Motoharu Sasaki

Characterization of stochastic noise and post-irradiation density growth for reflective-type radiochromic film in therapeutic photon beam dosimetry.

The aim of this study is to investigate the dosimetric uncertainty of stochastic noise and the post-irradiation density growth for reflective-type radiochromic film to obtain the appropriate dose from the exactly controlled film density. Film pieces were irradiated with 6-MV photon beams ranging from 0 to 400cGy. The pixel values (PVs) of these films were obtained using a flatbed scanner at elapsed times of 1min to 120h between the end of irradiation and the film scan. The means and standard deviations (SDs) of the PVs were calculated. The SDs of the converted dose scale, usd, and the dose increases resulting from the PV increases per ±29min at each elapsed time, utime, were computed. The combined dose uncertainties from these two factors, uc, were then calculated. A sharp increase in the PV occurred within the first 3h after irradiation, and a slight increase continued from 3h to 120h. usd was independent of post-irradiation elapsed time. Sharp decreases in utime were obtained within 1h after irradiation, and slight decreases in utime were observed from 1 to 24h after irradiation. uc first decreased 1h after irradiation and remained constant afterward. Assuming that the post-irradiation elapsed times of all of the related measurements are synchronized within ±29min, the elapsed time should be at least 1h in our system. It is important to optimize the scanning protocol for each institution with consideration of the required measurement uncertainty and acceptable latency time.

Influence of multi-leaf collimator leaf transmission on head and neck intensity-modulated radiation therapy and volumetric-modulated arc therapy planning

The aim of this study was to quantify the effect of multi-leaf collimators (MLCs) with different leaf widths on the planning of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Toward this objective, dose transmission through a high-definition 120-leaf MLC (HD120MLC) and 120-leaf Millennium MLC (M120MLC) was investigated, using it with a test case and clinical case studies. In test case, studies with IMRT and VMAT plans, the difference in MLC leaf width had a limited effect on planning target volumes (PTVs). Organs at risk (OARs) were more affected by a reduction in dose transmission through the MLC than by a reduction in MLC leaf width. The results of the test case studies and clinical case studies were mostly similar. In the latter, the different MLCs had no effect on the PTV regardless of the treatment method; however, the HD120MLC plans achieved dose reductions to OARs similar to or larger than the dose reduction of the M120MLC plans. The similar results of the test case and clinical case studies showed that despite a limitation of the irradiation field size, the HD120MLC plans were superior.

TU-AB-202-01: Multi-Institutional Validation Study of Commercially Available Deformable Image Registration Software for Thoracic Images

PURPOSE The purpose of this study was to assess the accuracy of commercially available deformable image registration (DIR) software for thoracic images on multi-institution. Furthermore, we determined the variation in the DIR accuracy among institutions due to different DIR algorithms and DIR procedures. METHODS Thoracic four-dimensional (4D) CT images of ten patients with esophagus or lung cancer were used. Datasets for these patients were provided by DIR-lab (dir-lab.com) and included a coordinate list of anatomical landmarks (300 bronchial bifurcations) that had been manually identified. DIR was performed between peak-inhale and peak-exhale images. DIR registration error was determined by calculating the difference at each landmark point between displacement calculated by DIR software and that calculated by the landmark. RESULTS Eleven institutions participated in this study: Four institutions used RayStation (RaySearch Laboratories, Stockholm, Sweden), five institutions used MIM software (MIM Software Inc, Cleveland, OH) and three institutions used Velocity (Varian Medical Systems, Palo Alto, CA). The range of average absolute registration errors over all cases were 0.48-1.51 mm (right-left), 0.53-2.86 mm (anterior-posterior), 0.85-4.46 mm (superiorinferior) and 1.26-6.20 mm (three-dimensional [3D]). For each DIR software, the average 3D registration error (range) was 3.28mm (1.26-3.91 mm) for RayStation; MIM was 3.29mm (2.17-3.61 mm); Velocity was 5.01mm (4.02-6.20 mm). These results showed that there was moderate variation among institutions, even though the DIR software was same. CONCLUSION We evaluated the commercially available DIR software using thoracic 4D CT images on multi-center. Our results demonstrated that DIR accuracy differed among institutions because it was dependent on both DIR software and DIR procedure. Our results could be helpful for establishment of prospective clinical trials and widespread use of DIR software. In addition, in clinical care, we should try to find the optimal DIR procedure, when DIR was performed using thoracic 4D-CT data to calculate the accumulated dose.

