G Mora

Conversion of CT numbers into tissue parameters for Monte Carlo dose calculations: a multi-centre study.

The conversion of computed tomography (CT) numbers into material composition and mass density data influences the accuracy of patient dose calculations in Monte Carlo treatment planning (MCTP). The aim of our work was to develop a CT conversion scheme by performing a stoichiometric CT calibration. Fourteen dosimetrically equivalent tissue subsets (bins), of which ten bone bins, were created. After validating the proposed CT conversion scheme on phantoms, it was compared to a conventional five bin scheme with only one bone bin. This resulted in dose distributions D(14) and D(5) for nine clinical patient cases in a European multi-centre study. The observed local relative differences in dose to medium were mostly smaller than 5%. The dose-volume histograms of both targets and organs at risk were comparable, although within bony structures D(14) was found to be slightly but systematically higher than D(5). Converting dose to medium to dose to water (D(14) to D(14wat) and D(5) to D(5wat)) resulted in larger local differences as D(5wat) became up to 10% higher than D(14wat). In conclusion, multiple bone bins need to be introduced when Monte Carlo (MC) calculations of patient dose distributions are converted to dose to water.

SU-F-T-649: Dosimetric Evaluation of Non-Coplanar Arc Therapy Using a Novel Rotating Gamma Ray System

PURPOSE Stereotactic intra and extra-cranial body radiation therapy has evolved with advances in treatment accuracy, effective radiation dose, and parameters necessary to maximize machine capabilities. Novel gamma systems with a ring type gantry were developed having the ability to perform oblique arcs. The aim of this study is to explore the dosimetric advantages of this new system. METHODS The rotating Gamma system is named CybeRay (Cyber Medical Corp., Xian, China). It has a treatment head of 16 cobalt-60 sources focused to the isocenter, which can rotate 360° on the ring gantry and swing 35° in the superior direction. Treatment plans were generated utilizing our in-house Monte Carlo treatment planning system. A cylindrical phantom was modeled with 2mm voxel size. Dose inside the cylindrical phantom was calculated for coplanar and non-coplanar arcs. Dosimetric differences between CybeRay cobalt beams and CyberKnife 6MV beams were compared in a lung phantom and for previously treated SBRT patients. RESULTS The full width at half maxima of cross profiles in the S-I direction for the coplanar setup matched the cone sizes, while for the non-coplanar setup, FWHM was larger by 2mm for a 10mm cone and about 5mm for larger cones. In the coronal and sagittal view, coplanar beams showed elliptical shaped isodose lines, while non-coplanar beams showed circular isodose lines. Thus proper selection of the oblique angle and cone size can aid optimal dose matching to the target volume. Comparing a single 5mm cone from CybeRay to that from CyberKnife showed similar penumbra in a lung phantom but CybeRay had significant lower doses beyond lung tissues. Comparable treatment plans were obtained with CybeRay as that from CyberKnife.ConclusionThe noncoplanar multiple source arrangement of CybeRay will be of great clinical benefits for stereotactic intra and extra-cranial radiation therapy.

SU‐G‐BRC‐08: Evaluation of Dose Mass Histogram as a More Representative Dose Description Method Than Dose Volume Histogram in Lung Cancer Patients

PURPOSE Dose-volume-histogram (DVH) is widely used for plan evaluation in radiation treatment. The concept of dose-mass-histogram (DMH) is expected to provide a more representative description as it accounts for heterogeneity in tissue density. This study is intended to assess the difference between DVH and DMH for evaluating treatment planning quality. METHODS 12 lung cancer treatment plans were exported from the treatment planning system. DVHs for the planning target volume (PTV), the normal lung and other structures of interest were calculated. DMHs were calculated in a similar way as DVHs expect that the voxel density converted from the CT number was used in tallying the dose histogram bins. The equivalent uniform dose (EUD) was calculated based on voxel volume and mass, respectively. The normal tissue complication probability (NTCP) in relation to the EUD was calculated for the normal lung to provide quantitative comparison of DVHs and DMHs for evaluating the radiobiological effect. RESULTS Large differences were observed between DVHs and DMHs for lungs and PTVs. For PTVs with dense tumor cores, DMHs are higher than DVHs due to larger mass weighing in the high dose conformal core regions. For the normal lungs, DMHs can either be higher or lower than DVHs depending on the target location within the lung. When the target is close to the lower lung, DMHs show higher values than DVHs because the lower lung has higher density than the central portion or the upper lung. DMHs are lower than DVHs for targets in the upper lung. The calculated NTCPs showed a large range of difference between DVHs and DMHs. CONCLUSION The heterogeneity of lung can be well considered using DMH for evaluating target coverage and normal lung pneumonitis. Further studies are warranted to quantify the benefits of DMH over DVH for plan quality evaluation.

