M Hossain

Output trends, characteristics, and measurements of three megavoltage radiotherapy linear accelerators.

The purpose of this study is to characterize and understand the long‐term behavior of the output from megavoltage radiotherapy linear accelerators. Output trends of nine beams from three linear accelerators over a period of more than three years are reported and analyzed. Output, taken during daily warm‐up, forms the basis of this study. The output is measured using devices having ion chambers. These are not calibrated by accredited dosimetry laboratory, but are baseline‐compared against monthly output which is measured using calibrated ion chambers. We consider the output from the daily check devices as it is, and sometimes normalized it by the actual output measured during the monthly calibration of the linacs. The data show noisy quasi‐periodic behavior. The output variation, if normalized by monthly measured “real’ output, is bounded between ± 3%. Beams of different energies from the same linac are correlated with a correlation coefficient as high as 0.97, for one particular linac, and as low as 0.44 for another. These maximum and minimum correlations drop to 0.78 and 0.25 when daily output is normalized by the monthly measurements. These results suggest that the origin of these correlations is both the linacs and the daily output check devices. Beams from different linacs, independent of their energies, have lower correlation coefficient, with a maximum of about 0.50 and a minimum of almost zero. The maximum correlation drops to almost zero if the output is normalized by the monthly measured output. Some scatter plots of pairs of beam output from the same linac show band‐like structures. These structures are blurred when the output is normalized by the monthly calibrated output. Fourier decomposition of the quasi‐periodic output is consistent with a 1/f power law. The output variation appears to come from a distorted normal distribution with a mean of slightly greater than unity. The quasi‐periodic behavior is manifested in the seasonally averaged output, showing annual variability with negative variations in the winter and positive in the summer. This trend is weakened when the daily output is normalized by the monthly calibrated output, indicating that the variation of the periodic component may be intrinsic to both the linacs and the daily measurement devices. Actual linac output was measured monthly. It needs to be adjusted once every three to six months for our tolerance and action levels. If these adjustments are artificially removed, then there is an increase in output of about 2%–4% per year. PACS numbers: 87.56bd, 87.56Fc, 87.55Qr

Dosimetric investigation of high dose rate, gated IMRT

Increasing the dose rate offers time saving for IMRT delivery but the dosimetric accuracy is a concern, especially in the case of treating a moving target. The objective of this work is to determine the effect of dose rate associated with organ motion and gated treatment using step-and-shoot IMRT delivery. Both measurements and analytical simulation on clinical plans are performed to study the dosimetric differences between high dose rate and low dose rate gated IMRT step-and-shoot delivery. Various sites of IMRT plans for liver, lung, pancreas, and breast cancers were delivered to a custom-made motorized phantom, which simulated sinusoidal movement. Repeated measurements were taken for gated and nongated delivery with different gating settings and three dose rates, 100, 500, and 1000 MU/min using ion chambers and extended dose range films. For the study of the residual motion effect for individual segment dose and composite dose of IMRT plans, our measurements with 30%-60% phase gating and without gating for various dose rates were compared. A small but clinically acceptable difference in delivered dose was observed between 1000, 500, and 100 MU/min at 30%-60% phase gating. A simulation is presented, which can be used for predicting dose profiles for patient cases in the presence of motion and gating to confirm that IMRT step-and-shoot delivery with gating for 1000 MU/min are not much different from 500 MU/min. Based on the authors sample plan analyses, our preliminary results suggest that using 1000 MU/Min dose rate is dosimetrically accurate and efficient for IMRT treatment delivery with gating. Nonetheless, for the concern of patient care and safety, a patient specific QA should be performed as usual for IMRT plans for high dose rate deliveries.

Effect of gold marker seeds on magnetic resonance spectroscopy of the prostate.

