Dose Histograms Research Articles

Feasibility of Stereotactic Body Radiation Therapy for Non–small Cell Lung Cancer Using a Linear Accelerator-Based Magnetic Resonance Image Guided Radiation Therapy System

To describe a linear accelerator (linac) based magnetic resonance image guided radiation therapy system (MR-IGRT) and demonstrate the feasibility of delivering stereotactic body radiation therapy (SBRT) for non-small cell lung cancer (NSCLC) treatment. The linac-based MR-IGRT (MRL) system currently being installed at our institution integrates a 0.35T MRI with a flattening-filter free (FFF) linac delivering 6MV at 600 cGy/min. A novel double-focused and double-stack multileaf collimator (MLC) with an effective leaf width of 4 mm provides modulation for step-and-shoot IMRT delivery. The ability of a similar Cobalt-based system for real time, real anatomy gating has already been demonstrated, and a number of patients with NSCLC were treated. Here, we investigate the feasibility of planning and delivery with the linac-based system for SBRT, primarily focusing on plan quality as compared to conventional linacs (CL) and delivery times. Four MRL plans were generated for NSCLC patients who were previously planned and treated on a CL using 6MV FFF beams with a total dose of 54 Gy in 3 fractions. MRL plans were compared to clinical plans (CP) by evaluating conformity numbers with 100% (CN100), 95% (CN95) of prescription dose (Rx), heterogeneity indices (HI), maximum dose and dose-volume histograms of organs-at-risks (OARs), and delivery times. All MRL plans achieved PTV coverage and OAR sparing within clinical constraints and were comparable to clinically-treated plans as indicated in Table 1. Three plans showed an average of 8% and 12% improvement on PTV conformity CN100 and CN95, respectively. There is no clear trend for treatment time comparison but in general they only differ by 3 mins for a 10-min long treatment. The MRL is capable of generating clinically acceptable SBRT plans targeting NSCLC. With real-time MR gating, novel double-stack MLC design and fast FFF delivery, the system is able to achieve a more conformal and precise dose to the target therefore having great potential for further lung tumor dose escalation and reduction of treated volumes. We expect that a higher dose rate version will be available soon to further reduce the treatment time.Abstract 3536IDPlanTumor LocationVPTV Rx (%)VPTV 95%Rx (%)HICN100CN95Max Cord (Gy)Max Esophagus (Gy)Max Heart (Gy)Max Carina (Gy)Mean Lung-ITV (Gy)V20 Lung-ITV (%)Treatment time (min)1MRLLeft upper lobe97.999.61.160.920.844.4910.5521.828.843.193.110.2CP97.399.91.230.860.785.779.6818.519.223.023.07.22MRLLeft lower lobe99.11001.160.800.641.268.3310.970.452.412.45.7CP98.71001.150.840.690.456.2210.010.281.872.27.83MRLRight upper lobe99.299.91.160.740.665.138.670.730.781.210.711.8CP99.81001.170.720.581.418.160.310.421.081.09.04MRLLeft lower lobe97.299.71.260.890.8313.8714.0712.8912.676.216.26.9CP96.6991.200.780.709.5218.0115.6816.576.319.310.0 Open table in a new tab

Dosimetria do Tl-201 em cintilografia de perfusão do miocárdio

Este trabalho conduz ao estudo da cintilografia de perfusão miocárdica, e investigação da dosimetria espacial do TI-201 distribuído no miocárdio. Métodos e Resultados: A manipulação de dados radiológicos foi realizado a fim de criar um modelo computacional do voxel do coração. Um conjunto de imagens obtidas por angiotomografia da aorta torácica e abdominal forneceu informações anatômicas e funcionais para a modelagem do coração no código SISCODES. A distribuição homogênea do TI-201 foi assumido no músculo cardíaco. Simulações do transporte de partículas através dos voxels e a interação com os tecidos do coração foram realizadas através do MCNP – Código Monte Carlo. A distribuição espacial de dose no modelo de coração é apresentado, bem como o histograma dose versus volume no músculo cardíaco. Para validação do estudo foi utilizado um modelo analítico. Conclusão: A presente ferramenta computacional gerou distribuição de dose espacial na cintilografia de perfusão do miocárdio. Especialmente, a dosimetria realizada elucida a dose transmitida no músculo cardíaco por unidade de atividade injetada de Tl-201, o que pode contribuir para futuras investigações de efeitos determinísticos no coração.

