Delivered Dose Distribution Research Articles

SU‐E‐J‐222: An Analysis of the Deep Inspiration Breath Hold (DIBH) Treatment Delivery Uncertainties

Purpose: Depends on how stable the patient can hold her breath during beam on time, there are always some degrees of uncertainties associated with the Deep Inspiration Breath Hold (DIBH) delivery. We herein analyze 1) how much motion patients have during delivery; 2) how much dosimetric uncertainties are resulted due to motion. Methods: The data from three tangential‐field DIBH breast patients were studied. AlignRT (Vision RT Ltd., London, UK), a 3D surface matching‐based system was used for the DIBH clinical implementation. AlignRT calculated and recorded, in realtime, the differences between the treatment surface and planned surface, including 3 translational and 3 rotational displacements (RTDs). We programmed our in‐house TPS to read RTDs and calculate the final dose distribution and DVHs patients have received. The received dose and DVHs were compared with the planned ones. We also calculated the mean (m) and standard deviation (σ) of the RTDs, trying to correlate them to the delivery uncertainties. Results: All patients had <0.2mm and 0.7° σ. Patient A had m_A = (−0.02mm, −0.05mm, 0.02mm, 1.11°, −0.36°, 0.45°). Her delivered dose distribution is slightly different than the plan with 3D max dose difference of 0.4%. No noticeable differences on the DVHs. Patients B and C had larger rotational displacement with m_B = (−0.07mm, 0.01mm, 0.12mm, 1.80°, 2.17°, 1.06°) and m_C = (−0.04mm, −0.04mm, −0.04mm, 2.09°, −0.22°, 0.97°). The resulted 3D max dose differences are both 1%. Although dose distributions in the center treatment regions are as planned, there are different hot spots and cold spots in the peripheral areas. Both patients had slightly lower received heart, lung, and target DVHs. Conclusion: No significant translational motion was observed, and its impact to the delivery uncertainties seems small. Larger rotational displacements seem to cause some dosimetric uncertainties. More data need to be collected for quantitative results.

Evaluation of two registration techniques applied to lung adaptive radiotherapy

During the course of a radiotherapy treatment, volumes of interest might change and the doses delivered to patients may not match the planned dose any more. The delivered dose distributions can be directly estimated from the Cone Beam Computed Tomography (CBCT), images acquired at each session. For this purpose, it is first necessary to perform a registration between the computed tomography (CT) planning images and the CBCT images to extract the contours of the structures of interest in order to calculate dose volume histograms. This study evaluates registration methods adapted to these imaging modalities in the case of lung stereotactic treatments. CBCT images acquired on the linear accelerator (Varian 2100 CD) are registered on planning CT acquisitions (GE Lightspeed RT 16). Phase-shift can be seen as the iterative solution of an optimization problem in which a similarity criterion is maximized. Digital processing has also been implemented to change the resolution of the two images. The goal of this preliminary study is to establish the best suited registration algorithm suited. Two methods were compared: diffeomorphic demons algorithm and B splines warping algorithm, both from the ITK open source library. The selection of these algorithms was based on their diffeomorphism property, required for dose matrix applications. Their evaluation is based on qualitative and quantitative methods. The evaluation between the two registration algorithms was based on ten patients. CBCT were performed at each session with fixed acquisition parameters. The results show the importance of the choice of interpolation technique. A tri-linear interpolation is not relevant for tumors located in the lower lobes where the respiratory cycle is more pronounced, while the two algorithms provide satisfactory results in terms of accuracy for tumors located in the upper lobes. Optimization of CT-CBCT registration was performed in this study. This registration will then be applied to matrices of dose distribution.

SU-D-141-04: Patient-Specific EPID Based High Resolution 3D VMAT QA

Purpose: Existing commercial 2D/3D array detectors for patient-specific VMAT QA are not suitable for stereotactic plans because of the poor spatial resolution of the embedded detectors. A pre-treatment QA method was developed to perform a complete 3D patient dose analysis of stereotactic VMAT plans using Sun Nuclear 3DVH software and a high resolution EPID detector in cine acquisition mode. Methods: Cine images are acquired continuously during the irradiation with the EPID panel while the delivery is monitored using the iCom messaging interface from Elekta linear accelerators to record the gantry angle and the delivered monitor units as a function of time. Then, acquired greyscale snapshots of the MLC apertures can be correlated to their associated gantry angles and converted into absolute dose. According to the sampling desired, the control points of the treatment plan are interpolated to produce planar dose maps at angles corresponding to those at which images were acquired. Disagreement between measured and calculated dose maps can be evaluated and the delivered dose distribution in the patient can be reconstructed by perturbing the calculated dose using errors detected in planar dose measurements (3DVH Planned Dose Perturbation algorithm). Results: Several stereotactic brain and lung plans from Pinnacle were analyzed using this method. Regarding the agreement between measured and calculated dose maps, the average passing rate for a 2%-2mm gamma criterion (threshold = 5%) is close to 97% ± 3%. The complete 3D patient dose analysis showed good agreement between treatment planning system and 3DVH dose volume histograms for both targets and organs at risk. Conclusion: In this study, we developed a practical approach based on EPID cine acquisition mode and the Elekta iCom interface to perform stereotactic VMAT QA. This approach takes advantage of the high spatial resolution of EPID panels to achieve an appropriate and accurate independent QA.

