Dosimetric verification of a commercial inverse treatment planning system

A commercial three-dimensional (3D) inverse treatment planning system, Corvus (Nomos Corporation, Sewickley, PA), was recently made available. This paper reports our preliminary results and experience with commissioning this system for clinical implementation. This system uses a simulated annealing inverse planning algorithm to calculate intensity-modulated fields. The intensity-modulated fields are divided into beam profiles that can be delivered by means of a sequence of leaf settings by a multileaf collimator (MLC). The treatments are delivered using a computer-controlled MLC. To test the dose calculation algorithm used by the Corvus software, the dose distributions for single rectangularly shaped fields were compared with water phantom scan data. The dose distributions predicted to be delivered by multiple fields were measured using an ion chamber that could be positioned in a rotatable cylindrical water phantom. Integrated charge collected by the ion chamber was used to check the absolute dose of single- and multifield intensity modulated treatments at various spatial points. The measured and predicted doses were found to agree to within 4% at all measurement points. Another set of measurements used a cubic polystyrene phantom with radiographic film to record the radiation dose distribution. The films were calibrated and scanned to yield two-dimensional isodose distributions. Finally, a beam imaging system (BIS) was used to measure the intensity-modulated x-ray beam patterns in the beam's-eye view. The BIS-measured images were then compared with a theoretical calculation based on the MLC leaf sequence files to verify that the treatment would be executed accurately and without machine faults. Excellent correlation (correlation coefficients > or = 0.96) was found for all cases. Treatment plans generated using intensity-modulated beams appear to be suitable for treatment of irregularly shaped tumours adjacent to critical structures. The results indicated that the system has potential for clinical radiation treatment planning and delivery and may in the future reduce treatment complexity.     

Dosimetric evaluation of a Monte Carlo IMRT treatment planning system incorporating the MIMiC

The high dose per fraction delivered to lung lesions in stereotactic body radiation therapy (SBRT) demands high dose calculation and delivery accuracy. The inhomogeneous density in the thoracic region along with the small fields used typically in intensity-modulated radiation therapy (IMRT) treatments poses a challenge in the accuracy of dose calculation. In this study we dosimetrically evaluated a pre-release version of a Monte Carlo planning system (PEREGRINE 1.6b, NOMOS Corp., Cranberry Township, PA), which incorporates the modeling of serial tomotherapy IMRT treatments with the binary multileaf intensity modulating collimator (MIMiC). The aim of this study is to show the validation process of PEREGRINE 1.6b since it was used as a benchmark to investigate the accuracy of doses calculated by a finite size pencil beam (FSPB) algorithm for lung lesions treated on the SBRT dose regime via serial tomotherapy in our previous study. Doses calculated by PEREGRINE were compared against measurements in homogeneous and inhomogeneous materials carried out on a Varian 600C with a 6 MV photon beam. Phantom studies simulating various sized lesions were also carried out to explain some of the large dose discrepancies seen in the dose calculations with small lesions. Doses calculated by PEREGRINE agreed to within 2% in water and up to 3% for measurements in an inhomogeneous phantom containing lung, bone and unit density tissue.     

Dosimetric evaluation of the clinical implementation of the first commercial IMRT Monte Carlo treatment planning system at 6 MV

In this work we dosimetrically evaluated the clinical implementation of a commercial Monte Carlo treatment planning software (PEREGRINE, North American Scientific, Cranberry Township, PA) intended for quality assurance (QA) of intensity modulated radiation therapy treatment plans. Dose profiles calculated in homogeneous and heterogeneous phantoms using this system were compared to both measurements and simulations using the EGSnrc Monte Carlo code for the 6 MV beam of a Varian CL21EX linear accelerator. For simple jaw-defined fields, calculations agree within 2% of the dose at d(max) with measurements in homogeneous phantoms with the exception of the buildup region where the calculations overestimate the dose by up to 8%. In heterogeneous lung and bone phantoms the agreement is within 3%, on average, up to 5% for a 1 x 1 cm2 field. We tested two consecutive implementations of the MLC model. After matching the calculated and measured MLC leakage, simulations of static and dynamic MLC-defined fields using the most recent MLC model agreed to within 2% with measurements.     

Dosimetric verification of a commercial 3D treatment planning system for conformal radiotherapy with a dynamic multileaf collimator

The dosimetric accuracy of a 3D treatment planning system (TPS) for conformal radiotherapy with a computer-assisted dynamic multileaf collimator (DMLC) was evaluated. The DMLC and the TPS have been developed for clinical applications where dynamic fields not greater than 10 x 10 cm2 and multiple non-coplanar arcs are required. Dosimetric verifications were performed by simulating conformal treatments of irregularly shaped targets using several arcs of irradiation with 6 MV x-rays and a spherical-shaped, tissue-simulating phantom. The accuracy of the delivered dose at the isocentre was verified using an ionization chamber placed in the centre of the phantom. Isodose distributions in the axial and sagittal planes passing through the centre of the phantom were measured using double-layer radiochromic films. Measured dose at the isocentre as well as isodose distributions were compared to those calculated by the TPS. The maximum percentage difference between measured and prescribed dose was less than 2.5% for all the simulated treatment plans. The mean (+/-SD) displacement between measured and calculated isodoses was, in the axial planes, 1.0 +/- 0.6 mm, 1.2 +/- 0.7 mm and 1.5 +/- 1.1 mm for 80%, 50% and 20% isodose curves, respectively, whereas in the sagittal planes it was 2.0 +/- 1.2 mm and 2.2 +/- 2 mm for 80% and 50% isodose curves, respectively. The results indicate that the accuracy of the 3D treatment planning system used with the DMLC is reasonably acceptable in clinical applications which require treatments with several non-coplanar arcs and small dynamic fields.     

