT) by panning/tilting the radiation beam to follow respiratory-induced tumor motion in real time. In this study, the panning/tilting motion is modeled in Monte Carlo (MC) for quality assurance (QA) of four-dimensional (4D) dose distributions created within the treatment planning system (TPS). Step-and-shoot intensity-modulated radiation therapy plans were optimized for 10 previously treated liver patients. These plans were recalculated on multiple phases of a 4D computed tomography (4DCT) scan using MC while modeling panning/tilting. The dose distributions on each phase were accumulated to create a respiratory-weighted 4D dose distribution. Differences between the TPS and MC modeled doses were examined. On average, 4D dose calculations in MC showed the maximum dose of an organ at risk (OAR) to be 10% greater than the TPS' three-dimensional dose calculation (collapsed cone [CC] convolution algorithm) predicted. MC's 4D dose calculations showed that 6 out of 24 OARs could exceed their specified dose limits, and calculated their maximum dose to be 4% higher on average (up to 13%) than the TPS' 4D dose calculations. Dose differences between MC and the TPS were greatest in the beam penumbra region. Modeling panning/tilting for DTT has been successfully modeled with MC and is a useful tool to QA respiratory-correlated 4D dose distributions. The dose differences between the TPS and MC calculations highlight the importance of using 4D MC to confirm the safety of OAR doses before DTT treatments.

Commissioning and clinical implementation of the first commercial independent Monte Carlo 3D dose calculation to replace CyberKnife M6™ patient-specific QA measurements.

PurposeTo report on the commissioning and clinical validation of the first commercially available independent Monte Carlo (MC) three‐dimensional (3D) dose calculation for CyberKnife robotic radiosurgery system® (Accuray, Sunnyvale, CA).MethodsThe independent dose calculation (IDC) by SciMoCa® (Scientific RT, Munich, Germany) was validated based on water measurements of output factors and dose profiles (unshielded diode, field‐size dependent corrections). A set of 84 patient‐specific quality assurance (QA) measurements for multi‐leaf collimator (MLC) plans, using an Octavius two‐dimensional SRS1000 array (PTW, Freiburg, Germany), was compared to results of respective calculations. Statistical process control (SPC) was used to detect plans outside action levels.ResultsOf all output factors for the three collimator systems of the CyberKnife, 99% agreed within 2% and 81% within 1%, with a maximum deviation of 3.2% for a 5‐mm fixed cone. The profiles were compared using a one‐dimensional gamma evaluation with 2% dose difference and 0.5 mm distance‐to‐agreement (Γ(2,0.5)). The off‐centre ratios showed an average pass rate >99% (92–100%). The agreement of the depth dose profiles depended on field size, with lowest pass rates for the smallest MLC field sizes. The average depth dose pass rate was 88% (35–99%). The IDCs showed a Γ(2,1) pass rate of 98%. Statistical process control detected six plans outside tolerance levels in the measurements, all of which could be attributed the measurement setup. Independent dose calculations showed problems in five plans, all due to differences in the algorithm between TPS and IDC. Based on these results changes were made in the class solution for treatment plans.ConclusionThe first commercially available MC 3D dose IDC was successfully commissioned and validated for the CyberKnife and replaced all routine patient‐specific QA measurements in our clinic.

Three-Dimensional Versus Four-Dimensional Dose Calculation for Breast Intensity-Modulated Radiation Therapy

Objective:To analyze the effect of intra- and interfractional motion during breast intensity modulated radiation therapy (IMRT) by calculating dose distribution based on four-dimensional computed t...

Targeted Marrow Radiation (TMI) Improves Therapeutic Efficacy of STAT3 Decoy Molecules By Augmenting Its Delivery and Immune Modulation in an AML Mouse Model