Esophageal wall dose-surface maps do not improve the predictive performance of a multivariable NTCP model for acute esophageal toxicity in advanced stage NSCLC patients treated with intensity-modulated (chemo-)radiotherapy

In our previous work, a multivariable normal-tissue complication probability (NTCP) model for acute esophageal toxicity (AET) Grade ⩾2 after highly conformal (chemo-)radiotherapy for non-small cell lung cancer (NSCLC) was developed using multivariable logistic regression analysis incorporating clinical parameters and mean esophageal dose (MED). Since the esophagus is a tubular organ, spatial information of the esophageal wall dose distribution may be important in predicting AET. We investigated whether the incorporation of esophageal wall dose-surface data with spatial information improves the predictive power of our established NTCP model. For 149 NSCLC patients treated with highly conformal radiation therapy esophageal wall dose-surface histograms (DSHs) and polar dose-surface maps (DSMs) were generated. DSMs were used to generate new DSHs and dose-length-histograms that incorporate spatial information of the dose-surface distribution. From these histograms dose parameters were derived and univariate logistic regression analysis showed that they correlated significantly with AET. Following our previous work, new multivariable NTCP models were developed using the most significant dose histogram parameters based on univariate analysis (19 in total). However, the 19 new models incorporating esophageal wall dose-surface data with spatial information did not show improved predictive performance (area under the curve, AUC range 0.79−0.84) over the established multivariable NTCP model based on conventional dose-volume data (AUC = 0.84). For prediction of AET, based on the proposed multivariable statistical approach, spatial information of the esophageal wall dose distribution is of no added value and it is sufficient to only consider MED as a predictive dosimetric parameter.

Instrumental Quality Control of Therapeutic Linear Accelerator Performance

The objective of the article was to assess therapeutic linear accelerator performance. Material & method used were quality control tools, direct measurement & theoretical calculation methods. The analysis of results showed that: shift of machine isocenter was 1 mm then increases up to 2 mm through the gantry angles 0 to 300° and 300 to 360 respectively. The diaphragm rotation isocenter clock & anti-clock wise was 1mm. the light and radiation fields showed concise matching up to 9×9 cm, then for 10×10, 14×14 and 16×16 cm there were incongruence by 0.25, 0.3 and 0.41 cm respectively. The increment of the field sizes (2×2, 4×4 - 20×20) cm following SSD increment fitted with the inverse square law significantly (R2 = 1). The theoretical (calculation method) field size was greater than the measured (practical) field size relative to SSD by 0.2 cm. The system output in Gy/Mu increases significantly (R2 = 0.9) as the field size increases in logarithmic equation; while it decreases as SSD increases. The measured output on phantom surface was greater (0.8Gy/MU) than that calculated theoretically which was (0.5 Gy/MU). A significant (R2 = 0.8) reduction in output reading following the increment of temperature for Linac 10 MV and 6 MV respectively, while the pressure lead to significant (0.6) increment of system output reading. TLD showed narrow penumbra extension as 0.32 and 0.2 cm for lianc 6MV and 10MV respectively compared with 0.5 and 0.3 cm at maximum depth dose when obtained from dose histogram.

Estimation of Secondary Skin Cancer Risk Due To Electron Contamination in 18-MV LINAC-Based Prostate Radiotherapy

Introduction Accurate estimation of the skin-absorbed dose in external radiation therapy is essential to estimating the probability of secondary carcinogenesis induction Materials and Methods Electron contamination in prostate radiotherapy was investigated using the Monte Carlo (MC) code calculation. In addition, field size dependence of the skin dose was assessed. Excess cancer risk induced by electron contamination was determined for the skin, surface dose, and prostate dose-volume histogram (DVH) using MC calculation and analytical methods. Results MC calculations indicated that up to 80% of total electron contamination fluence was produced in the linear accelerator. At 5 mm below the skin surface, surface dose was estimated at 6%, 13%, 27%, and 38% for 5×5 cm2, 10×10 cm2, 20×20 cm2, and 40×40 cm2 field sizes, respectively. Relative dose at Dmax was calculated at 0.92% and 5.42% of the maximum dose for 5×5 cm2 and 40×40 cm2 field sizes, respectively. Excess absolute skin cancer risk was obtained at 2.96×10-4 (PY) -1 for total 72 Gy. Differences in prostate and skin DVHs were 1.01% and 1.38%, respectively. Conclusion According to the results of this study, non-negligible doses are absorbed from contaminant electrons by the skin, which is associated with an excess risk of cancer induction.