SU‐E‐T‐168: A Feasibility Study to Use Gamma‐Histogram Analysis in Assisting Quality Assurance Criteria for Evaluating Volumetric Modulated Arc Therapy Treatment Plans

Purpose: To investigate if gamma‐histogram statistical analysis can be used as criteria in establishing clinical action limits for pre‐treatment quality assurance (QA) agreement between planned and delivered dose distributions in volumetric‐modulated‐arc therapy (VMAT). Methods: Radiotherapy patient‐treatment plans were calculated using AAA dose‐calculation algorithm implemented in Varian Eclipse system (v.10). 6 and 10 MV photon beams from a Varian‐Trilogy Linac were investigated. Pre‐treatment QA dosimetry on 102 VMAT plans was performed using an electronic‐portal‐imaging device (EPID) and an ion‐chamber‐based array (MatriXX). Evaluation was done by comparing planned and measured dose distributions using gamma‐histogram analysis for plans grouped by site (head and neck, prostate, brain and lung). Different dose‐difference/distance‐to‐agreement gamma criteria from 3%/1mm to 5%/3mm were evaluated considering only dose levels higher than 10% of the maximum dose. Statistical analysis of gamma histograms included mean γ values (γmean), γ values corresponding to Δ = mean + 1.5 SD (γΔ), and areal percentages with gamma values less than 1.0, 1.5, and greater than 2.0. Average values and standard deviations for each of these five parameters were calculated per site to determine specific action limits. Results: The γmean over all analyzed plans with EPID was 0.42 +/− 0.10 for 3%/1mm and 0.29+/− 0.06 for 3%/3mm, while the corresponding average percentage of points with gamma less than 1 was 95.0% +/− 5.0 and 99.1% +/− 1.6, respectively. The results showed that parameters under the 3%/1mm and 3%/3mm criteria are plan‐site specific while the 4%/3mm and 5%/3mm criteria did not provide statistically significant differences among sites for EPID. Similar trends were observed for analysis using the MatriXX. Conclusion: Statistical analysis of γ histogram parameters is useful to perform pre‐treatment QA for VMAT plans. A specific selected criterion can be used to establish clinical action levels based on clinical acceptability and machine delivery accuracy.

SU‐E‐T‐209: Deconvolution of An Integral Film Exposure in a Cylindrical Phantom for VMAT QA

Purpose: Present an algorithm that is capable of deconvoluting a single radiochromic film exposed in a cylindrical phantom into the basic control point components. Changes noted in the measured control points can be incorporated into a delivered treatment plan using a commercial treatment planning system to compare the delivered dose with the planned dose using DVH tools. Methods: A 180 degree VMAT verification plan was subdivided into control points every 5 degrees. Dose matrices for the individual control points were exported for analysis to a PC with MATLAB software. The dose in the center ring of the cylindrical phantom represents the simulated film measurement. The plan dose is equal to the sum of the control points. Mathematically, the control points are the basis vectors of thedelivered plan. The deconvolution algorithm utilizes linear optimization to first determine the coefficients of the vectors and then to check for changes in the individual control points. Changes in the control points were introduced to test the deconvolution algorithm. For development purposes only a single plane was investigated. Measurement of both the entrance and exit doses improves the accuracy and is also used to compute the control point angle. Results: (1) Measured film has superior resolution compared to electronic detectors, (2) the deconvolution algorithm can determine the individual delivered control point parameters, (3) the measured control points can be used to calculate the delivered dose distribution, and (4) the treatment planning system can be used to compare the plan and delivered dose distribution within the patient using its DVH tools. Conclusion: Deconvolution of an integral film in a cylindrical phantom for a VMAT plan represents the delivered control points that can be entered into the treatment planning system for patient specific VMAT QA utilizing DVH tools.