Introduction Acute myeloid leukemia (AML) is a highly aggressive form of leukemia with poor long-term survival. Our clinical development of a targeted radiation treatment for relapsed/refractory AML disease led to an impressive two-year overall survival (OS) rate of 41% (Stein et al., 2019) in contrast to <10% reported for similar patients (Duval et al., 2019). Unfortunately, a lack of a preclinical model for TMI has impaired further scientific advancement. Ionizing radiation therapy (RT) has been shown to have immune modulatory functions; however, previous studies demonstrated that its efficacy is limited by activation of the radiation-induced tolerogenic TLR9/IL-6/STAT3 signaling axis in myeloid immune cells (Gao et al., 2013; Won et al., 2017). TLR9+ myeloid cells (macrophages, AML, etc) can be efficiently targeted using TLR9 agonists (CpG) conjugated with oligonucleotide (ODN) drugs (e.g., siRNA) to eliminate STAT3 signaling. Such a strategy has been shown to synergize with local tumor RT to prevent tumor recurrence (Gao et al., 2013). A CPG-STAT3-dODN inhibitor (CSI) has been previously reported as an effective immunotherapy for AML (Zhang et al., 2016), but the efficacy of immunotherapeutic agents is drastically reduced at high disease burden (Pai et al., 2019). Therefore, we hypothesized that the immunomodulatory effects of low dose RT can be augmented by blocking the STAT3 signaling in AML cells, enhancing the efficacy of CSI in a high-disease burden model. Methods We first developed a TMI treatment methodology using a Precision X-RAD small animal irradiator. A whole body CT image was acquired, and three-dimensional (3D) dose calculations were performed using a Monte Carlo dose engine-based SmART-Plan treatment system (van Hoof et al, 2013; Downes et al., 2009; Faddegon et al., 2009). Before treatment, mice were further imaged to verify target anatomy prior to delivering precise radiation doses to the entire skeletal system and spleen while sparing vital organs (lungs, liver, gut). We used a Cbfb-MYH11/Mpl (CMM)-induced mice leukemia model. At 5-10% CMM-GFP cells in peripheral blood (~20-30% in bone marrow; high disease burden), the mice were treated with three doses of CSI (2.5mg/kg) on alternate days with or without 4 Gy radiation (TMI or total body irradiation [TBI]). Cy3 labeled CSI uptake studies were carried out 48h post RT by flow cytometry, and immune cell trafficking and activation studies were conducted by harvesting bone marrow and spleen cells on day 8 post RT. Results Dose volume histogram (DVH) and radiation dose painting show that the TMI treatment plan significantly reduced doses to critical organs (lungs, liver, gut) while maintianing the same dose to the skeleton and spleen, unlike TBI (same dose to entire body) (Figure 1A-D). In TMI-treated mice, the CSI-Cy3 uptake was significantly higher in CMM cells than in the CMM cells of mice treated with CSI alone, indicating improved delivery post RT (Figure 1E, F). The trafficking of T helper cells (CD4+) and cytotoxic T lymphocytes (CD8+; CTLs) as well as effector (CD62L-CD44+) and effector memory T cells (CD62L+CD44+) is significantly augmented in the bone marrow of TMI+CSI treated mice compared to levels in mice treated with TBI+CSI or CSI alone (Figure 1G, H). An increased total number of IFNγ secreting CTLs, and a higher CD8:Treg (CD4+CD25+FOXP3+) ratio indicate enhanced anti-tumerogenic activity in TMI+CSI treated mice over that of mice treated with TBI+CSI or CSI alone (Figure 1I, J). Similarly, myeloid cell trafficking and activation was augmented in TMI-treated bone marrow (data not shown). The benefit of augmented immune modulation in TMI+CSI combinatorial therapy is reflected in a significantly increased survival (~39 median survival days) over untreated mice and those treated with CSI or TMI alone ( ~8-9 days median survival) (Figure 1K). Conclusion This is the first report of a novel radio-immunotherapy using a systemic targeted precise RT (TMI) in combination with STAT3 down-regulation (CSI) in AML. As hypothesized, low-dose targeted RT in combination with blocking of STAT3 signaling improved immune cell trafficking and activation, thereby enhancing the efficacy of this combinatorial therapy in high-disease burden. Further, newly developed low-dose TMI shows enhanced immune modulation over conventional TBI, suggesting the benefits of localized targeted RT in hematological malignancies. Disclosures Hui: Janssen Research & Development, LLC: Consultancy, Honoraria.