Influence of voxel S factors on three-dimensional internal dosimetry calculations

Internal dosimetry is a fundamental instrument for the personalization of nuclear medicine therapies, to maximize the therapeutic effect while minimizing the radiation burden to other organs. Three-dimensional (3D) dosimetry can quantify the impact of heterogeneous radiopharmaceutical distributions in organs, lesions and tissues.We analysed the influence of radionuclide voxel S factors in 3D dosimetry of 111In, 177Lu and 90Y, the most used radionuclides in Peptide Receptor Radionuclide Therapy (PRRT). Calculations were carried out for kidneys on a workstation equipped with a software for 3D dosimetry (Imalytics STRATOS, Philips AG), adopting a computational anthropomorphic phantom and, retrospectively, the SPECT-CT image series of a clinical case of PRRT.Two sets of voxel S factors were adopted: the pre-loaded Philips kernels, calculated by direct Monte Carlo simulation, and the ones calculated through a previously proposed analytical approach. Philips 111In kernel did not account for mono-energetic Auger or Conversion electrons.Results indicate a difference of about −32% in voxel S factors for 111In in 4.42mm voxel size and around −35% in 4.80mm voxel size, particularly self-dose values; this lead to significant shift in dose histograms and average doses. For 177Lu and 90Y, differences are about 2% and 12% for 4.42mm voxels and about −8% and 9% for 4.80mm voxels, respectively, attributable to the different calculation methods of the voxel S factors; this does not lead to significant discrepancies between the two dose histograms. Consequently, voxel S factors must account accurately for all radiations emitted by the nuclide.

SU-G-TeP1-11: Predictors of Cardiac and Lung Dose Sparing in DIBH for Left Breast Treatment

Purpose: This retrospective study of left sided whole breast radiation therapy (RT) patients investigates possible predictive parameters correlating to cardiac and left lung dose sparing by deep inspiration breath-hold (DIBH) technique compared to free-breathing (FB). Methods: Thirty-one patients having both DIBH and FB CT scans were included in the study. All patients were planned with a standard step-and-shoot tangential technique using MV photons, with prescription of 50Gy or 50.4Gy. The displacement of the breath hold sternal mark during DIBH, the cardiac contact distances of the axial (CCDax) and parasagittal (CCDps) planes, and lateral-heart-to-chest (LHC) distance on FB CT scans were measured. Lung volumes, mean dose and dose-volume histograms (V5, V10 and V20) were analyzed and compared for heart and left lung for both FB and DIBH techniques. Correlation analysis was performed to identify the predictors for heart and left lung dose sparing. Two-tailed Student's t-test and linear regression were used for data analysis with significance level of P≤0.05. Results: All dosimetric metrics for the heart and left lung were significantly reduced (P<0.01) with DIBH. Breath hold sternal mark displacement ranged from 0.4–1.8 cm and correlated with mean (P=0.05) and V5 (P=0.02) of heart dose reduction by DIBH. FB lung volume showed correlation with mean lung dose reduction by DIBH (P<0.01). The FB-CCDps and FB-LHC distance had strong positive and negative correlation with FB mean heart dose (P<0.01) and mean heart dose reduction by DIBH (P<0.01), respectively. FB-CCDax showed no correlation with dosimetric changes. Conclusion: DIBH technique has been shown to reduce dose to the heart and left lung. In this patient cohort, FB-CCDps, FB-LHC distance, and FB lung volume served as significant predictors for heart and left lung. These parameters can be further investigated to be used as a tool to better select patients who will benefit from DIBH.

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.

Defining Action Levels for In Vivo Dosimetry in Intraoperative Electron Radiotherapy.