SU-E-T-335: Dosimetric Investigation of An Advanced Rotating Gamma Ray System for Imaged Guided Radiation Therapy

Purpose: Co-60 beams have unique dosimetric properties for cranial treatments and thoracic cancers. The conventional concern about the high surface dose is overcome by modern system designs with rotational treatment techniques. This work investigates a novel rotational Gamma ray system for image-guided, external beam radiotherapy. Methods: The CybeRT system (Cyber Medical Corp., China) consists of a ring gantry with either one or two treatment heads containing a Gamma source and a multileaf collimator (MLC). The MLC has 60 paired leaves, and the maximum field size is either 40cmx40cm (40 pairs of 0.5cm central leaves, 20 pairs of 1cm outer leaves), or 22cmx40cm (32 pairs of 0.25cm central leaves, 28 pairs of 0.5cm outer leaves). The treatment head(s) can swing 35° superiorly and 8° inferiorly, allowing a total of 43° non-coplanar beam incident. The treatment couch provides 6-degrees-of-freedom motion compensation and the kV cone-beam CT system has a spatial resolution of 0.4mm. Monte Carlo simulations were used to compute dose distributions and compare with measurements. A retrospective study of 98 previously treated patients was performed to compare CybeRT with existing RT systems. Results: Monte Carlo results confirmed the CybeRT design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient was similar to or better than that of 6MV photon beams and its isocenter accuracy is 0.3mm. Co-60 beams produce lower-energy secondary electrons that exhibit better dose properties in low-density lung tissues. Because of their rapid depth dose falloff, Co-60 beams are favorable for peripheral lung tumors with half-arc arrangements to spare the opposite lung and critical structures. Superior dose distributions were obtained for head and neck, breast, spine and lung tumors. Conclusion: Because of its accurate dose delivery and unique dosimetric properties of C-60 sources, CybeRT is ideally suited for advanced SBRT as well as conventional RT. This work was partially supported by Cyber Medical Corp.

SU-E-T-605: A New Design for a Rotating Gamma Knife. Monte Carlo Simulation

PURPOSE to determine the characteristics of the 60Co beam emerging from a new design of a rotating Gamma Knife system and to calculate 3D dose distributions at the isocenter for different source configurations and collimator openings. METHODS We employed the BEAM-Monte Carlo code to realistically model the geometry design, including 30 60Co source capsules, two circular primary collimators (diameter of 6.6mm and 6.1mm) and four different changeable collimators. The shielding of the head design was also simulated. The sources (2.8mm diameter) are distributed in six groups in the spherical geometry. Each source is individually collimated to obtained four different circular fields at the isocenter (3mm, 3.5mm, 6mm and 8mm). The phase-space particles reaching the scoring plane below the primary collimation assembly were recorded and the BEAMDP code was used to determine the fluence and energy spectra of the particles emerging from each source-collimator configuration. The dose distributions at the isocenter plane (397.6mm from the source) were calculated in a spherical component module for the circular field sizes studied. RESULTS The energy spectra below the head assembly and primary collimators have been obtained, which exhibited the typical 60Co peaks and a small low-energy tail due to scattered photons (from about 200keV to 1MeV). The scattered component of the spectra represents about 8 % of the total number of photons reaching the scoring plane. The radial photon fluence does not vary significantly inside the collimator openings. The spectra of particles from different source groups are compared. CONCLUSION The 60Co beam emerging from each source configuration was characterized, which can be used to establish a generic source model for all the sources for fast MC dose calculation. Further investigations are needed to determine the dose variations as a result of partial switching on/off different groups of sources for advanced Gamma Knife SRS/SBRT planning.