EP-1283 Three-dimensional versus four-dimensional dose calculation for breast IMRT

EP-1283 Three-dimensional versus four-dimensional dose calculation for breast IMRT

Surface scanning for 3D dose calculation in intraoperative electron radiation therapy

BackgroundDose calculations in intraoperative electron radiation therapy (IOERT) rely on the conventional assumption of water-equivalent tissues at the applicator end, which defines a flat irradiation surface. However, the shape of the irradiation surface modifies the dose distribution. Our study explores, for the first time, the use of surface scanning methods for three-dimensional dose calculation of IOERT.MethodsTwo different three-dimensional scanning technologies were evaluated in a simulated IOERT scenario: a tracked conoscopic holography sensor (ConoProbe) and a structured-light three-dimensional scanner (Artec). Dose distributions obtained from computed tomography studies of the surgical field (gold standard) were compared with those calculated under the conventional assumption or from pseudo-computed tomography studies based on surfaces.ResultsIn the simulated IOERT scenario, the conventional assumption led to an average gamma pass rate of 39.9% for dose values greater than 10% (two configurations, with and without blood in the surgical field). Results improved when considering surfaces in the dose calculation (88.5% for ConoProbe and 92.9% for Artec).ConclusionsMore accurate three-dimensional dose distributions were obtained when considering surfaces in the dose calculation of the simulated surgical field. The structured-light three-dimensional scanner provided the best results in terms of dose distributions. The findings obtained in this specific experimental setup warrant further research on surface scanning in the IOERT context owing to the clinical interest of improving the documentation of the actual IOERT scenario.

Improvement of conformal arc plans by using deformable margin delineation method for stereotactic lung radiotherapy.

PurposeStereotactic body radiotherapy (SBRT) is an established treatment technique in the management of medically inoperable early stage non–small cell lung cancer (NSCLC). Different techniques such as volumetric modulated arc (VMAT) and three‐dimensional conformal arc (DCA) can be used in SBRT. Previously, it has been shown that VMAT is superior to DCA technique in terms of plan evaluation parameters. However, DCA technique has several advantages such as ease of use and considerable shortening of the treatment time. DCA technique usually results in worse conformity which is not possible to ameliorate by inverse optimization. In this study, we aimed to analyze whether a simple method – deformable margin delineation (DMD) – improves the quality of the DCA technique, reaching similar results to VMAT in terms of plan evaluation parameters.MethodsTwenty stage I–II (T1‐2, N0, M0) NSCLC patients were included in this retrospective dosimetric study. Noncoplanar VMAT and conventional DCA plans were generated using 6 MV and 10 MV with flattening filter free (FFF) photon energies. The DCA plan with 6FFF was calculated and 95% of the PTV was covered by the prescription isodose line. Hot dose regions (receiving dose over 100% of prescription dose) outside PTV and cold dose regions (receiving dose under 100% of prescription dose) inside PTV were identified. A new PTV (PTV‐DMD) was delineated by deforming PTV margin with respect to hot and cold spot regions obtained from conventional DCA plans. Dynamic multileaf collimators (MLC) were set to PTV‐DMD beam eye view (BEV) positions and the new DCA plans (DCA‐DMD) with 6FFF were generated. Three‐dimensional (3D) dose calculations were computed for PTV‐DMD volume. However, the prescription isodose was specified and normalized to cover 95% volume of original PTV. Several conformity indices and lung doses were compared for different treatment techniques.Results DCA‐DMD method significantly achieved a superior conformity index (CI), conformity number (CIP addick), gradient index (R50%), isodose at 2 cm (D2 cm) and external index (CΔ) with respect to VMAT and conventional DCA plans (P < 0.05 for all comparisons). CI ranged between 1.00–1.07 (Mean: 1.02); 1.00–1.18 (Mean: 1.06); 1.01–1.23 (Mean 1.08); 1.03–1.29 (Mean: 1.15); 1.04–1.29 (Mean: 1.18) for DCA‐DMD‐6FFF, VMAT‐6FFF, VMAT‐10FFF DCA‐6FFF and DCA‐10FFF respectively. DCA‐DMD‐6FFF technique resulted significantly better CI compared to others (P = 0.002; < 0.001; < 0.001; < 0.001). R50% ranged between 3.22–4.74 (Mean: 3.99); 3.24–5.92 (Mean: 4.15) for DCA‐DMD‐6FFF, VMAT‐6FFF, respectively. DCA‐DMD‐6FFF technique resulted lower intermediate dose spillage compared to VMAT‐6FFF, though the difference was statistically insignificant (P = 0.32). D2 cm ranged between 35.7% and 67.0% (Mean: 53.2%); 42.1%–79.2% (Mean: 57.8%) for DCA‐DMD‐6FFF, VMAT‐6FFF respectively. DCA‐DMD‐6FFF have significantly better and sharp falloff gradient 2 cm away from PTV compared to VMAT‐6FFF (P = 0.009). CΔ ranged between 0.052 and 0.140 (Mean: 0.085); 0,056–0,311 (Mean: 0.120) for DCA‐DMD, VMAT‐6FFF, respectively. DCA‐DMD‐6FFF have significantly improved CΔ (P = 0.002). VMAT‐ V20 Gy, V2.5 Gy and mean lung dose (MLD) indices are calculated to be 4.03%, 23.83%, 3.42 Gy and 4.19%, 27.88%,3.72 Gy, for DCA‐DMD‐6FFF and DCA techniques, respectively. DCA‐DMD‐6FFF achieved superior lung sparing compared to DCA technique. DCA‐DMD‐6FFF method reduced MUs 44% and 33% with respect to VMAT‐6FFF and 10FFF, respectively, without sacrificing dose conformity (P < 0.001; P < 0.001).ConclusionsOur results demonstrated that DCA plan evaluation parameters can be ameliorated by using the DMD method. This new method improves DCA plan quality and reaches similar results with VMAT in terms of dosimetric parameters. We believe that DCA‐DMD is a simple and effective technique for SBRT and can be preferred due to shorter treatment and planning time.