Non-invasive monitoring of therapeutic carbon ion beams in a homogeneous phantom by tracking of secondary ions

Radiotherapy with narrow scanned carbon ion beams enables a highly accurate treatment of tumours while sparing the surrounding healthy tissue. Changes in the patient’s geometry can alter the actual ion range in tissue and result in unfavourable changes in the dose distribution. Consequently, it is desired to verify the actual beam delivery within the patient. Real-time and non-invasive measurement methods are preferable. Currently, the only technically feasible method to monitor the delivered dose distribution within the patient is based on tissue activation measurements by means of positron emission tomography (PET). An alternative monitoring method based on tracking of prompt secondary ions leaving a patient irradiated with carbon ion beams has been previously suggested. It is expected to help in overcoming the limitations of the PET-based technique like physiological washout of the beam induced activity, low signal and to allow for real-time measurements. In this paper, measurements of secondary charged particle tracks around a head-sized homogeneous PMMA phantom irradiated with pencil-like carbon ion beams are presented. The investigated energies and beam widths are within the therapeutically used range. The aim of the study is to deduce properties of the primary beam from the distribution of the secondary charged particles. Experiments were performed at the Heidelberg Ion Beam Therapy Center, Germany. The directions of secondary charged particles emerging from the PMMA phantom were measured using an arrangement of two parallel pixelated silicon detectors (Timepix). The distribution of the registered particle tracks was analysed to deduce its dependence on clinically important beam parameters: beam range, width and position. Distinct dependencies of the secondary particle tracks on the properties of the primary carbon ion beam were observed. In the particular experimental set-up used, beam range differences of 1.3 mm were detectable. In addition, variations in the beam width could be measured with a precision of 0.9 mm. Furthermore, shifts of the lateral beam position could be monitored with a sub-millimetre precision. The presented investigations demonstrate experimentally that the non-invasive measurement and analysis of secondary ion distributions around head-sized homogeneous objects provide information on the actual beam delivery. Beam range, width and position could be monitored with a precision attractive for therapeutic situations.

Utilization of cone‐beam CT for reconstruction of dose distribution delivered in image‐guided radiotherapy of prostate carcinoma — bony landmark setup compared to fiducial markers setup

The purpose of this study was to compare two different styles of prostate IGRT: bony landmark (BL) setup vs. fiducial markers (FM) setup. Twenty‐nine prostate patients were treated with daily BL setup and 30 patients with daily FM setup. Delivered dose distribution was reconstructed on cone‐beam CT (CBCT) acquired once a week immediately after the alignment. Target dose coverage was evaluated by the proportion of the CTV encompassed by the 95% isodose. Original plans employed 1 cm safety margin. Alternative plans assuming smaller 7 mm margin between CTV and PTV were evaluated in the same way. Rectal and bladder volumes were compared with initial ones. While the margin reduction in case of BL setup makes the prostate coverage significantly worse (p=0.0003, McNemar's test), in case of FM setup with the reduced 7 mm margin, the prostate coverage is even better compared to BL setup with 10 mm margin (p=0.049, Fisher's exact test). Moreover, partial volumes of organs at risk irradiated with a specific dose can be significantly lowered (p<0.0001, unpaired t‐test). Reducing of safety margin is not acceptable in case of BL setup, while the margin can be lowered from 10 mm to 7 mm in case of FM setup.PACS numbers: 87.55.dk, 87.55.km, 87.55.tm

Accuracy and precision of cone-beam computed tomography guided intensity modulated radiation therapy

PurposeTo assess the accuracy and precision of cone-beam computed tomography (CBCT)-guided intensity modulated radiation therapy (IMRT). Methods and MaterialsA 7-field intensity modulated radiation therapy plan was constructed for an anthropomorphic head phantom loaded with a custom cassette containing radiochromic film. The phantom was positioned on the treatment table at 9 locations: 1 “correct” position and 8 “misaligned” positions along 3 orthogonal axes. A commercial kilovoltage cone-beam computed tomography (kV-CBCT) system (VolumeView, Elekta AB, Stockholm, Sweden) was then used to align the phantom prior to plan delivery. The treatment plan was delivered using the radiation therapy delivery system (Infinity; Elekta AB) 3 times for each of the 9 positions, allowing film measurement of the delivered dose distribution in 3 orthogonal planes. Comparison of the planned and delivered dose profiles along the major axes provided an estimate of the accuracy and precision of CBCT-guided IMRT. ResultsOn average, targeting accuracy was found to be within 1 mm in all 3 major anatomic planes. Over all 54 measured dose profiles, the means and standard errors of the displacement of the center of the field between the measured and calculated profiles for each of the right-left, anterior-posterior, and superior-inferior axes were +0.08 ± 0.07 mm, +0.60 ± 0.08 mm, and +0.78 ± 0.16 mm, respectively. Agreement between planned and measured 80% profiles was less than 0.4 mm on either side along the right-left axis. A systematic shift of the measured profile of slightly less than 1 mm in anterior and superior directions was noted along the anterior-posterior and superior-inferior axes, respectively. ConclusionsSubmillimeter targeting accuracy can be achieved using a commercial kV-CBCT IGRT system.

