F Su

A graphic user interface toolkit for specification, report and comparison of dose-response relations and treatment plans using the biologically effective uniform dose

A toolkit (BEUDcal) has been developed for evaluating the effectiveness and for predicting the outcome of treatment plans by calculating the biologically effective uniform dose (BEUD) and complication-free tumor control probability. The input for the BEUDcal is the differential dose-volume histograms of organs exported from the treatment planning system. A clinical database is built for the dose-response parameters of different tumors and normal tissues. Dose-response probabilities of all the examined organs are illustrated together with the corresponding BEUDs and the P(+) values. Furthermore, BEUDcal is able to generate a report that simultaneously presents the radiobiological evaluation together with the physical dose indices, showing the complementary relation between the physical and radiobiological treatment plan analysis performed by BEUDcal. Comparisons between treatment plans for helical tomotherapy and multileaf collimator-based intensity modulated radiotherapy of a lung patient were demonstrated to show the versatility of BEUDcal in the assessment and report of dose-response relations.

Radiobiological and Dosimetric Analysis of Daily Megavoltage CT Registration on Adaptive Radiotherapy with Helical Tomotherapy

Pre-treatment patient repositioning in highly conformal image-guided radiation therapy modalities is a prerequisite for reducing setup uncertainties. In Helical Tomotherapy (HT) treatment, a megavoltage CT (MVCT) image is usually acquired to evaluate daily changes in the patients internal anatomy and setup position. This MVCT image is subsequently compared to the kilovoltage CT (kVCT) study that was used for dosimetric planning, by applying a registration process. This study aims at investigating the expected effect of patient setup correction using the Hi-Art tomotherapy system by employing radiobiological measures such as the biologically effective uniform dose () and the complication-free tumor control probability (P+). A new module of the Tomotherapy software (TomoTherapy, Inc, Madison, WI) called Planned Adaptive is employed in this study. In this process the delivered dose can be calculated by using the sinogram for each delivered fraction and the registered MVCT image set that corresponds to the patients position and anatomical distribution for that fraction. In this study, patients treated for lung, pancreas and prostate carcinomas are evaluated by this method. For each cancer type, a Helical Tomotherapy plan was developed. In each cancer case, two dose distributions were calculated using the MVCT image sets before and after the patient setup correction. The fractional dose distributions were added and renormalized to the total number of fractions planned. The dosimetric and radiobiological differences of the dose distributions with and without patient setup correction were calculated. By using common statistical measures of the dose distributions and the P+ and concepts and plotting the tissue response probabilities vs. a more comprehensive comparison was performed based on radiobiological measures. For the lung cancer case, at the clinically prescribed dose levels of the dose distributions, with and without patient setup correction, the complication-free tumor control probabilities, P+ are 48.5% and 48.9% for a of 53.3 Gy. The respective total control probabilities, PB are 56.3% and 56.5%, whereas the corresponding total complication probabilities, PI are 7.9% and 7.5%. For the pancreas cancer case, at the prescribed dose levels of the two dose distributions, the P+ values are 53.7% and 45.7% for a of 54.7 Gy and 53.8 Gy, respectively. The respective PB values are 53.7% and 45.8%, whereas the corresponding PI values are ~0.0% and 0.1%. For the prostate cancer case, at the prescribed dose levels of the two dose distributions, the P+ values are 10.9% for a of 75.2 Gy and 11.9% for a of 75.4 Gy, respectively. The respective PB values are 14.5% and 15.3%, whereas the corresponding PI values are 3.6% and 3.4%. Our analysis showed that the very good daily patient setup and dose delivery were very close to the intended ones. With the exception of the pancreas cancer case, the deviations observed between the dose distributions with and without patient setup correction were within ±2% in terms of P+. In the radiobiologically optimized dose distributions, the role of patient setup correction using MVCT images could appear to be more important than in the cases of dosimetrically optimized treatment plans were the individual tissue radiosensitivities are not precisely considered.