TLD measurements and Monte Carlo calculations of head and neck organ and effective doses for cone beam computed tomography using 3D Accuitomo 170.

In dentistry, the use of cone beam CT has steadily increased over the last few years. The aim of this study was to measure organ doses and to perform dose calculations based on Monte Carlo (MC) simulations to work out a basis for full three-dimensional (3D) dose calculations for any patient examination performed with the machine used in this study. TLD-100 LiF detectors were placed at 71 measurement positions on the surface and within a RT-Humanoid phantom to cover all relevant radiosensitive organs and tissues. Three examinations with different protocols were performed with the 3D Accuitomo® and dose calculations with MC simulations were carried out for the same three protocols using the EGSnrc MC transport code system. Field of views of 140 × 100, 80 × 50 and 40 × 40 mm2 were selected, the mean organ doses were measured as 5.2, 2.75 and 1.5 mGy and the effective doses were determined as 250, 97 and 48 µSv. For the MC simulation of organ doses and the thermoluminescent dosemeter measurements, an overall agreement within ±10.1% (two standard deviations) was achieved. The measured dose values for 3D Accuitomo® were about a factor 2 lower when compared with conventional CT examinations. Reliable results for the organ doses as well as effective dose values were achieved with thermoluminescent dosemeter measurements in the RT-Humanoid phantom. This study provides the basis for the application of MC simulations for further dose determinations of cone beam CT machines. The MC calculation may therefore be a valuable tool to support the dentists in the evaluation of the trade-off between additional information that may be relevant to the choice of therapy and the additional dose given to the patient.

Predictors of chest wall toxicity after stereotactic ablative radiotherapy using real-time tumor tracking for lung tumors

BackgroundTo evaluate the incidence of chest wall toxicity after lung stereotactic ablative radiotherapy (SABR) and identify risk factors for the development of rib fracture.MethodsThirty-nine patients with 49 lesions underwent SABR for primary or metastatic lung tumors using Cyberknife® with tumor tracking systems. Patient characteristics, treatment factors and variables obtained from dose-volume histograms (DVHs) were analyzed to find the association with chest wall toxicity. Four-dimensional (4D) dose calculations were done to investigate the effect of respiratory motion on dose to the ribs.ResultsAfter follow-up of median 26.7 months (range: 8.4 – 80.0), 8 patients (20.5%) experienced rib fractures and among these patients, three (37.5%) had chest wall pain at 2–3 months after SABR. Median time to rib fracture was 13.4 months (range: 8.0 – 38.5) and the 2-year actuarial risk of rib fracture was 12.2%. Dose to the 4.6 cc of the ribs (D4.6cc) and rib volume received 160 Gy or more (V160) were significant predictor for rib fracture. No significant differences between three-dimensional (3D) and 4D dose calculations were found.ConclusionsParameters from DVH are useful in predicting the risk of chest wall toxicity after SABR for lung tumors. Efforts should be made to reduce the risk of the rib fracture after lung SABR.