Radiobiological Evaluation of Breast Cancer Radiotherapy Accounting for the Effects of Patient Positioning and Breathing in Dose Delivery. A Meta Analysis

In breast cancer radiotherapy, significant discrepancies in dose delivery can contribute to underdosage of the tumor or overdosage of normal tissue, which is potentially related to a reduction of local tumor control and an increase of side effects. To study the impact of these factors in breast cancer radiotherapy, a meta analysis of the clinical data reported by Mavroidis et al. (2002) in Acta Oncol (41:471-85), showing the patient setup and breathing uncertainties characterizing three different irradiation techniques, were employed. The uncertainties in dose delivery are simulated based on fifteen breast cancer patients (5 mastectomized, 5 resected with negative node involvement (R-) and 5 resected with positive node involvement (R1)), who were treated by three different irradiation techniques, respectively. The positioning and breathing effects were taken into consideration in the determination of the real dose distributions delivered to the CTV and lung in each patient. The combined frequency distributions of the positioning and breathing distributions were obtained by convolution. For each patient the effectiveness of the dose distribution applied is calculated by the Poisson and relative seriality models and a set of parameters that describe the dose-response relations of the target and lung. The three representative radiation techniques are compared based on radiobiological measures by using the complication-free tumor control probability, P(+) and the biologically effective uniform dose, (BEUD)concepts. For the Mastectomy case, the average P(+) values of the planned and delivered dose distributions are 93.8% for a (BEUD)(CTV) of 51.8 Gy and 85.0% for a (BEUD)(CTV) of 50.3 Gy, respectively. The respective total control probabilities, P(B) values are 94.8% and 92.5%, whereas the corresponding total complication probabilities, P(1) values are 0.9% and 7.4%. For the R- case, the average P(+) values are 89.4% for a (BEUD)(CTV) of 48.9 Gy and 88.6% for a (BEUD)(CTV) of 49.2 Gy and 85.5% for a (BEUD)(CTV) of 49.1 Gy, respectively. The respective PB values are 90.2% and 90.1%, whereas the corresponding P(+) values are 4.1% and 4.6%. The combined effects of positioning uncertainties and breathing can introduce a significant deviation between the planned and delivered dose distributions in lung in breast cancer radiotherapy. The positioning and breathing uncertainties do not affect much the dose distribution to the CTV. The simulated delivered dose distributions show larger lung complication probabilities than the treatment plans. This means that in clinical practice the true expected complications are underestimated. Radiation pneumonitis of Grade 1-2 is more frequent and any radiotherapy optimization should use this as a more clinically relevant endpoint.

Cumulative dose on fractional delivery of tomotherapy to periodically moving organ: A phantom QA suggestion

This study was conducted to evaluate the cumulative dosimetric error that occurs in both target and surrounding normal tissues when treating a moving target in multifractional treatment with tomotherapy. An experiment was devised to measure cumulative error in multifractional treatments delivered to a horseshoe-shaped clinical target volume (CTV) surrounding a cylinder shape of organ at risk (OAR). Treatments differed in jaw size (1.05 vs 2.5cm), pitch (0.287 vs 0.660), and modulation factor (1.5 vs 2.5), and tumor motion characteristics differing in amplitude (1 to 3cm), period (3 to 5 second), and regularity (sinusoidal vs irregular) were tested. Treatment plans were delivered to a moving phantom up to 5-times exposure. Dose distribution on central coronal plane from 1 to 5 times exposure was measured with GAFCHROMIC EBT film. Dose differences occurring across 1 to 5 times exposure of treatment and between treatment plans were evaluated by analyzing measurements of gamma index, gamma index histogram, histogram changes, and dose at the center of the OAR. The experiment showed dose distortion due to organ motion increased between multiexposure 1 to 3 times but plateaued and remained constant after 3-times exposure. In addition, although larger motion amplitude and a longer period of motion both increased dosimetric error, the dose at the OAR was more significantly affected by motion amplitude rather than motion period. Irregularity of motion did not contribute significantly to dosimetric error when compared with other motion parameters. Restriction of organ motion to have small amplitude and short motion period together with larger jaw size and small modulation factor (with small pitch) is effective in reducing dosimetric error. Pretreatment measurements for 3-times exposure of treatment to a moving phantom with patient-specific tumor motion would provide a good estimation of the delivered dose distribution.