SU-F-T-332: Dose Impact of Rectal Gas On Prostate VMAT

PURPOSE The aim of this study to evaluate influence of rectal gas on dose distribution during prostate VMAT. METHODS Our subjects were 10 patients who have received VMAT for prostate cancer at our hospital. In this study, we made four types of VMAT plan. The angles of rotation were as follows: clockwise from 181-179° and 179-181° (Full arc), clockwise from 200-160° and counter-clockwise from 160-200° (Partial arc1), clockwise from 220- 140° and counter-clockwise from 140-220° (Partial arc2), clockwise from 240-120° and counter-clockwise from 120-240° (Partial arc3). The rectal contour used for the reference treatment plan each had 5% or less rectal gas. In order to evaluate the effects of the rectal gas on the dose distribution, we created a rectal contour for assessment separate from the contour used for the reference treatment plans. In the contour for evaluation, the Hounsfield unit (HU) value of the gas was assigned for the total volume of the rectal contour. A HU value of -950 was adopted for simulating the rectal gas. The 3DVH version 2.2 was used for evaluation, and evaluation was performed based on the concordance rate between the contour being evaluated and that of the reference treatment plans. The dose difference (DD), distance to agreement (DTA), and gamma analysis (GA) were used to obtain the concordance rate. The contours being evaluated were CTV, PTV, rectum and bladder. RESULTS The results of DD, DTA, and GA showed that the rectum, the CTV and rectum had the lowest concordance rates. Irrespective of DD, DTA, or GA, the treatment plan based on full arc had a higher concordance rate. CONCLUSION With respect to the effect of rectal gas on the dose distribution in prostate VMAT, it was shown that full arc might be the least susceptible.

SU-F-T-681: Does the Biophysical Modeling for Immunological Aspects in Radiotherapy Precisely Predict Tumor and Normal Tissue Responses?

PURPOSE Recent advances in immunotherapy make possible to combine with radiotherapy. The aim of this study was to assess the TCP/NTCP model with immunological aspects including stochastic distribution as intercellular uncertainties. METHODS In the clinical treatment planning system (Eclipse ver.11.0, Varian medical systems, US), biological parameters such as α/β, D50, γ, n, m, TD50 including repair parameters (bi-exponential repair) can be set as any given values to calculate the TCP/NTCP. Using a prostate cancer patient data with VMAT commissioned as a 6-MV photon beam of Novalis-Tx (BrainLab, US) in clinical use, the fraction schedule were hypothesized as 70-78Gy/35-39fr, 72-81Gy/40-45fr, 52.5-66Gy/16-22fr, 35-40Gy/5fr of 5-7 fractions in a week. By use of stochastic biological model applying for Gaussian distribution, the effects of the TCP/NTCP variation of repair parameters of the immune system as well as the intercellular uncertainty of tumor and normal tissues have been evaluated. RESULTS As respect to the difference of the α/β, the changes of the TCP/NTCP were increased in hypo-fraction regimens. The difference between the values of n and m affect the variation of the NTCP with the fraction schedules, independently. The elongation of repair half-time (long) increased the TCP/NTCP twice or much higher in the case of hypo-fraction scheme. For tumor, the repopulation parameters such as Tpot and Tstart, which is immunologically working to the tumor, improved TCP. CONCLUSION Compared to default fixed value, which has affected by the probability of cell death and cure, hypo-fractionation schemes seemed to have advantages for the variations of the values of m. The possibility of an increase of the α/β or TD50 and repair parameters in tumor and normal tissue by immunological aspects were highly expected. For more precise prediction, treatment planning systems should be incorporated the complicated biological optimization in clinical practice combined with basic experiments data.

SU-E-T-146: Effects of Uncertainties of Radiation Sensitivity of Biological Modelling for Treatment Planning

PURPOSE The aim of this study was to evaluate the distribution of uncertainty of cell survival by radiation, and assesses the usefulness of stochastic biological model applying for gaussian distribution. METHODS For single cell experiments, exponentially growing cells were harvested from the standard cell culture dishes by trypsinization, and suspended in test tubes containing 1 ml of MEM(2×106 cells/ml). The hypoxic cultures were treated with 95% N2 -5% CO2 gas for 30 minutes. In vitro radiosensitization was also measured in EMT6/KU single cells to add radiosensitizer under hypoxic conditions. X-ray irradiation was carried out by using an Xray unit (Hitachi X-ray unit, model MBR-1505R3) with 0.5 mm Al/1.0 mm Cu filter, 150 kV, 4 Gy/min). In vitro assay, cells on the dish were irradiated with 1 Gy to 24 Gy, respectively. After irradiation, colony formation assays were performed. Variations of biological parameters were investigated at standard cell culture(n=16), hypoxic cell culture(n=45) and hypoxic cell culture(n=21) with radiosensitizers, respectively. The data were obtained by separate schedule to take account for the variation of radiation sensitivity of cell cycle. RESULTS At standard cell culture, hypoxic cell culture and hypoxic cell culture with radiosensitizers, median and standard deviation of alpha/beta ratio were 37.1±73.4 Gy, 9.8±23.7 Gy, 20.7±21.9 Gy, respectively. Average and standard deviation of D5 0 were 2.5±2.5 Gy, 6.1±2.2 Gy, 3.6±1.3 Gy, respectively. CONCLUSION In this study, we have challenged to apply these uncertainties of parameters for the biological model. The variation of alpha values, beta values, D5 0 as well as cell culture might have highly affected by probability of cell death. Further research is in progress for precise prediction of the cell death as well as tumor control probability for treatment planning.