Comparing cone-beam CT intensity correction methods for dose recalculation in adaptive intensity-modulated photon and proton therapy for head and neck cancer

ABSTRACTBackground. Adaptive intensity-modulated photon and proton radiotherapy (IMRT and IMPT) of head and neck (H&N) cancer requires frequent three-dimensional (3D) dose calculation. We compared two approaches for dose recalculation on the basis of intensity-corrected cone-beam (CB) x-ray computed tomography (CT) images.Material and methods. For nine H&N tumor patients, virtual CTs (vCT) were generated by deformable image registration of the planning CT (pCT) to the CBCT. The second intensity correction approach used population-based lookup tables for scaling CBCT intensities to the pCT HU range (CBCTLUT). IMRT and IMPT plans were generated with a commercial treatment planning system. Dose recalculations on vCT and CBCTLUT were analyzed using a (3%, 3 mm) gamma-index analysis and comparison of normal tissue and tumor dose/volume parameters. A replanning CT (rpCT) acquired within three days of the CBCT served as reference. Single field uniform dose (SFUD) proton plans were created and recalculated on vCT and CBCTLUT for proton range comparison.Results. Dose/volume parameters showed minor differences between rpCT, vCT and CBCTLUT in IMRT, but clinically relevant deviations between CBCTLUT and rpCT in the spinal cord for IMPT. Gamma-index pass-rates were found increased for vCT with respect to CBCTLUT in IMPT (by up to 21 percentage points) and IMRT (by up to 9 percentage points) for most cases. The SFUD-based proton range assessment showed improved agreement of vCT and rpCT, with 88–99% of the depth dose profiles in beam's eye view agreeing within 3 mm. For CBCTLUT, only 80–94% of the profiles fulfilled this criterion.Conclusion. vCT and CBCTLUT are suitable options for dose recalculation in adaptive IMRT. In the scope of IMPT, the vCT approach is preferable.

Three-dimensional versus four-dimensional dose calculation for volumetric modulated arc therapy of hypofractionated treatments

PurposeRespiratory motion is a non-negligible source of uncertainty in radiotherapy. A common approach is to delineate the target volume in all respiratory phases (ITV) and to calculate a treatment plan using the average reconstruction of the four-dimensional computed tomography (4DCT) scans. In this study the extent of the interplay effect caused by interaction between dynamic dose delivery and respiratory tumor motion, as well as other motion effects were investigated. These effects are often ignored when the ITV concept is used. Methods and MaterialsNine previously treated patients with in ten abdominal or thoracic cancer lesions (3 liver, 3 adrenal glands and 4 lung lesions) were selected for this planning study. For all patients, phase-sorted respiration-correlated 4DCT scans were taken, and volumetric modulated arc therapy (VMAT) treatments were planned using the ITV concept. Margins from ITV to planning target volume (PTV) of 3-10mm were used. Plans were optimized and dose distributions were calculated on the average reconstruction of the 4DCT. 4D dose distributions were calculated to evaluate motion effects, caused by the interference of dynamic treatment delivery with respiratory tumor motion and inhomogeneously planned target dose. These calculations were performed on the phase-sorted CT series with a respiration-correlated assignment of the treatment plan's monitor units (MU) to the respiration phases of the 4DCT. The 4D dose was accumulated with rigid as well as deformable registrations of the CT series and compared to the original 3D dose distribution. Maximum, minimum and mean doses to ITV and PTV, and maximum or mean doses to organs at risk (OAR), were compared after rigid accumulation. The dose variation in the gross tumor volume (GTV) was compared after deformable registration. ResultsUsing rigid registrations, variations in the investigated dose parameters between 3D and 4D dose calculations were found to be within -2.1% to 1.4% for all target volumes and within -0.8% to 1.7% in OAR. Using deformable registrations, dose differences in the GTV were below 3.8% for dose accumulation of lung and adrenal gland patients. For liver patients the used deformable registrations were not considered to be robust enough. It was also shown that a major part of the dose differences originates from the Hounsfield unit differences between 3D and 4D calculations, regardless of the interplay effect. ConclusionThe evaluated motion effects during VMAT treatments resulted in negligible dose variability. Therefore, the approximation of calculating the dose on the average reconstruction of the 4DCT (3D dose calculation), instead of calculating on the respiration-correlated phase CTs (4D dose calculation) with assignment of the corresponding MUs, gives acceptable results.