Assessing Four-dimensional Radiotherapy Planning and Respiratory Motion-induced Dose Difference Based on Biologically Effective Uniform Dose

Four-dimensional (4D) radiotherapy is considered as a feasible and ideal solution to accommodate intra-fractional respiratory motion during conformal radiation therapy. With explicit inclusion of the temporal changes in anatomy during the imaging, planning, and delivery of radiotherapy, 4D treatment planning in principle provides better dose conformity. However, the clinical benefits of developing 4D treatment plans in terms of tumor control rate and normal tissue complication probability as compared to other treatment plans based on CT images of a fixed respiratory phase remains mostly unproven. The aim of our study is to comprehensively evaluate 4D treatment planning for nine lung tumor cases with both physical and biological measures using biologically effective uniform dose (D̿) together with complication-free tumor control probability, P+. Based on the examined lung cancer patients and PTV margin applied, we found similar but not identical curves of DVH, and slightly different mean doses in tumor (up to 1.5%) and normal tissue in all cases when comparing 4D, P0%, and P50% plans. When it comes to biological evaluations, we did not observe definitively PTV size dependence in P+ among these nine lung cancer patients with various sizes of PTV. Moreover, it is not necessary that 4D plans would have better target coverage or higher P+ as compared to a fixed phase IMRT plan. However, on the contrary to significant deviations in P+ (up to 14.7%) observed if delivering the IMRT plan made at end-inhalation incorrectly at end-exhalation phase, we estimated the overall P+, PB, and PI for 4D composite plans that have accounted for intra-fractional respiratory motion.

SU-FF-T-84: Application of GAFCHROMIC® EBT Film for in Vivo Dosimetry with Total Body Irradiation (TBI) Radiotherapy

Purpose: To demonstrate the viability of GAFCHROMIC® EBT film as an in vivo, dosimeter for total body irradiation (TBI) procedures. Method and Materials:Dosimetry results are reported for six anatomical regions (i.e. head, shoulder, chest, umbilicus, hip and thigh) on an anthropomorphic phantom irradiated using a standard procedure of lateral TBI technique in our facility. The films and thermoluminescencedosimeters(TLD) were taped to the phantom with adequate bolus to ensure that the dosimeters were at a depth of electronic equilibrium. To establish the film calibration curve, film samples (2.5 × 1.5 cm2 in size) were placed at the isocenter of the accelerator (SAD=100 cm) at depth of 10cm in a water equivalent solid phantom. The GAFCHROMIC® EBT films were exposed to doses from 0 to 400 cGy (with 50 cGy increment) using a 20 × 20 cm2field size. The films were scanned with the Vidar VXR‐16 scanner and analyzed with Image J software. Results: The results indicated that the measured doses at all points as obtained by the GAFCHROMIC® EBT films are in good agreement with the prescribed doses, varying from −3.15% to 1.46%. The comparison of the GAFCHROMIC® EBT dosimetry against the TLD analysis showed a similar agreement (variance from −4.08% to 0.55%). Conclusions: GAFCHROMIC® EBT film is a viable dosimetry tool providing comparable results as those obtained using TLDs. The elimination of the film processor that usually introduces uncertainties, the almost water equivalent density, the water resistance, and insensitivity to room light are some of the advantages of the EBT films, which make the films easier to handle and a good candidate for in vivodosimetry in TBI and possibly other radiotherapy applications.

SU‐E‐T‐118: Small Dynamic Field Dosimetry by Gfachromic Film (EBT3) and 2D‐Array Diode

PURPOSE Small dynamic multi-leaves collimator (MLC) dosimetry presents challenges to clinical radiological measurements. This study was to evaluate the small dynamic field dosimetry using one-scan film algorithm (FilmQA Pro/Ashland Inc.) and MapCheck 2D-array (Sun Nuclear). Commercial diode-array would provide practical needs for dynamic field QA (e.g. IMRT or VMAT). Alternatively Chromic film dosimetry serves the same purposes as diode array with much better spatial resolution, wider dose range and less energy dependency. One-scan algorithm minimizes the problems of conventional film dosimetry, which suffered time-consuming process, numerous non-radiation artifacts. METHODS Both dosimeters along with chamber measurements (TN31010/PTW, 0.125cm3) at central-axis (CAX) as absolute dose comparisons were tested by 6MeV photon with 120HD-MLC under a Varian iX LINAC using dynamic MLC (with gaps of 1, 5, 10, 20, 50, 100mm width) sweeping through a 5.0cm jaws-opening size. The experiments were arranged by a 2cm solid-water buildup above tested films, and the 2cm water-equivalent intrinsic buildup over the MapCheck diode array at 100cm SAD setup. Each film-measurement was calibrated by 4 dose-points in the same film by one-scan algorithm. RESULTS CAX absolute doses from both dosimeters showed very close results (+/-0.3% deviation) from the according chamber measurements for relatively large dynamic MLC fields (gap size > 20mm). Otherwise each dosimeter presented different problems with mixed deviations of 4∼8% from CAX chamber measurements. Energy-dependence (from scatters) and low-dose bias had been well-known for diode performance, which was enhanced by small gap-width (< 1cm). Film dosimetry suffered from systematic noises, especially in low-dose (8∼10 cGy), even with the one-scan algorithm corrections. The problem of one-scan protocol may be improved by the cares of the choice for calibration dose values to avoid over-correction in low-dose. CONCLUSION Film provided much better spatial details (0.35mm/pixel), e.g. MLC interleaved leakage, than 2D diode-array, and similar overall dose accuracy compared to diode array.