PURPOSE Magnetic resonance stereoscopic imaging (MRSI) of the prostate is an emerging technique that may enhance targeting and assessment in radiotherapy. Current practices in radiotherapy invariably involve image guidance. Gold seed fiducial markers are often used to perform daily prostate localization. If MRSI is to be used in targeting prostate cancer and therapy assessment, the impact of gold seeds on MRSI must be investigated. The purpose of this study was to quantify the effects of gold seeds on the quality of MRSI data acquired in phantom experiments. METHODS AND MATERIALS A cylindrical plastic phantom with a spherical cavity 10 centimeters in diameter wss filled with water solution containing choline, creatine, and citrate. A gold seed fiducial marker was put near the center of the phantom mounted on a plastic stem. Spectra were acquired at 1.5 Tesla by use of a clinical MRSI sequence. The ratios of choline + creatine to citrate (CC/Ci) were compared in the presence and absence of gold seeds. Spectra in the vicinity of the gold seed were analyzed. RESULTS The maximum coefficient of variation of CC/Ci induced by the gold seed was found to be 10% in phantom experiments at 1.5 T. CONCLUSION MRSI can be used in prostate radiotherapy in the presence of gold seed markers. Gold seeds cause small effects (in the order of the standard deviation) on the ratio of the metabolites CC/Ci in the phantom study done on a 1.5-T scanner. It is expected that gold seed markers will have similar negligible effect on spectra from prostate patients. The maximum of 10% of variation in CC/Ci found in the phantom study also sets a limit on the threshold accuracy of CC/Ci values for deciding whether the tissue characterized by a local spectrum is considered malignant and whether it is a candidate for local boost in radiotherapy dose.

SU‐GG‐J‐101: Investigation of Intrafractional Prostate Rotation and Its Effect on PTV Margin Evaluation

Purpose: The intrafractional prostate rotation has been rarely studied due to the limitation of real‐time imaging of the prostate during treatment. The Calypso 4D Localization System provides the capability of real‐time tracking of implanted transponders for prostate patients. Based on the transponder locations, intrafractional prostate rotation can be investigated to facilitate prostate PTV margin evaluation.Method and Materials: With three transponders implanted in the prostate, the system log file recorded intrafractional 3D coordinates of transponders for 10 fractions of 5 patients enrolled in the study. To minimize noise, the 3D coordinates were first downsampled following a size of 5 median filtering. Based on the transponder positions in the planning CT, the rotational and translational motion of the prostate was calculated using a point‐based algorithm. To check the CTV coverage for a particular fraction we overlapped the rotated prostate CTV with the PTV. Results: For 9 fractions, the maximum rotation angles around x (Lateral), y (SI) and z (AP) were 7°, 6° and 3°, respectively. Although the prostate may not rotate around all 3 axes simultaneously we tested this worst case scenario and found that the CTV was still covered by the PTV when the prostate rotated by the maximum angles. For one fraction, large rotational angles were observed (39.4°, −22° and 12° in x, y and z) along with large transponder distance variations (>1cm). In this case, 20.5% of the CTV was outside of the PTV. The rotational angles for this patient need further investigation since large transponder migration may affect the rotation calculation significantly as we reported before. Conclusion: For most cases, the prostate exhibits relatively small intrafractional rotations and the CTV is covered by the PTV. For patients with larger transponder migration, a CBCT or other imaging modality is recommended for further investigation.

TH‐D‐AUD B‐04: Developing Hardware and Software Tools for Advanced Mixed Beam Radiotherapy

Purpose: Intensity‐modulated radiation therapy(IMRT) provides excellent lateral dose conformity while energy‐ and intensity‐modulated electron therapy (MBRT) can spare distal critical structures for shallow targets. This work aims to combine IMRT and MERT for advanced mixed beam therapy (MBRT) of breast and head and neck cancers.Method and Materials: We have acquired a motorized electron‐specific multileaf collimator (eMLC) for accurate beam delivery for both conventional electron therapy and MERT. The eMLC is retractable to provide large apertures for efficient photon and electron beamdelivery for MBRT. Extensive measurements were performed to verify dose distributions collimated by the eMLC and to validate MBRT treatment plans. Monte Carlo based dose calculation, treatment optimization and leaf sequencing algorithms were investigated for efficient and accurate beam delivery. This technique is being implemented clinically for scalp, head and neck, and breast treatment through pilot studies that are specially designed for dose escalation and hypofractionation. Results: The eMLC provides similar electron beam characteristics to that obtained with a conventional electron applicator/cutout. The leakage is 1.8% for 16MeV electron beams and is less than 1% for other lower electron energies. A typical MERT has a 2–3 modulation‐scaling factor, which results in a maximum 5% leakage dose that is similar to that from IMRT. The measurement results agreed with the planned dose distributions to 3%/3mm for both uniform and heterogeneous phantoms. Conclusion: We have developed a MBRT system for the treatment of shallow targets that consists of hardware tools and software tools for accurate and efficient beam delivery. The technique is being implemented clinically for partial breast, scalp and head and neck treatments.