MO-F-108-10: Implementation of 4Pi Non-Coplanar Converging (4PiNC) Beams Using a Conical Delivery Geometry

Purpose: To study the feasibility of a proposed conical delivery system for implementing 4PiNC beams. Methods: In the proposed novel 4PiNC beam delivery system, the x-ray source aims at the isocenter but rotates around a point on the superior-inferior axis in a conical fashion. Mathematically, each rotation forms a cone where the vertex is the isocenter and the base is the area bounded by the circular trajectory of the x-ray source. By changing the cone angle (90 - vertex angle), the proposed 4PiNC system allows hundreds of non-coplanar beams to be delivered without couch rotation. We developed a simple treatment planning system to investigate the dosimetry for an elliptic cylinder phantom. The planning target volume was a sphere inside the phantom. The three-dimensional dose calculation was calculated using the TMR table, off-axis ratios and output factors of a 6 x-ray beam commissioned for radiosurgery cones. Planning parameters (depth, off-axis distance…) were calculated analytically. The dose grid contained 100×100×100 voxels with a voxel size of 2×2×2 mm^3. Results: Dose distributions were calculated for (A) 263 beams using cone angles 0, ±20, ±40, ±60, ±80 degree, (B) 153 beam using cone angles 0, 20, 40, 60, 80 degree and (C) 179 beams using cone angles 0, ±20, ±40 degree. Highly conformed dose distributions were achieved with conformity indexes 1.12, 1.11 and 1.19 for case (A), (B) and (C), respectively. The dose falloff (80%–20% of the central dose profile) was 8 mm, 8 mm and 11 mm, respectively. Conclusion: We proved in principle that the proposed 4PiNC beam delivery system can provide excellent dose distribution (low conformity index and fast dose falloff) without couch rotation. The proposed system can significantly improve the treatment plan quality and avoid couch-gantry collisions, a common patient safety concern of c-arm linear accelerators, when delivering non-coplanar beams. This work was partially supported by a DOD grant DOD W81XWH1010862.

A Dosimetric Effects of 4-dimensional and Monte Carlo Calculation Methods on Treatment Plans for Mobile Lung Cancer in Robotic Radiosurgery

Recent advances in computational technique have allowed use of sophisticated dose-calculation methods in robotic radiosurgery, namely a four-dimensional (4D) dynamic dose calculation combined with deformable image registration method and a full Monte-Carlo algorithm. In this work, we systematically evaluated the dosimetric effects of these advanced techniques relative to the conventional treatment methods, a three-dimensional (3D) dose calculation and a simple ray-tracing algorithm, using actual patient's 4DCT data.

Poster - Thur Eve - 70: Quantification of tumour dose enhancement at kilo-voltage energies due to the presence of gold nanoparticles during radiation therapy: EGSnrcMP Monte Carlo study.

One of the greatest challenges in radiation therapy is the ability to deliver a lethal dose of radiation to a tumour while sparing the surrounding normal tissues. In theory, the dose delivered to a tumour during photon-based radiation therapy can be enhanced by loading high atomic number (Z) materials into the tumour, which results in greater photoelectric absorption and hence increased photoelectron fluence within the tumour than in surrounding tissues. The EGSnrcMP Monte Carlo code, together with DOSXYZnrc, a three-dimensional voxel dose calculation module has been used to study the macroscopic dose enhancement factor (MDEF) in a tumour infused with gold nanoparticles at the kilo-voltage energies. We observed that gold nanoparticles infused in a tumour irradiated with kilo-voltage energies has the potential to enhance the tumour dose by a factor ranging from 0.25 to about 5 depending on the mean energy of the beam and the concentration of gold nanoparticles in the tumour. The increase in dose can be attributed to the significant increase in the photoelectron fluence within the tumour loaded with gold particles during the irradiation. Future studies will involve the characterization of the MDEF at megavoltage energies.