A phantom-based evaluation of a real-time tracking micro MLC delivery

A cutting edge real-time tracking delivery system was evaluated for respiratory-correlated radiation therapy. The performed evaluations included Multi-Leaf Collimator (MLC) leaf position accuracy, MLC motion time delay, dosimetric performance, and tracking failure. The experiment-based results indicated that the dynamic MLC was capable of achieving the position accuracy in 1~2 mm and the MLC time delay in the range of 100 μs. The dosimetric comparison evidenced the significant advantage when utilising real-time tracking. The analysis of experiments also indicated the potential fail of tracking due to the impact of patient breathing patterns and the visibility of tumour.

SU‐E‐J‐29: A Dosimetric Assessment of Rectum and Bladder Using Deformable Registration in Image‐Guided Adaptive Prostate IMRT

Purpose: To determine the impact of the deformation of rectum and bladder on the assessment of dose‐volume constraints for these organs at risk (OARs) in adaptive prostate IMRT. Methods: Five prostate cancer patients treated with both prostate and seminal vesicle as the initial PTV and with prostate only as the boost PTV were selected for this study. All patients received two CT simulations with the first CT scanned for the initial treatment volume followed by a second CT simulation after 45 Gy. All patients were treated to a total dose of 79.2 Gy in 1.8 Gy fractions delivered with seven‐coplanner IMRT beams. MIM‐Adaptive deformation workflow (MIMVista, Cleveland, OH) was used to perform deformation image registration of the two CTimage sets. For evaluation of the accumulative dose delivered to the OARs in presence of organ deformation, MIM‐Dose deformation tool was used to register the deformed dose matrices of two CT sets. The dose‐volume constraints, D25%, calculated for rectum and bladder with and without consideration of organ deformation were compared. Results: Differences in rectal D25% of up to 9.8% (from −9.8% to +2.8%) and in bladder D25% of up to 19.5% (from −19.5% to +9.0%) were observed. The deformation of OARs during IMRT caused the values of D25% for rectum to change from −646 cGy to +173 cGy and for bladder from −831 to +541 cGy from patient to patient. Conclusions: Discrepancies in OAR dose‐volume constraints can be calculated using modern deformable registration tools with and without consideration of organ volume change and deformation and should be carefully considered in the evaluation of OAR complication risks in adaptive prostate IMRT. MIMVista work station which was used to conduct this research was provided by MIM software Inc.

Evaluation on Lung Cancer Patients’ Four-dimensional Treatment Plans Utilizing Biologically Effective Uniform Dose

Four-dimensional (4D) radiotherapy is considered to be a feasible and ideal solution to accommodate intrafractional respiratory motion during conformal radiation therapy. With explicit inclusion of the temporal changes in anatomy during the imaging, planning, and delivery of radiotherapy, 4D treatment planning in principle provides better dose conformity. However, the clinical benefits of developing 4D treatment plans in terms of tumor control rate and normal tissue complication probability as compared to other treatment plans based on CT images of a fixed respiratory phase remain mostly unproven. The aim of our study is to comprehensively evaluate 4D treatment planning for nine lung tumor cases with both physical and biological measures using biologically effective uniform dose ( Open image in new window ) together with complication-free tumor control probability, P + . Based on the examined lung cancer patients and PTV margin applied, we found similar but not identical curves of DVH, and slightly different mean doses in tumor and normal tissues in all cases when comparing 4D, P0% and P50% plans. When it comes to biological evaluations, we did not observe definitively PTV size dependence in P + among these nine lung cancer patients with various sizes of PTV. Moreover, it is not necessary that 4D plans would have better target coverage or higher P + as compared to a fixed phase IMRT plan.