In vivo dosimetry is recommended in intraoperative electron radiotherapy (IOERT). To perform real-time treatment monitoring, action levels (ALs) have to be calculated. Empirical approaches based on observation of samples have been reported previously, however, our aim is to present a predictive model for calculating ALs and to verify their validity with our experimental data. We considered the range of absorbed doses delivered to our detector by means of the percentage depth dose for the electron beams used. Then, we calculated the absorbed dose histograms and convoluted them with detector responses to obtain probability density functions in order to find ALs as certain probability levels. Our in vivo dosimeters were reinforced TN-502RDM-H mobile metal-oxide-semiconductor field-effect transistors (MOSFETs). Our experimental data came from 30 measurements carried out in patients undergoing IOERT for rectal, breast, sarcoma, and pancreas cancers, among others. The prescribed dose to the tumor bed was 90%, and the maximum absorbed dose was 100%. The theoretical mean absorbed dose was 90.3% and the measured mean was 93.9%. Associated confidence intervals at P = .05 were 89.2% and 91.4% and 91.6% and 96.4%, respectively. With regard to individual comparisons between the model and the experiment, 37% of MOSFET measurements lay outside particular ranges defined by the derived ALs. Calculated confidence intervals at P = .05 ranged from 8.6% to 14.7%. The model can describe global results successfully but cannot match all the experimental data reported. In terms of accuracy, this suggests an eventual underestimation of tumor bed bleeding or detector alignment. In terms of precision, it will be necessary to reduce positioning uncertainties for a wide set of location and treatment postures, and more precise detectors will be required. Planning and imaging tools currently under development will play a fundamental role.

Quantitative comparison between the commercial software STRATOS® by Philips and a homemade software for voxel-dosimetry in radiopeptide therapy

BackgroundTargeted radionuclide therapy is a rapidly growing modality. A few commercial treatment planning systems are entering the market. However, some in-house systems are currently developed for a more flexible and customized dosimetry calculation at voxel-level. For this purpose, we developed a novel software, VoxelMed, and performed a comparison with the software STRATOS. MethodsThe validation of both of them was undertaken using radioactive phantoms with different volume inserts. A cohort of 10 patients was also studied after a therapeutic administration of 177Lu-labelled radiopeptides. The activity, number of disintegrations, absorbed dose and dose-volume histogram (DVH) were calculated for the phantoms and the kidneys in patients, which were the main critical organs at risk in this study. ResultsIn phantoms the absorbed doses computed with VoxelMed and STRATOS agree within 5%. In patients at the voxel-level the absorbed dose to kidneys (VoxelMed: mean 0.66 Gy/GBq) showed a limited difference of 5%, but with a remarkable range (−40%, +60%) between the two software packages. Voxel-dosimetry allows to estimate the dose non-homogeneities in volumes, which may be evaluated through DVHs. ConclusionThis study demonstrates that a fully 3D voxel-dosimetry with multiple SPECT images is feasible by using home-made or commercial software package and absorbed dose results obtained are similar. The main difference between the studied tools was observed in the activity integration method (effective vs physical half-time to time activity curve tail). We believe that an effective half-time integration method produces a more accurate approximation of clinical uptake and resultant dosimetry.

Two non-parametric methods for derivation of constraints from radiotherapy dose–histogram data

Dose constraints based on histograms provide a convenient and widely-used method for informing and guiding radiotherapy treatment planning. Methods of derivation of such constraints are often poorly described. Two non-parametric methods for derivation of constraints are described and investigated in the context of determination of dose-specific cut-points—values of the free parameter (e.g., percentage volume of the irradiated organ) which best reflect resulting changes in complication incidence. A method based on receiver operating characteristic (ROC) analysis and one based on a maximally-selected standardized rank sum are described and compared using rectal toxicity data from a prostate radiotherapy trial. Multiple test corrections are applied using a free step-down resampling algorithm, which accounts for the large number of tests undertaken to search for optimal cut-points and the inherent correlation between dose–histogram points. Both methods provide consistent significant cut-point values, with the rank sum method displaying some sensitivity to the underlying data. The ROC method is simple to implement and can utilize a complication atlas, though an advantage of the rank sum method is the ability to incorporate all complication grades without the need for grade dichotomization.