Statistical simulations to estimate motion-inclusive dose-volume histograms for prediction of rectal morbidity following radiotherapy

Background and purpose. Internal organ motion over a course of radiotherapy (RT) leads to uncertainties in the actual delivered dose distributions. In studies predicting RT morbidity, the single estimate of the delivered dose provided by the treatment planning computed tomography (pCT) is typically assumed to be representative of the dose distribution throughout the course of RT. In this paper, a simple model for describing organ motion is introduced, and is associated to late rectal morbidity data, with the aim of improving morbidity prediction. Material and methods. Organ motion was described by normally distributed translational motion, with its magnitude characterised by the standard deviation (SD) of this distribution. Simulations of both isotropic and anisotropic (anterior-posterior only) motion patterns were performed, as were random, systematic or combined random and systematic motion. The associations between late rectal morbidity and motion-inclusive delivered dose-volume histograms (dDVHs) were quantified using Spearman's rank correlation coefficient (Rs) in a series of 232 prostate cancer patients, and were compared to the associations obtained with the static/planned DVH (pDVH). Results. For both isotropic and anisotropic motion, different associations with rectal morbidity were seen with the dDVHs relative to the pDVHs. The differences were most pronounced in the mid-dose region (40–60 Gy). The associations were dependent on the applied motion patterns, with the strongest association with morbidity obtained by applying random motion with an SD in the range 0.2–0.8 cm. Conclusion. In this study we have introduced a simple model for describing organ motion occurring during RT. Differing and, for some cases, stronger dose-volume dependencies were found between the motion-inclusive dose distributions and rectal morbidity as compared to the associations with the planned dose distributions. This indicates that rectal organ motion during RT influences the efforts to model the risk of morbidity using planning distributions alone.

Quantitative carbon ion beam radiography and tomography with a flat-panel detector

High dose gradients are inherent to ion beam therapy. This results in high sensitivity to discrepancies between planned and delivered dose distributions. Therefore an accurate knowledge of the ion stopping power of the traversed tissue is critical. One proposed method to ensure high quality dose deposition is to measure the stopping power by ion radiography. Although the idea of imaging with highly energetic ions is more than forty years old, there is a lack of simple detectors suitable for this purpose. In this study the performance of an amorphous silicon flat-panel detector, originally designed for photon imaging, was investigated for quantitative carbon ion radiography and tomography. The flat-panel detector was exploited to measure the water equivalent thickness (WET) and water equivalent path length (WEPL) of a phantom at the Heidelberg Ion–Beam Therapy Center (HIT). To do so, the ambiguous correlation of detector signal to particle energy was overcome by active or passive variation of carbon ion beam energy and measurement of the signal-to-beam energy correlation. The active method enables one to determine the WET of the imaged object with an uncertainty of 0.5 mm WET. For tomographic WEPL measurements the passive method was exploited resulting in an accuracy of 0.01 WEPL. The developed imaging technique presents a method to measure the two-dimensional maps of WET and WEPL of phantoms with a simple and commercially available detector. High spatial resolution of 0.8 × 0.8 mm2 is given by the detector design. In the future this powerful tool will be used to evaluate the performance of the treatment planning algorithm by studying WET uncertainties.

Adaptive MRI Guided Brachytherapy of Cervix Cancer: Impact of Reference Image Selection on Dose Accumulation