SU‐E‐T‐240: Monte Carlo Modelling of SMC Proton Nozzles Using TOPAS

PURPOSE To simulate the 60Co beam from a novel Gamma-Tomo SBRT system and to calculate the output factors and dose rates for different source configurations and collimator sizes. METHODS The BEAM-Monte Carlo code is used to realistically model the system geometry, including 32 source capsules, source housing, primary collimator and 4 different changeable collimators. The sources (6.22 mm of diameter) are located in 2 rows with different angles and distances to the longitudinal axis and the beams can be collimated to obtain four different circular field sizes at the isocenter (35 mm, 16 mm, 7mm, and 3.5mm). Since the dose distributions will be determined in a spherical polystyrene phantom of 8cm radius centered at the isocenter, a new geometry module is designed for the dose calculation. RESULTS The caracteristics of the particle spectra emerging from each source are determined and the influence of the source position on the spectra reaching the isocenter is found to be minimal (<2%). We found that the scattered photons represent about 15% of the total number of photons reaching the scoring plane immediately (1 mm) below the source and primary collimator. The radial photon fluence does not vary significantly inside the collimator opening and decreases quickly beyond the 3cm radius (projected at the isocenter plane). CONCLUSION Our preliminary results indicate that the position of the source and primary collimator assembly does not affect the energy spectra and fluence distributions reaching the isocenter significantly. This allows the use of a well-represented source model for all the sources to perform the dose calculations for all the changeable collimators for this novel Gamma-Tomo SBRT system.

SU-E-T-238: Monte Carlo Estimation of Cerenkov Dose for Photo-Dynamic Radiotherapy

PURPOSE Estimation of Cerenkov dose from high-energy megavoltage photon and electron beams in tissue and its impact on the radiosensitization using Protoporphyrine IX (PpIX) for tumor targeting enhancement in radiotherapy. METHODS The GEPTS Monte Carlo code is used to generate dose distributions from 18MV Varian photon beam and generic high-energy (45-MV) photon and (45-MeV) electron beams in a voxel-based tissueequivalent phantom. In addition to calculating the ionization dose, the code scores Cerenkov energy released in the wavelength range 375-425 nm corresponding to the pick of the PpIX absorption spectrum (Fig. 1) using the Frank-Tamm formula. RESULTS The simulations shows that the produced Cerenkov dose suitable for activating PpIX is 4000 to 5500 times lower than the overall radiation dose for all considered beams (18MV, 45 MV and 45 MeV). These results were contradictory to the recent experimental studies by Axelsson et al. (Med. Phys. 38 (2011) p 4127), where Cerenkov dose was reported to be only two orders of magnitude lower than the radiation dose. Note that our simulation results can be corroborated by a simple model where the Frank and Tamm formula is applied for electrons with 2 MeV/cm stopping power generating Cerenkov photons in the 375-425 nm range and assuming these photons have less than 1mm penetration in tissue. CONCLUSION The Cerenkov dose generated by high-energy photon and electron beams may produce minimal clinical effect in comparison with the photon fluence (or dose) commonly used for photo-dynamic therapy. At the present time, it is unclear whether Cerenkov radiation is a significant contributor to the recently observed tumor regression for patients receiving radiotherapy and PpIX versus patients receiving radiotherapy only. The ongoing study will include animal experimentation and investigation of dose rate effects on PpIX response.

SU‐E‐T‐518: Effect of Statistic Uncertainty On DVH and Dose Mass Histogram (DMH) Derived Dose Indices (DI) for Head and Neck IMRT‐Monte Carlo(MC) Treatments