Dose impact in radiographic lung injury following lung SBRT: Statistical analysis and geometric interpretation

To demonstrate a new method of evaluating dose response of treatment-induced lung radiographic injury post-SBRT (stereotactic body radiotherapy) treatment and the discovery of bimodal dose behavior within clinically identified injury volumes. Follow-up CT scans at 3, 6, and 12 months were acquired from 24 patients treated with SBRT for stage-1 primary lung cancers or oligometastic lesions. Injury regions in these scans were propagated to the planning CT coordinates by performing deformable registration of the follow-ups to the planning CTs. A bimodal behavior was repeatedly observed from the probability distribution for dose values within the deformed injury regions. Based on a mixture-Gaussian assumption, an Expectation-Maximization (EM) algorithm was used to obtain characteristic parameters for such distribution. Geometric analysis was performed to interpret such parameters and infer the critical dose level that is potentially inductive of post-SBRT lung injury. The Gaussian mixture obtained from the EM algorithm closely approximates the empirical dose histogram within the injury volume with good consistency. The average Kullback-Leibler divergence values between the empirical differential dose volume histogram and the EM-obtained Gaussian mixture distribution were calculated to be 0.069, 0.063, and 0.092 for the 3, 6, and 12 month follow-up groups, respectively. The lower Gaussian component was located at approximately 70% prescription dose (35 Gy) for all three follow-up time points. The higher Gaussian component, contributed by the dose received by planning target volume, was located at around 107% of the prescription dose. Geometrical analysis suggests the mean of the lower Gaussian component, located at 35 Gy, as a possible indicator for a critical dose that induces lung injury after SBRT. An innovative and improved method for analyzing the correspondence between lung radiographic injury and SBRT treatment dose has been demonstrated. Bimodal behavior was observed in the dose distribution of lung injury after SBRT. Novel statistical and geometrical analysis has shown that the systematically quantified low-dose peak at approximately 35 Gy, or 70% prescription dose, is a good indication of a critical dose for injury. The determined critical dose of 35 Gy resembles the critical dose volume limit of 30 Gy for ipsilateral bronchus in RTOG 0618 and results from previous studies. The authors seek to further extend this improved analysis method to a larger cohort to better understand the interpatient variation in radiographic lung injury dose response post-SBRT.

SU‐C‐141‐01: Dose Impact in Lung Fibrosis Following Lung SBRT: Statistical Analysis and Geometric Interpretation

Purpose: To test the hypothesis that certain dosimetric parameters prognosticate treatment‐induced lung fibrosis. To obtain estimates for most prognostic dose values and to utilize them in developing new planning strategies to reduce injury in future SBRT. Methods: Follow‐up CT scans at 6 and 12 months were acquired from patients treated with SBRT (18Gy/Fx*3Fx or 12.5Gy/Fx*4Fx) for stage‐1 primary lung cancers or oligometastic lesions. Fibrosis regions were identified in these scans and propagated to the planning CT coordinates by rigidly registering the follow‐up and the planning CTs. Among a cohort of 8 properly registered cases, a bimodal behavior was repeatedly observed from the probability distribution for dose values within fibrosis regions. Based on a mixture‐Gaussian assumption, an Expectation‐Maximization (EM) algorithm was used to obtain characteristic parameters for such distribution. Geometric analysis was performed to interpret such parameters and infer the dose level that is potentially inductive of post‐SBRT fibrosis. Results: The Gaussian mixture obtained from the EM algorithm closely approximates the empirical dose histogram within the fibrosis volume with good consistency. The Kolmogorov‐Smirnov test yields a goodness of fit value 0.039. The higher Gaussian component, contributed by the dose received by PTV, was located around the prescription dose (50 or 54 Gy), as expected. Geometrical analysis suggests the mean of the lower Gaussian component as a possible indicator for a threshold dose that induces fibrosis after SBRT. Conclusion: Bimodal behavior was observed in the dose distribution of lung fibrosis volumes after SBRT. Novel statistical and geometrical analysis has shown that the systematically quantified low‐dose peak is a good indication of a threshold dose for injury. We seek to further improve the quality of registration to extend this analysis to a larger cohort for validation, and to explore planning strategies to reduce post‐SBRT injury by controlling the exposure above the threshold value.