Online imaging and deformable image registration (DIR) are required for an accurate dose accumulation of fractionated treatments. Dosimetry may depend on the selection of reference image, to which fractional doses are accumulated. This study reviews the impact of reference image selection on the accumulated dosimetry for adaptive MRI guided brachytherapy of cervix cancer. Twelve cervical cancer patients underwent one planning and two daily pelvic MRI scans during 3 day-long PDR brachytherapy. A brachytherapy plan was optimized on planning MRI respecting GEC-ESTRO recommendations and the plan was repeatedly used on following daily MRIs for delivered dose computation, resulting in three delivered dose distributions on each patient. To evaluate plan quality, D2cc of rectum/bladder wall and D90 of HRCTV/IRCTV were estimated by (1) simply taking average of each D2cc and D90 from three delivered dose distributions and more accurately measured from (2) DIR based dose accumulation using one of the MRI images as a reference (totaling three accumulations doses). Standard deviations of D2cc and D90 from three accumulations were considered as a measure of accumulation dose uncertainty. The evaluation parameters were compared among planned, averaged, and accumulated dose distributions. Relative to the clinical plan, the accumulated D90s of HRCTV/IRCTV varied by, on average, 2.1% (range 0.2∼3.0%) and 3.2% (0.8∼9.3%) respectively, depending on the selection of the reference image. The differences from planned dose were, on average, 3.3% (-12.3∼20.4%) and 1.8% (-9.5∼18.8%), respectively. Averaged D90s of HRCTV/IRCTV were less than the accumulated dose by 7.6% (3.4∼17.0%) and 7.7% (2.7∼13.5%), demonstrating a good conservative estimate for target coverage. Averaged D2cc estimated the accumulated D2cc well and were found within 2 STDs in all except one patient. 6 patients out of 12 had larger accumulated dose variation than difference from planned dose in at least one evaluation parameter. Accumulated dose using DIR was significantly different from the planned dose (p < 0.05, paired t-test) in PDR brachytherapy plans, demonstrating the need for adaptive planning. However, care should be taken in the interpretation of the result due to the dosimetric uncertainty of accumulation process depending on the selection of reference image. Averaging showed conservative estimate for target coverage and good D2cc agreement relative to accumulation.Poster Viewing Abstract 3485; TableEvaluation parameters on 12 patients relative to the clinical plan (%)Average∗Averaged from three delivered dose distributionsAccum 1†Accum n is DIR based dose accumulation using n-th MRI as a reference imageAccum 2Accum 3Mean of accumSTD of accum‡Standard deviation of accumulated dose in each patient.HRCTV D9089.2 (73.5∼100.0)98.8 (82.4∼115.6)95.7 (78.3∼111.7)95.8 (78.3∼109.7)96.7(79.7∼112.3)2.1 (0.2∼3.2)IRCTV D9090.6 (74.7∼99.7)100.8 (84.6∼108.2)96.9 (78.3∼106.5)97.0 (80.8∼114.5)98.2 (81.2∼109.5)3.2 (0.8∼9.3)Bladder wall D2cc95.0 (71.9∼119.4)93.7 (74.1∼111.8)94.7(73.8∼135.5)92.8 (69.8∼115.9)93.7 (73.0∼115.6)6.7 (1.7∼18.4)Rectum wall D2cc109.9 (93.0∼143.3)109.2(93.5∼124.0)109.2 (87.1∼139.3)108.7 (91.2∼152.8)109.1 (90.6∼138.7)4.8 (1.7∼14.4)() Shows range of the data∗ Averaged from three delivered dose distributions† Accum n is DIR based dose accumulation using n-th MRI as a reference image‡ Standard deviation of accumulated dose in each patient. Open table in a new tab

Treatment Dose Verification in Liver SBRT Using a Deformable Respiratory Motion Model and Treatment Beam's-Eye-View Fluoroscopy With Tumor Tracking

• A patient with hepatocellular carcinoma was treated with SBRT 40Gy/4fx. Gold fiducial markers were implanted to mark the tumor location. • 4D-CT was used for ITV definition/planning and modeling of deformable motion between respiratory phases. • In the TPS, beams were applied to each phase’s 3DCT. Each resulting dose distribution was spatially transformed via the motion model onto a common anatomical space (planning CT). • MVF with fiducial tracking yielded tumor motion during radiation delivery. Motion analysis determined the duty cycle for each respiratory phase. A duty cycle weighted average of the deformed dose distributions yielded the composite delivered dose distribution. • Breathing variability in SBRT causes deviation of delivered dose distribution from the planned dose. • The goal was to calculate the actual dose delivered to a moving tumor using measurements of implanted fiducial marker position obtained using beam’s-eye-view MV fluoroscopy (MVF).