SU‐E‐T‐484: Impact of Multileaf Collimator Leaf Positioning Accuracy on Intensity Modulated Radiation Therapy

Purpose: It is reported to have an impact on dose due to uncertainty of MLC drive control in IMRT using MLC. It is reported to have displacement of about 0.5mm in a month as changes over time of MLC drive control accuracy installed in LINAC made by SIEMENS. There is fear that these changes over time contribute to dose distribution when SMLC‐IMRT is practiced. Methods: For LINAC, We used PRIMUS High‐Energy KD2 7467 (Siemens Medical Systems) that generates 10MV‐X rays. MLC installed in this therapy machine is MLC‐20A (Toshiba Medical Systems) with lower collimator replaced by MLC of 29 pairs and adopts double focus that does focusing with two aspects in a structure of MLCs leaf tip and side contacting always parallel to dose angle. We used Kodak Extended Dose Range2 (Carestream health Inc.) for film, D.D.system (R‐TECH Inc.) for film analyzer, flat bed scanner ES‐10000G (EPSON Corp.) for film reader and Xio‐version4.50.00 (ELEKTA) for RTP. We studied the impact of MLC drive control accuracy on dose evaluation (gamma analysis) measuring IMRTdose distribution as well as evaluating MLC drive control accuracy (resting positional accuracy and position reproducibility) once a week for 60 days. Results: MLC positional accuracy tended to expand by 0.1–0.15mm in one week accompanied by changes over time and tended to expand by about 1mm in 60 days. The reproducibility was within 0.2mm for roughly over 95%. For prostate gland IMRT, I did not see a significant difference in pass rate of y analysis if the resting positional accuracy of MLC is about 1 mm. Conclusions: It was suggested that it would be an effective index to continue IMRT safely in the future by practicing regular management upon setting an acceptable value by MLC positional accuracy test. This study was supported, in part, by a grant of the Japanese Society of Radiologocal Technology(JSRT)

Use of a diagnostic positron emission tomography-computed tomography system for planning radiotherapy positioning: distortion of the tabletop

PurposeThe aim of this study was to evaluate distortion of the tabletop in a diagnostic positron emission tomography-computed tomography (PET-CT) system to determine its suitability for planning radiotherapy positioning.Materials and methodsDistortion of the tabletop was compared among PET-CT, lineac CT, and CT simulator systems. A phantom or angiography catheter was fixed to the tabletop and imaged after iron plate weight loading. The acquired images were analyzed using radiotherapy planning software. Distortion of the tabletop was measured based on the displayed coordinates.ResultsSinking represented the greatest distortion of the tabletop in all systems. Using the same baseline, the maximum sinking were −0.4, −0.2, and +0.4 cm, respectively. The distortion of the tabletop in the PET-CT system was more similar to that in the lineac CT than in the CT simulator system.ConclusionDistortion of the tabletop in a diagnostic PET-CT system may be within the acceptable range to allow its use for planning radiotherapy positioning.

536 Quality control of CT on rail system (FOCAL unit) with a micro-multi leaf collimator (mMLC) using new Gafchromic film for stereotactic radiotherapy

PURPOSE Recent years, CT on rail system was reported to be useful as a tool for image-guided radiotherapy (IGRT). This system was clinically developed with the aim of stereotactic irradiation (STI) for brain, lung, liver, prostate and other sites. Quality assurance and quality control (QC) is an important issue in CT on rail system to assure geometric accuracies. The purpose of this study is to estimate the geometric accuracies of our CT on rail system using a detachable micro-multi leaf collimator (mMLC) with new type radiochromic films. Carrying out our original QC program, translational errors, setup reproducibility, beam misalignment and beam characteristics were evaluated. METHODS AND MATERIALS We have studied with CT on rail system (FOCAL unit, Toshiba Medical systems, Tokyo, Japan) and mMLC unit (Accuknife, Direx Inc., Tokyo, Japan). We have developed original alignment phantom and small steel markers (2 mm phi) were implanted on its surface at certain intervals. Firstly, we have evaluated the accuracy of self-moving CT gantry and CT resolutions for cranio-caudal directions by changing slice thickness. And then using the phantom, we have measured the accuracy and reproducibility of geometric isocenter of the linac side and the CT gantry side by scanning the phantom. We have also measured the geometric changes of the common treatment couch by weight-loaded test (up to 135 kgw). To estimate dosimetric and geometric accuracies with the mMLC unit, the misalignment of the beam axes (gantry, collimator and couch rotation axis), mMLC leaf positions, and dose distributions for the verification plan were measured with new type GafChromic films (GafChromic-RTQA, ISP Inc., USA) and cylindrical phantom. The dose characteristics of the GafChromic film were also evaluated. RESULTS The reproducibility of the self-moving CT gantry have a good agreement within 1 mm. Weight-load test have shown a good reliability within 2 mm at the common treatment couch. The translational precision of the common treatment couch was 0.0 +/- 0.1 mm at linac side and -0.2 +/- 0.5 mm at CT gantry side. The misalignments of beam axes have been kept within 0.4 mm at maximum. Gap test have shown the accuracies of the mMLC leaf positions, which is needed to keep within 1 mm by a routine calibration. CONCLUSIONS To practice quality control program for the FOCAL unit and the mMLC unit is essential for a regular interval to reduce systematic errors. New type radiochromic film would be useful for a verification tool as alternative to conventional film.