Dose distribution changes with shielding disc misalignments and wrong orientations in breast IOERT: a Monte Carlo – GEANT4 and experimental study

One of the most relevant risks in breast intraoperative electron radiotherapy (IOERT) is the incorrect positioning of the shielding disc. If such a setup error occurs, the treatment zone could receive a nonuniform dose delivery, and a considerable part of the electron beam could hit — and irradiate — the patient's healthy tissue. However misalignment and tilt angle of the shielding disc can be evaluated, but it is not possible to measure the corresponding in vivo dose distribution. This led us to develop a simulation using the Geant4 Monte Carlo toolkit to study the effects of disc configuration on dose distribution. Some parameters were investigated: the shielding factor (SF), the radiation back scattering factor (BSF), the volume–dose histogram in the treatment zone, and the maximum leakage dose (MLD) in normal tissue. A lateral shift of the disc (in the plane perpendicular to the beam axis) causes a decrease in SF (from 4% for a misalignment of 5 mm to 40% for a misalignment of 40 mm), but no relevant dose variations were found for a tilt angle until 10°. In the same uncorrected disc positions, the BSF shows no significant change. MLD rises to 3.45 Gy for a 14 mm misalignment and 4.60 Gy for 30° tilt angle when the prescribed dose is 21 Gy. The simulation results are compared with the experimental ones, and allow an a posteriori estimation of the dose distribution in the breast target and underlying healthy tissue. This information could help the surgical team choose a more correct clinical setup, and assist in quantifying the degree of success or failure of an IOERT breast treatment.PACS number: 87.53.Jw, 87.55.dk, 87.55.Gh, 87.55.K‐, 87.55.N‐

SU-E-T-477: Influence of Eye Size on Radiation Absorbed Dose Delivered to Non- Targeted Tissues during Stereotactic Radiosurgery for Age-Related Macular Degeneration.

This work determines how variations in eye size will influence the radiation absorbed dose delivered to non-targeted tissues within the eye during stereotactic radiosurgery of age-related macular degeneration (AMD) using the IRay™ treatment. Stylized models of the eye were created with axial lengths of 20, 22, 24, 26, and 28mm. Each model was based upon the reference eye model from NCRP Report 130 and then scaled appropriately for each axial length. Models were incorporated with MCNPX radiation transport code in order to simulate the three beam IRay™ delivery system. Simulation results were assessed for both the mean absorbed dose and dose-volume histograms (DVH) for both target (macula) and non- targeted eye tissues, including the lens, retina, central retinal artery, and optic nerve. For each of the three beams, an average dose of 8Gy was delivered to the macula resulting in a total average dose of 24Gy for each eye model. The lens of the eye received a total average dose ranging from 146 to 189mGy, with the larger doses occurring in the smaller eye models since the beams traverse through the sciera closer to the limbus. The distal tip (1.5mm) of the central retinal artery received a total average dose ranging from 499 to 567mGy, with the larger doses occurring in the larger eye models due to increased scatter resulting from longer tissue path length to the nominal target. The optic nerve received a total average dose ranging from 207 to 225mGy, with the larger doses occurring in the smaller eye models. The small variation in dose to the lens, central retinal artery, and optic nerve suggests that eye size does not significantly affect radiation dose to non-targeted eye tissues. This work was sponsored by Oraya Therapeutics.

A dosimetric evaluation of dose escalation for the radical treatment of locally advanced vulvar cancer by intensity-modulated radiation therapy