Partial Breast Extracranial Radiosurgery: A Dosimetric Evaluation of Treatment Delivery Accuracy

Partial breast extra-cranial radiosurgery was proposed to treat early stage breast cancer patient preoperatively. Cone-beam CT (CBCT) acquired prior to treatment represents the actual patient position during the treatment. This study evaluates dosimetric variations between the delivered plan (DP) based on CBCT and the original plan (OP) based on the planning CT. Seven patients treated to 15 Gy in the prone position were included in this study. The gross tumor volume (GTV) (volume: 0.5 cc to 2.7 cc) was defined based on fused CT and breast MR images. The clinical target volume (CTV) encompassed the GTV + a 1.5-cm isotropic margin. An additional 0.3-cm isotropic margin was added to form the planning target volume (PTV). Both CTV and PTV excluded the outermost 0.5 cm of the patient surface. A 3 mm thick region along the ipsilateral breast surface was defined as skin to analyze dose to the subcutaneous tissue. Four to five non-coplanar photon beams were arranged to avoid heart, lungs and contralateral breast and intensity modulated radiation therapy (IMRT) provided further optimization. The plans were normalized to deliver 100% of the prescription to at least 95% of the CTV. For treatment, target localization was achieved based on both skin surface and soft-tissue tumor and/or biopsy markers using CBCT. To standardize the CBCT images, the CT numbers were assigned - breast to -122 HU, lungs to -719 HU, and all others to 40 HU. The GTV, CTV, and PTV were copied to CBCT based on the target localization while the skin, breasts, and lungs were manually contoured. The delivered dose distribution was re-calculated on the CBCT. Dosimetric parameters of OP and DP were compared. The average CTV receiving 100% of the prescription (V100%) was 95.5% ± 0.4 vs 93.2% ± 5.7 (OP vs. DP). The difference in V100% between OP and DP was > 10% for 2 patients (DP ≤ OP). However, the difference in V96% between OP and DP was < 1% for all 7 patients. The average maximum dose to skin was 76.7% ± 13.9 vs. 77.4% ± 11.7. The difference in maximum skin dose between OP and DP was > 8% for 3 patients (DP ≥ OP for 2 patients and ≤ OP for 1 patient). The lung V5Gy was ≤ 1% for all patients in both OP and DP. The contralateral breast V1Gy was 8.1% vs 12.9% for 1 patient, 6 patients had V1Gy < 1% in both OP and DP. The heart V5Gy was zero in OP for all patients, and it was not delineated in CBCT due to difficulties with image quality. Partial breast radiosurgery in this cohort was delivered with a high level of accuracy to the target coverage (CTV V96% achieved for all patients within 1%). Similarly, doses to critical organs were essentially as planned. However, small differences in skin doses were seen due to the effect of set-up variation on breast shape.

Evaluation of Delivered Dose Distributions and Metrics: A New Patient-specific QA Protocol

Patient specific IMRT QA is performed to detect systematic errors in the treatment plan, transfer, and delivery. However, traditional 2D IMRT QA metrics have poor correlation with clinically relevant patient dose errors. This work aims to evaluate the impact of MLC leaf bank offset errors on dose delivery as measured by a next generation 3D IMRT QA tool that provides clinically relevant dose metrics and per-structure 3D gamma analysis.

Dosimetric effect of intrafraction tumor motion in phase gated lung stereotactic body radiotherapy

A major concern for lung intensity modulated radiation therapy delivery is the deviation of actually delivered dose distribution from the planned one due to simultaneous movements of multileaf collimator (MLC) leaves and tumor. For gated lung stereotactic body radiotherapy treatment (SBRT), the situation becomes even more complicated because of SBRT's characteristics such as fewer fractions, smaller target volume, higher dose rate, and extended fractional treatment time. The purpose of this work is to investigate the dosimetric effect of intrafraction tumor motion during gated lung SBRT delivery by reconstructing the delivered dose distribution with real-time tumor motion considered. The tumor motion data were retrieved from six lung patients. Each of them received three fractions of stereotactic radiotherapy treatments with Cyberknife Synchrony (Accuray, Sunnyvale, CA). Phase gating through an external surrogate was simulated with a gating window of 5 mm. The resulting residual tumor motion curves during gating (beam-on) were retrieved. Planning target volume (PTV) was defined as physician-contoured clinical target volume (CTV) surrounded by an isotropic 5 mm margin. Each patient was prescribed with 60 Gy∕3 fractions. The authors developed an algorithm to reconstruct the delivered dose with tumor motion. The DMLC segments, mainly leaf position and segment weighting factor, were recalculated according to the probability density function of tumor motion curve. The new DMLC sequence file was imported back to treatment planning system to reconstruct the dose distribution. Half of the patients in the study group experienced PTV D95% deviation up to 26% for fractional dose and 14% for total dose. CTV mean dose dropped by 1% with tumor motion. Although CTV is almost covered by prescribed dose with 5 mm margin, qualitative comparison on the dose distributions reveals that CTV is on the verge of underdose. The discrepancy happens due to tumor excursion outside of the gating window, which, for our study group, is mainly caused by baseline shift, i.e., the change in general trend of the motion curve during extended period of treatment time. The dose deviation in PTV and CTV due to target motion is not always negligible in gated SBRT. Although CTVs are covered sufficiently with prescribed dose in most cases, some are on the verge of underdose due to large tumor excursion caused by factors such as baseline shift.