When and how to treat an IMRT patient on a second accelerator without replanning

When a linear accelerator is unavailable for treatment, a clinical decision is imminent regarding whether a patient should be treated on a linear accelerator other than the machine the patient was scheduled on, or whether treatment should be postponed until the original Linac becomes available. This work investigates the feasibility of switching patients to different accelerators for intensity-modulated radiation therapy (IMRT). We have performed Monte Carlo simulations of photon beams from different Linac models and vendors. Prostate and head and neck (H&N) treatment plans for Siemens Primus, Primart, and Varian 21EX accelerators are studied in this work. Dose distributions for given plans are recalculated using different beam data with the same nominal energy from different Linacs. We have compared dose-volume histograms (DVHs) and the maximum, the minimum, and the mean doses to the target and critical structures because of switching accelerators. In the process of switching a treatment plan to a different accelerator, issues exist, including optimum penumbra compensation, dose distribution at the boundary of target and critical structures, and multileaf collimator (MLC) leaf-width effects, which need to be considered and verified with measurements. Our Monte Carlo simulation results confirm that, for the cases we tested, the dose received by 95% of the planning target volume differs by 0.2% to 1.5% between Siemens Primus and Varian 21EX Linacs. The discrepancy is within our clinical acceptance criteria of 3% for IMRT treatments. In making the final decision on whether to switch machines or not, the tumor control probabilities (TCPs) based on a linear-quadratic model are compared. Based on the analyses performed in this work, it is therapeutically more beneficial to switch a patient to a different machine than to postpone a treatment until the original machine is available, especially for fast-growing tumors such as H&N cancers.

SU-E-T-522: Investigation of Underdosage of Total Body Irradiation with Bilateral Irradiation Scheme

Purpose: Patient in-vivo measurements report lower readings than those predicted from TMR-based treatment planning on TBI patient knees and ankles where rice was placed to fill the gap between patient’s legs. This study is to understand and correct the under dosage of Total Body Irradiation(TBI) with rice tissue equivalent bolus placement at TBI treatment patient setup. Methods: Bilateral TBI scheme was investigated with rice bags bolus placing between patient’s two legs acting as missing tissue. In-house TMR based treatment planning system was commissioned with measurements under TBI condition at 10MV, i.e. source-to-reference distance 383.4cm with 40×40cm field size with 1cm thickness Lucite. Predictions of patient specific dose points are reported at different sites with 200cGy prescription at patient umbilicus point. Solid water and rice bag phantoms are used at TBI conditions for the attenuation factor verification and CT scanned to verify the CT number and electron density. Results: We found that the rice bag bolus overall density is 11% lower than the water; however, the attenuation factor of rice bags could become 15% lower than that of water at TBI condition. This overestimate of rice bag electron density could cause the lack of lateral scatter and the lack of backscatter. This could Result in an overestimate of dose at in-vivo dosimeter measurement points with TMR-based treatment planning systems. Observations of patient specific optically stimulated luminescent dosimeters(OSLDs) were used to confirm this overestimation. Measurements of setups with increasing the rice bag filled patient leg separation were performed to demonstrate eliminating the overdose issue. Conclusion: Rice bolus has a lower electron density than water does(11%) but results in 15% lower in attenuation factor at TBI condition. This effect was observed in patient delivery with OSLD measurements and can be corrected by increasing the filling rice bolus thickness with 15% longer of separation.

SU-E-T-385: 4D Radiobiology

PURPOSE The linear-quadratic model is the most prevalent model for planning dose fractionation in radiation therapy in the low dose per fraction regimens. However for high-dose fractions, used in SRS/SBRT/HDR treatments the LQ model does not yield accurate predictions, due to neglecting the reduction in the number of sublethal lesions as a result of their conversion to lethal lesions with subsequent irradiation. Proper accounting for this reduction in the number of sublethally damaged lesions leads to the dependence of the survival fraction on the temporal structure of the dose. The main objective of this work is to show that the functional dependence of the dose rate on time in each voxel is an important additional factor that can significantly influence the TCP. METHODS Two SBRT lung plans have been used to calculate the TCPs for the same patient. One plan is a 3D conformal plan and the other is an IMRT plan. Both plans are normalized so that 99.5% of PTV volume receives the same prescription dose of 50 Gy in 5 fractions. The dose rate in each individual voxel is calculated as a function of treatment time and subsequently used in the calculation of TCP. RESULTS The calculated TCPs show that shorter delivery times lead to greater TCP, despite all delivery times being short compared to the repair half-time for sublethal lesions. Furthermore, calculated TCP(IMRT) =0.308 for the IMRT plan is smaller than TCP(3D) =0.425 for 3D conformal, even though it shows greater tumor hot spots and equal PTV coverage. The calculated TCPs are considerably lower compared to those based on the LQ model for which TCP=1 for both plans. CONCLUSION The functional dependence of the voxel-by-voxel dose rate on time may be an important factor in predicting the treatment outcome and cannot be neglected in radiobiological modeling.