The purpose of this planning study was to determine whether intensity-modulated radiation therapy (IMRT) reduces the radiation dose to organs at risk (OAR) when compared with 3D conventional radiation therapy (3D-CRT) in patients with vulvar cancer treated by irradiation. This study also investigated the use of sequential IMRT boost (seq-IMRT) and simultaneous integrated boost (SIB-IMRT) for dose escalation in the treatment of locally advanced vulvar cancer. Five vulvar cancer patients treated in the postoperative setting and 5 patients treated with definitive intent (def-group) were evaluated. For the postoperative group, 3D-CRT and IMRT plans to a total dose (TD) of 45 Gy were generated. For the def-group, 4 plans were generated: a 3D-CRT and an IMRT plan to a TD of 56.4 Gy, a SIB-IMRT plan to a TD of 56 Gy, and a SIB-IMRT with dose escalation (SIB-IMRT-esc): TD of 67.2 Gy. Mean dose and dose-volume histograms were compared using Student's t-test. IMRT significantly (all p < 0.05) reduced the Dmean, V30, and V40 for all OAR in the adjuvant setting. The V45 was also significantly reduced for all OAR except the bladder. For patients treated in the def-group, all IMRT techniques significantly reduced the Dmean, V40, and V45 for all OAR. The mean femur doses with SIB-IMRT and SIB-IMRT-esc were 47% and 49% lower compared with 3D-CRT. SIB-IMRT-esc reduced the doses to the OAR compared with seq-3D-CRT but increased the Dmax. for the small bowel, rectum, and bladder. IMRT reduces the dose to the OAR compared with 3D-CRT in patients with vulvar cancer receiving irradiation to a volume covering the vulvar region and nodal areas without compromising the dosimetric coverage of the target volume. IMRT for vulvar cancer is feasible and an attractive option for dose escalation studies.

SU-E-T-398: 3D Dosimetry Verification for Prostate Implant Treatment Plans

Purpose: A 3D dose calculation program (PIDOSE) for prostate implant was developed for comparison to treatment plans from a commercial treatment planning system (Variseed V7.1, Varian Medical System, Palo Alto, CA). Methods: Although a simple dose verification (one source) was done when Variseed was commissioned, we believe more complicated treatment plans should also be verified. These verifications include 3D dose distributions, dose histograms, isodose lines, maximum, median and minimal doses as a second hand check. A file with needle‐source information was exported from the Variseed planning system to PIDOSE. A 3D dose matrix was created using a tri‐exponential fitting function for the seed model (1). All single‐source doses were summed and selected isodose lines were superimposed on the ultrasoundimage slices together with those from Variseed. Dose‐volume histograms for prostate, urethra and partial rectum were calculated and compared. Results: : Although maximum dose, mean dose and standard deviation were slightly different, there were no large deviations from typical isodose lines (< 1 mm), minimum dose (<2%), median dose (<2%) and DVHs. Conclusions: The 3D dose verification as a second hand check confirms the accuracy of dose calculations provided by the Variseed Planning System. The small differences between maximum dose, mean dose and standard deviation are due to the differences in dose calculation resolution, dose volume interpolation, and radial dose fitting functions. The maximum doses from PIDOSE at some calculation points near the seeds were high due to the exponential fitting function.

SU‐E‐T‐19: The Use of Probability Volume Histograms (PVH) and Iso‐Probability of Response Charts in Treatment Plan Evaluation

Purpose: This study demonstrates new methods for treatment plan evaluation and comparison. The clinical importance of these methods is examined in relation with standard radiobiological and dosimetric treatment plan evaluation criteria. Methods: Typical head and neck, breast and prostate cancer cases were selected to quantify the benefits of the proposed treatment plan evaluation method. In each case, conformal radiotherapy (CRT) and IMRT treatment plans were produced. The different treatment plans were evaluated based on the dose distribution converted to D2Gy, iso‐probabilty and iso‐BED dose charts and corresponding probability volume histograms. Results: For the head &amp; neck cancer case, at the prescribed dose level of the CRT and IMRT dose distributions, the P+ values are 0.0% and 24.9% for a BEUD to the target of 54.6 Gy and 63.5%, respectively. For the breast cancer case, the P+ values are 88.0% and 89.1% for a BEUD of 61.6 Gy and 61.4%, respectively. For the prostate cancer case, the P+ values are 28.7% and 51.3% for a BEUD of 69.3 Gy and 70.7%, respectively. The use of local iso‐probability charts is very useful for plan evaluation since the visual information focuses on the doses that may cause an effect in a particular organ by considering its radiosensitivity. Also, since these low doses may not have an actual clinical effect, the dosimetric information provided by low doses in the normal tissues is reduced eliminating some visual noise. At the same time, cold spots in the target volumes and hot spots in the normal tissues are immediately visualized. Conclusions: The proposed D2Gy, iso‐probabilty and iso‐BED dose charts and histograms illustrated better the difference in the effectiveness of the different treatment plans, which shows the benefits that these evaluation tools can offer in the process of treatment planning.