Optimization of 3D Planning Dosimetry in a Breast Phantom for the Match Region of Supraclavicular and Tangential Fields

Introduction: The complex geometry of breast and also lung and heart inhomogeneities near the planning target volume (PTV) result in perturbations in dose distribution. This problem can result in overdosage or underdosage in the match region of the three treatment fields. The purpose of this study is to create a homogeneous dose distribution in the match region between the supraclavicular and tangential fields, utilizing Gafchromic EBT film for dosimetry. Material and Methods: In this study, a slab phantom was designed with lung- and heart-equivalent inhomogetenities. Our measurements were carried out using Gafchromic EBT film. Irradiations were performed using the 6 MV photon beam of a Varian 2100 Clinac linear accelerator. Three-dimensional treatment planning and dose calculations were carried out using the RT Dose Plan software. The conventional method with source-to-skin distance match and a 3D method with source-to-axis distance match under different combinations of parameters such as gantry, collimator and couch rotations as well as asymmetric fields were utilized. Results: The results demonstrated differences between the 3D and conventional techniques. The best results in the match region were observed using the 3D method with asymmetric technique (maximum area of overlap = 0.43 cm2, depth of overlap = 3.55 cm, and average width of overlap = 0.75 cm). Discussion and Conclusion: This study shows that the EBT film is a suitable tool for two-dimensional distribution dosimetry and relative dose measurements. The study shows difficulties in achieving a homogenous dose distribution in the match region of supraclavicular and tangential fields and also demonstrates the effects of setup parameters on matching of treatment fields.

Dosimetric evaluation of a three-dimensional treatment planning system

The computerized treatment planning system plays a major role in radiation therapy in delivering correct radiation dose to the patients within ±5% as recommended by the ICRU. To evaluate the dosimetric performance of the Treatment Planning system (TPS) with three-dimensional dose calculation algorithm using the basic beam data measured for 6 MV X-rays. Eleven numbers of test cases were created according to the Technical Report Series-430 (TRS 430) and are used to evaluate the TPS in a homogeneous water phantom. These cases involve simple field arrangements as well as the presence of a low-density material in the beam to resemble an air in-homogeneity. Absolute dose measurements were performed for the each case with the MU calculation given by the TPS, and the measured dose is compared with the corresponding TPS calculated dose values. The result yields a percentage difference maximum of 2.38% for all simple test cases. For complex test cases in the presence of in-homogeneity, beam modifiers or beam modifiers with asymmetric fields a maximum percentage difference of 5.94% was observed. This study ensures that the dosimetric calculations performed by the TPS are within the accuracy of ±5% which is very much warranted in patient dose delivery. The test procedures are simple, not only during the installation of TPS, but also repeated at periodic intervals.

A Treatment Planning Method for Sequentially Combining Radiopharmaceutical Therapy and External Radiation Therapy

Effective cancer treatment generally requires combination therapy. The combination of external beam therapy (XRT) with radiopharmaceutical therapy (RPT) requires accurate three-dimensional dose calculations to avoid toxicity and evaluate efficacy. We have developed and tested a treatment planning method, using the patient-specific three-dimensional dosimetry package 3D-RD, for sequentially combined RPT/XRT therapy designed to limit toxicity to organs at risk.The biologic effective dose (BED) was used to translate voxelized RPT absorbed dose (D(RPT)) values into a normalized total dose (or equivalent 2-Gy-fraction XRT absorbed dose), NTD(RPT) map. The BED was calculated numerically using an algorithmic approach, which enabled a more accurate calculation of BED and NTD(RPT). A treatment plan from the combined Samarium-153 and external beam was designed that would deliver a tumoricidal dose while delivering no more than 50 Gy of NTD(sum) to the spinal cord of a patient with a paraspinal tumor.The average voxel NTD(RPT) to tumor from RPT was 22.6 Gy (range, 1-85 Gy); the maximum spinal cord voxel NTD(RPT) from RPT was 6.8 Gy. The combined therapy NTD(sum) to tumor was 71.5 Gy (range, 40-135 Gy) for a maximum voxel spinal cord NTD(sum) equal to the maximum tolerated dose of 50 Gy.A method that enables real-time treatment planning of combined RPT-XRT has been developed. By implementing a more generalized conversion between the dose values from the two modalities and an activity-based treatment of partial volume effects, the reliability of combination therapy treatment planning has been expanded.