SU‐D‐103‐01: TG 142 Imaging Modalities QA ‐ Three Year Experience

PURPOSE This study is initiated to check if the frequency of imaging QA suggested by AAPM Task Group Report 142(TG142) is necessary. TG142 presents recommendations for QA criteria of IGRT treatment. It is an update to AAPM TG40 and has added recommendations for imaging devices that are integrated with IGRT Linacs. The imaging devices studied include kilovoltage(KV) X-ray imaging, megavoltage(MV) portal imaging, and cone-beam CT(CBCT) with kV CBCT for Varian and MV CBCT for Siemens. METHOD AND MATERIALS This study uses VarianIX2100 and Siemens Artiste Linacs to perform QAs on KV,MV,CBCT modalities. The QA was designed following under the recommendations of TG142. This study reports the daily imaging positioning/repositioning and imaging and treatment coordinate coincidence. QA results on kV, MV and CBCT from 4/7/2010∼12/14/2012 are analyzed. A radio opaque marker was implanted into the center of a 5cm cube. KV,MV,CBCT images are taken with the cube localized at the isocenter. Digital graticule is used in the software to verify the isocenter position. Another image is taken with the cube placed at 1cm superior, lateral and anterior of the isocenter. In-line fusion software is used to verify the contrived shift. Digital ruler provided at the on-board-imaging software or adaptive-targeting software was used to measure the position differences. The position differences were recorded at AP,LR,SI directions. RESULTS Thirty-two-month records on kV,MV,CBCT show the shifts in all three directions are within the tolerance of 1mm suggested in TG142 for stereotactic radiation treatment(SRS/SRT). There is no occasion where shifts are outside 1mm tolerance. CONCLUSIONS The daily imaging QA suggested in TG142 is useful in ensuring the accuracy needed for SRS/SRT in IGRT. Thirty-two-month measurements presented suggest that decreasing the frequency of imaging QA may be acceptable, in particular for institutions reporting no violation of tolerance over periods of few years.

SU‐E‐T‐557: Dosimetric Evaluation of Elekta‐XiO Superposition Convolution Algorithm for Siemens MultiLeaf Collimators

PURPOSE Elekta-XiO treatment planning system(TPS) uses the same machine layout to simulate both two level jaw system Linacs like Siemens and three level jaw system like Varian Linacs. The purpose of this study is to perform a systematic investigation of the dose calculated from TPS using superposition convolution algorithm and compare that to the measurements for these two types of Linacs. METHOD AND MATERIALS Siemens Primus and Artiste accelerators with multi-leaf collimators as lower jaws and Varian Trilogy accelerators with tertiary multileaf collimators were used in this study. A series of specially-designed T-shaped MLC fields with different field-aperture-opening-ratios(FAOR) and several clinically used 3DCRT MLC-blocked fields were calculated using the XiO superposition convolution and Clarkson methods. These plans were delivered on the respective Linacs and measurements were compared to the calculations. Several TMR based dose calculations were performed as independent checks on both superposition convolution and Clarkson methods. Monte Carlo simulations were performed to verify the dose calculation and to study the sources of scattering contribution. RESULTS A larger discrepancy was found between the dose calculated by XiO using superposition convolution algorithm and the dose measured for the Siemens Linacs than Varian Linacs. An average difference of 1.8% between Siemens and Varian Linacs was found for the situation of FAOR >0.33, possibly resulted from modeling the non-existant lower jaws for Siemens Linacs in the XiO convolution algorithm. TMR based dose calculations using a combined collimator scatter and phantom scatter factors with a XiO back-projected-blocked equivalent square(BES) can estimate the delivered dose accurately(0.12% of measured values) including extreme cases for Siemens(FAOR<0.33). CONCLUSIONS A correction of magnitude of 1.8% should be applied to Linacs that use MLCs to replace a pair of jaws in the XiO superposition convolution algorithm when FAOR is >0.33. For FAOR<0.33, the algorithm cannot predict the delivered doses accurately.