The energy margin strategy for reducing dose variation due to setup uncertainty in intensity modulated proton therapy (IMPT) delivered with distal edge tracking (DET)

Intensity‐modulated proton therapy (IMPT) can produce plans with similar target dose conformity but lower normal tissue dose than intensity‐modulated X‐ray therapy (IMXT). However, due to the finite range of proton beams in tissue, proton therapy treatment plans are usually more sensitive to setup uncertainties than X‐ray therapy plans. In this work, the energy margin (EM) concept, which was initially developed for passive scattering proton therapy, was generalized to apply to IMPT treatment planning. The effectiveness of the EM method was evaluated on five head‐and‐neck cancer patients with distal edge tracking (DET) treatment plans by comparing the original plans (ORG) without an EM to those with an EM. Three beam arrangements were considered: 24 beams delivered over a 360° arc, 12 beams delivered over a 180° arc, and 12 beams delivered over two 90° fan angles. Setup uncertainty was modeled by sampling rigid translational shifts from a Gaussian distribution with a mean of 0 mm and standard deviation of 2 mm in all directions. Delivered dose distributions for all 30 fractions were recalculated using the Geant4 Monte Carlo code. Normalized total dose (NTD) for both the CTV and a ring structure surrounding the PTV were recorded. The plan quality comparison revealed that EM plans had the same CTV coverage but higher dose to the normal tissue than ORG plans. After the simulated delivery, ORG plans resulted in more than 3% underdosage to 5% of the CTV volume in all three beam arrangements, whereas the EM plans did not. Both ORG and EM plans did not produce more than 5% overdose to D2% of the ring structure. The use of an EM for IMPT treatment planning can substantially reduce sensitivity of the resulting dose distributions to setup uncertainty.PACS number: 87.55.K‐

A GEM-based dose imaging detector with optical readout for proton radiotherapy

New techniques in proton radiation therapy and advances in beam delivery systems design such as beam scanning require accurate 2D dosimetry systems to verify the delivered dose distribution. Dose imaging detectors based on gas electron multipliers (GEMs) are capable of providing high sensitivity, improved dose measurement linearity, position resolution, fast response and accurate characterization of depth-dose distributions. In this work, we report on the development of a GEM-based dose imaging detector with optical readout using a CCD camera. A 10×10cm2 detector has been tested in a 205MeV proton beam in single- and double-GEM configurations. The detector demonstrates linearity in dose rate up to 100Gy/min and position resolution (σ) of 0.42mm. Transverse non-uniformity of the detector response is ≤10% before correction and the stability of the detector output throughout the day is within ±1%, with day-to-day reproducibility of about 10%. The depth-dose response of the detector is close to that of a wide-aperture air-filled ionization chamber and is in good agreement with Monte Carlo simulations.

The impact of modeling nuclear fragmentation on delivered dose and radiobiology in ion therapy

The importance of nuclear interactions for ion therapy arises from the influence of the particle spectrum on, first, radiobiology and therefore also on treatment planning, second, the accuracy of measuring dose and, third, the delivered dose distribution. This study tries to determine the qualitative as well as the quantitative influence of the modeling of inelastic nuclear interactions on ion therapy. Thereby, three key disciplines are investigated, namely dose delivery, dose assessment and radiobiology. In order to perform a quantitative analysis, a relative comparison between six different descriptions of nuclear interactions is carried out for carbon ions. The particle transport is simulated with the Monte Carlo code SHIELD-HIT10A while dose planning and radiobiology are covered by the analytic treatment planning program for particles TRiP, which determines the relative biological effectiveness (RBE) with the local effect model. The obtained results show that the physical dose distribution can in principle be significantly influenced by the modeling of fragmentation (about 10% for a 20% change in all inelastic nuclear cross sections for a target volume ranging from 15 to 25 cm). While the impact of nuclear fragmentation on stopping power ratios can be neglected, the fluence correction factor may be influenced by the applied nuclear models. In contrast to the results for the physical dose, the variation of the RBE is only small (about 1% for a 20% change in all inelastic nuclear cross sections) suggesting a relatively weak dependence of radiobiology on the detailed composition of the particle energy spectrum of the mixed radiation field. Also, no significant change (about 0.2 mm) of the lateral penumbra of the RBE-weighted dose is observed.