SU-E-T-587: A CT Based Monte Carlo Dosimetry for Post Implant LDR Brachytherapy

Purpose: Introducing a new CT based Monte Carlo calculation for dose evaluation following LDR brachytherapy and comparing the results with conventional TG‐43 calculation Methods: We adapted a voxel based mathematical phantom of patient's body modeling in MCNP using conventional DICOM images from post implant of prostate brachytherapy. With the aid of Image processingsoftware, the implanted seed and their position are detected and conveyed to Monte Carlo SDEF source definition. The results of MCNP dosimetry is presented to the physicists in a GUI medium. Results: The developed software could aid the physicist to browse, explore or modify patient's CT and explore the results of Monte Carlodosimetry. The software is designed to evaluate the absorbed dose(dose distribution contours) and the volumetric dose histograms. The software is used to evaluate the dose distribution of HDR seeds in a gel phantom [1] and the results are within 3% of experimental data. Conclusions: The developed image processing and mathematical voxel phantom generation software package is successful in producing accurate dosimetry data based on exact patient body information. Besides producing dose distribution in CTV and PTV, the dose distributions of adjacent sensitive organs are readily available with Monte Carlo accuracy. This is an advantage of the developed software compared with conventional TG‐43 based software which ignores patient's body heterogeneity in dose evaluation

Treatment plans optimization for contrast‐enhanced synchrotron stereotactic radiotherapy

Synchrotron stereotactic radiotherapy (SSRT) is a treatment that involves the targeting of high-Z elements into tumors followed by stereotactic irradiation with monochromatic x-rays from a synchrotron source, tuned at an optimal energy. The irradiation geometry, as well as the secondary particles generated at a higher yield by the medium energy x-rays on the high-Z atoms (characteristic x-rays, photoelectrons, and Auger electrons), produces a localized dose enhancement in the tumor. Iodine-enhanced SSRT with systemic injections of iodinated contrast agents has been successfully developed in the past six years in the team, and is currently being transferred to clinical trials. The purpose of this work is to study the impact on the SSRT treatment of the contrast agent type, the beam quality, the irradiation geometry, and the beam weighting for defining an optimized SSRT treatment plan. Theoretical dosimetry was performed using the MCNPX particle transport code. The simulated geometry was an idealized phantom representing a human head. A virtual target was positioned in the central part of the phantom or off-centered by 4 cm. The authors investigated the dosimetric characteristics of SSRT for various contrast agents: Iodine, gadolinium, and gold; and for different beam qualities: Monochromatic x-ray beams from a synchrotron source (30-120 keV), polychromatic x-ray beams from an x-ray tube (80, 120, and 180 kVp), and a 6 MV x-ray beam from a linear accelerator. Three irradiation geometries were studied: One arc or three noncoplanar arcs dynamic arc therapy, and an irradiation with a finite number of beams. The resulting dose enhancements, beam profiles, and histograms dose volumes were compared for iodine-enhanced SSRT. An attempt to optimize the irradiation scheme by weighing the finite x-ray beams was performed. Finally, the optimization was studied on patient specific 3D CT data after contrast agent infusion. It was demonstrated in this study that an 80 keV beam energy was a good compromise for treating human brain tumors with iodine-enhanced SSRT, resulting in a still high dose enhancement factor (about 2) and a superior bone sparing in comparison with lower energy x-rays. This beam could easily be produced at the European Synchrotron Radiation Facility medical beamline. Moreover, there was a significant diminution of dose delivered to the bone when using monochromatic x-rays rather than polychromatic x-rays from a conventional tube. The data showed that iodine SSRT exhibits a superior sparing of brain healthy tissue in comparison to high energy treatment. The beam weighting optimization significantly improved the treatment plans for off-centered tumors, when compared to nonweighted irradiations. This study demonstrated the feasibility of realistic clinical plans for low energy monochromatic x-rays contrast-enhanced radiotherapy, suitable for the first clinical trials on brain metastasis with a homogeneous iodine uptake.