Dosimetric variation due to the photon beam energy in the small‐animal irradiation: A Monte Carlo study

The impact of photon beam energy and tissue heterogeneities on dose distributions and dosimetric characteristics such as point dose, mean dose, and maximum dose was investigated in the context of small-animal irradiation using Monte Carlo simulations based on the EGSnrc code. Three Monte Carlo mouse phantoms, namely, heterogeneous, homogeneous, and bone homogeneous were generated based on the same mouse computed tomography image set. These phantoms were generated by overriding the tissue type of none of the voxels (heterogeneous), all voxels (homogeneous), and only the bone voxels (bone homogeneous) to that of soft tissue. Phase space files of the 100 and 225 kVp photon beams based on a small-animal irradiator (XRad225Cx, Precision X-Ray Inc., North Branford, CT) were generated using BEAMnrc. A 360 degrees photon arc was simulated and three-dimensional (3D) dose calculations were carried out using the DOSXYZnrc code through DOSCTP in the above three phantoms. For comparison, the 3D dose distributions, dose profiles, mean, maximum, and point doses at different locations such as the isocenter, lung, rib, and spine were determined in the three phantoms. The dose gradient resulting from the 225 kVp arc was found to be steeper than for the 100 kVp arc. The mean dose was found to be 1.29 and 1.14 times higher for the heterogeneous phantom when compared to the mean dose in the homogeneous phantom using the 100 and 225 kVp photon arcs, respectively. The bone doses (rib and spine) in the heterogeneous mouse phantom were about five (100 kVp) and three (225 kVp) times higher when compared to the homogeneous phantom. However, the lung dose did not vary significantly between the heterogeneous, homogeneous, and bone homogeneous phantom for the 225 kVp compared to the 100 kVp photon beams. A significant bone dose enhancement was found when the 100 and 225 kVp photon beams were used in small-animal irradiation. This dosimetric effect, due to the presence of the bone heterogeneity, was more significant than that due to the lung heterogeneity. Hence, for kV photon energies of the range used in small-animal irradiation, the increase of the mean and bone dose due to the photoelectric effect could be a dosimetric concern.

A Pilot Study on the Feasibility of Real-Time Calculation of Three-Dimensional Dose Distribution for 153Sm-EDTMP Radionuclide Therapy Based on the Voxel S-Values

Using (153)Sm-EDTMP therapy for bone metastases from nasopharyngeal cancer (NPC), we attempted a real-time three-dimensional (3D) dose calculation based on the S-value for voxels. The 3D radionuclide uptake data on SPECT images were factored into the precalculations by the Monte Carlo (MC) method. For the nuclide (153)Sm, the S-value for voxels of size 3.45 x 3.45 x 3.45 mm(3) in a soft tissue phantom was precalculated on a self-developed program built from the MC program EGS4. Based on the SPECT/CT image of the patient, the 3D dose rate distributions were calculated with the S-value method and compared with the direct MC calculation results. The total volume of the whole calculation region and the bone were chosen as the regions of interest (ROIs); additionally, the dose rate volume histograms (DVHs) for the ROIs were also calculated. The iso-dose was administered based on the scan images obtained at 6 hours following injection with the nuclide. In the calculation region, the maximum dose rate was 5.98 x 10(-5) and 6.26 x 10(-5) mGy/(MBq S) for S-value and direct MC calculations, respectively. Once the dose rate was normalized to the maximum dose rate point, the iso-dose curves for both methods of calculation were similar in most regions and reasonably matched the functional image as well. The DVHs of the ROIs indicated that the dose rate distributions were nonuniform, that is, approximately 30% of the bone-ROI volume received 10% of the maximum dose rate; however, only 3.8% volume received 50% of the maximum dose rate. Using (153)Sm-EDTMP therapy for bone metastases from NPC, we attempted to supplement the current dosimetry work at the image-level by a pragmatic and real-time dosimetry calculation based on S-value and functional imaging. More accurate dose calculations for patients undergoing radionuclide therapy will depend on the development of higher image resolution in nuclear medicine and warrant further studies to optimize the pharmacokinetics model.