PURPOSE The use of DMH as an alternative method to validate the treatment plans of the head and neck region has been investigated in the past[1].For patient geometry contains large air cavities,high statistical uncertainties were found in dose distributions calculated using MC method in air cavities and tissues that surround air cavities.These previous studies reported that the shape of DMH is less affected by the statistical uncertainty than DVH. But the authors did not verify the differences between DIs derived from DVH and DMH.In the present work we compare the DIs derived from DVHs and DMHs based on dose distributions calculated using MC method and different levels of statistical precision. METHODS Six IMRT plans were calculated using the EGS4 based MCSIM code.MCSHOW allows to built DVHs and DMHs from dose distributions calculated with different statistical uncertainty(from 0.5% to 10%). The DIs were derived from the DVHs and DMHs for the target and critical structures (cord, chiasm, larynx,brainstem and parotid). RESULTS Comparing the values (averaged over 6 patients) of target DMH (0.5%) and DMH (2%) we found that all DIs (D05, D95, Mp, Dmax) do not differ in more than 0.7%. By comparison of target DVH (0.5%) and target DVH (2%) we observed differences of about 14% on the value of Dmax and V95 changes in more than 3%. We verified that the statistical effect is not significant (less than 1%) for the DVH of critical structures. CONCLUSION The DIs derived from DMHs are not strong dependent on the statistical uncertainty of the dose values as those derived from DVHs. Our results confirmed the previous idea that DMH is a better parameter than DVH for evaluating treatment plans calculated by MC simulations in low density regions. [1]-G.Mora et al., MCTP 2009,Cardiff 2009.

SU‐GG‐T‐397: Voxel Size Effect on Dose Mass Histograms of Head and Neck‐ IMRT Monte Carlo Treatments

Purpose: Previous studies [1] demonstrated that Dose Volume Histogram (DMH) is a better parameter than DVH for evaluating treatment plans calculated by Monte Carlo(MC) simulations that may have large statistical uncertainties in low‐density regions such as air cavities and lungtissues. On other hand, it has been showed [2] that the voxel size may affect the dose distributions calculated by MC for head and neck region. In the present work we asses the voxel size effect on DMH for head and neck MC treatments. Method and Materials: Four IMRT head and neck cases were included in this study. MC patient simulation phantoms of different voxel sizes (2–9mm), were built from the same patient CT data. The EGS4 based MCSIM code and photon source models for 6 MV beams were used to calculate the isodose distributions, DVH and DMHs for both, PTV and critical structures. Results: Our results show significant differences between the DMHs calculated in the simulation phantoms of different voxel sizes (2–9mm). The effect is more significant for critical structures (up to 10%). DMH is more sensitive than DVH to the voxel size effect, however the accurate calculation of the DMH requires significantly less CPU time than the calculation of DVH. Conclusion: Our results suggest that 2 mm voxels should be used for the calculation of DMHs for head and neck for accurate treatment evaluation. Since the voxel size effect reduces the abiltiy of DMH to properly validate the plan, it may have a negative impact on the treatment outcome.

SU‐GG‐T‐351: Using Dose Mass Histograms (DMH) for the Evaluation of Head and Neck IMRT Plans Calculated by Monte Carlo

Purpose:Dose volume histograms (DVH) represent an important tool for the clinical evaluation of radiotherapy treatment plans. For head and neck regions, however, the presence of air cavities makes the DVH tool less adequate for Monte Carlo calculated IMRT plans. The air cavities may introduce high dose uncertainties in both air cavities and surrounding tissues (interface effects). Since the dose to air is clinically irrelevant DVH becomes less clinically representative in the presence of large air cavities, and hence is no longer a good parameter for plan evaluation. In this work we assess the limitations of DVH for head and neck plan evaluation and investigate the dose mass histogram (DMH) as an alternative to overcome those limitations. Method and Materials: Seven IMRT head and neck cases were included in this study. The Monte Carlo simulation geometry (2mm voxels) was built from patient CT data. Patient dose calculations were performed using the EGS4 based MCSIM code and photon source models for 6 and 10MV beams. Isodose distributions and DVH (including air) were generated for these plans. A new feature was created in order to calculate DMH for both the target (PTV) and critical structures. In addition, DVH excluding air were also generated. Results: Our results for the 7 patients show significant differences (up to 10%) between the DVH calculated including and excluding air for the PTV and critical structures. DMH, on the other hand, eliminates the effect of large Monte Carlo statistical uncertainties in air cavities and is a better parameter than DVH for evaluating head and neck IMRT treatment plans. Conclusion: DMH is a better parameter than DVH for evaluating treatment plans calculated by Monte Carlo simulations that may have large statistical uncertainties in low‐density regions such as air cavities and lungtissues.