BEUD/SECONDARY MALIGNANCY ANALYSIS: COMPARISON OF HT, MLC-BASED IMRT, AND CRT IN PROSTATE TREATMENT PLANNING

Purpose: To show that radiobiological analysis offers a better clinical assessment of treatment outcome compared to physical dosimetric analysis and to apply this radiobiological analysis to characterize possible benefits of tumor tracking over conventional radiotherapy. Materials: 4DCT data from six (n=6) patients with tumor motion in the range of 2-10mmwere used. Ten image sets corresponding to various phases of the respiratory cycle were derived from each 4DCT image set. A single clinician contoured the target and organs at risk on all ten phase-dependent image sets. Two plans were developed per patient based on tracking and conventional radiotherapy. In the latter, larger margins were used to account for organ motion. First, a reference image set was chosen and a plan developed based on a composite target. The plan parameters were then exported to the remaining phases and 9 other plans were created. By applying a validated deformable image registration algorithm, a deformation field was derived between each phase and the reference image which was then used to deform the dose from each of the phases to the reference phase where a weighted sum of the dose was computed to constitute the 4D dose from which a 4D DVH was developed.Tracking was characterized by reduced margins as ten different plans were developed based on target location per image set. The 4D dose and DVH were derived from all plans in a similar manner.Radiobiological assessment was based on the linear quadratic-Poisson and relative seriality models. The radiobiological parameters for each tissue were the D50, which is the dose that cause response to 50% of the patients, γ, which is the steepness of the dose-response curve, and s, which is the relative seriality. These parameters are specific for each organ and injury type and have been calculated from clinical data. The radiobiological effectiveness of each plan was quantified by the complication free tumor control probability P+ = PB PI where PB and PI are the total target response (benefit) and normal tissue complication (injury) probabilities, respectively. Four different fractionation schemes were considered; 10Gy, 6Gy, 3Gy and 2Gy per fraction for 6, 10, 20 and 30 fractions, respectively. Results: The advantages of tracking over conventional delivery were best observed using radiobiological analysis (fig 1). Higher P+ values for tracking, which decreased towards hyperfractionated schedules, were observed across all patients. For patients with tumor motion extent greater than 5mm, the relative increase in P+ were 25%±10%; 15%±7%; 8%±4%; 6%±3%, respectively for the plans with 6, 10, 20 and 30 fractions. The corresponding values for two patients with tumor motion extent less than 5mm were 10%±5%; 7%±3%; 3%±1%; 2%±2%. Conclusions: Radiobiological analysis offers a more sensitive assessment of clinical outcome and show that tracking is advantageous over conventional method with significant improvements for hypofractionated schedules. 653 poster (Physics Track)

SU‐FF‐T‐526: Radiobiological Evaluation of Inhomogenity Corrections in Tissue for Lung Cancer Patients

Purpose: To radiobiologically quantify dose difference induced by the inhomogeneity corrections in tissues for lungcancer patients using complication‐free tumor control probability (P +). Method and Materials: One lungcancer patient was retrospectively selected to quantify the difference in clinical effectiveness due to inhomogeneity correction in treatment planning. Four 3D conformal plans were generated for this patient. These treatment plans include two plans with and without tissue inhomogeneity corrections in tissues using 6 MV photon beams. Also, the other two plans were produced with and without inhomogeneity corrections using 18 MV photon beams. The common prescription for four plans was 40 Gy delivered in 20 fractions. The uniform dose that causes the same tumor control probability or normal tissue complication rate as the actual dose given to the patient was evaluated using the biologically effective uniform dose,BEUD. Results: From physical dose evaluation, the treatment plans without inhomogeneity corrections in tissues had up to 8% lower mean dose estimated in the target. From radiobiological assessment, for the plans generated using 18 MV photon beams, the under‐estimated dose of 1 Gy in the mean dose of the target resulted in 3% lower P + in the plans without inhomogeneity corrections. For the plans generated using 6 MV photon beams, the under‐estimated dose of 2 Gy in the mean dose of the target results in 5% lower P + in the plans without inhomogeneity corrections. Conclusion: The dose deviation resulting from the lack of inhomogeneity corrections in tissues for lungcancer patients can be quantified into difference in clinical effectiveness using P +. Treatment plans without inhomogeneity corrections tend to under estimate the P +. Therefore, treatment plans that have limit access to account for tissue inhomogeneity should be reviewed with caution regarding the accuracy in the physical dose and the resulting clinical outcome of P +.