Monte Carlo verification of IMRT dose distributions from a commercial treatment planning optimization system

The purpose of this work was to use Monte Carlo simulations to verify the accuracy of the dose distributions from a commercial treatment planning optimization system (Corvus, Nomos Corp., Sewickley, PA) for intensity-modulated radiotherapy (IMRT). A Monte Carlo treatment planning system has been implemented clinically to improve and verify the accuracy of radiotherapy dose calculations. Further modifications to the system were made to compute the dose in a patient for multiple fixed-gantry IMRT fields. The dose distributions in the experimental phantoms and in the patients were calculated and used to verify the optimized treatment plans generated by the Corvus system. The Monte Carlo calculated IMRT dose distributions agreed with the measurements to within 2% of the maximum dose for all the beam energies and field sizes for both the homogeneous and heterogeneous phantoms. The dose distributions predicted by the Corvus system, which employs a finite-size pencil beam (FSPB) algorithm, agreed with the Monte Carlo simulations and measurements to within 4% in a cylindrical water phantom with various hypothetical target shapes. Discrepancies of more than 5% (relative to the prescribed target dose) in the target region and over 20% in the critical structures were found in some IMRT patient calculations. The FSPB algorithm as implemented in the Corvus system is adequate for homogeneous phantoms (such as prostate) but may result in significant under or over-estimation of the dose in some cases involving heterogeneities such as the air-tissue, lung-tissue and tissue-bone interfaces.     

Dosimetric verification of IMRT treatment planning using Monte Carlo simulations for prostate cancer

The purpose of this work is to investigate the accuracy of dose calculation of a commercial treatment planning system (Corvus, Normos Corp., Sewickley, PA). In this study, 30 prostate intensity-modulated radiotherapy (IMRT) treatment plans from the commercial treatment planning system were recalculated using the Monte Carlo method. Dose-volume histograms and isodose distributions were compared. Other quantities such as minimum dose to the target (D(min)), the dose received by 98% of the target volume (D98), dose at the isocentre (D(iso)), mean target dose (D(mean)) and the maximum critical structure dose (D(max)) were also evaluated based on our clinical criteria. For coplanar plans, the dose differences between Monte Carlo and the commercial treatment planning system with and without heterogeneity correction were not significant. The differences in the isocentre dose between the commercial treatment planning system and Monte Carlo simulations were less than 3% for all coplanar cases. The differences on D98 were less than 2% on average. The differences in the mean dose to the target between the commercial system and Monte Carlo results were within 3%. The differences in the maximum bladder dose were within 3% for most cases. The maximum dose differences for the rectum were less than 4% for all the cases. For non-coplanar plans, the difference in the minimum target dose between the treatment planning system and Monte Carlo calculations was up to 9% if the heterogeneity correction was not applied in Corvus. This was caused by the excessive attenuation of the non-coplanar beams by the femurs. When the heterogeneity correction was applied in Corvus, the differences were reduced significantly. These results suggest that heterogeneity correction should be used in dose calculation for prostate cancer with non-coplanar beam arrangements.     

Dosimetric evaluation of a commercial 3D treatment planning system using the AAPM Task Group 23 test package

The accuracy of the dose calculation algorithm is one of the most critical steps in assessing the radiotherapy treatment to achieve the 5% accuracy in dose delivery, which represents the suggested limit to increase the complication-free local control of tumor. We have used the AAPM Task Group 23 (TG-23) test package for clinical photon external beam therapy to evaluate the accuracy of the new version of the PLATO TPS algorithm. The comparison between tabulated values and calculated ones has been performed for 266 and 297 dose values for the 4 and 18 MV photon beams, respectively. Dose deviations less than 2% were found in the 98.5%- and 90.6% analyzed dose points for the two considered energies, respectively. Larger deviations were obtained for both energies, in large dose gradients, such as the build-up region or near the field edges and blocks. As far as the radiological field width is concerned, 64 points were analyzed for both the energies: 53 points (83%) and 64 points (100%) were within +/-2 millimeters for the 4 and 18 MV photon beams, respectively. The results show the good accuracy of the algorithm either in simple geometry beam conditions or in complex ones, in homogeneous medium, and in the presence of inhomogeneities, for low and high energy beams. Our results fit well the data reported by several authors related to the calculation accuracy of different treatment planning systems (TPSs) (within a mean value of 0.7% and 1.2% for 4 and 18 MV respectively). The TG-23 test package can be considered a powerful instrument to evaluate dose calculation accuracy, and as such may play an important role in a quality assurance program related to the commissioning of a new TPS.     

Dosimetric IMRT verification with a flat-panel EPID

A convolution-based calibration procedure has been developed to use an amorphous silicon flat-panel electronic portal imaging device (EPID) for accurate dosimetric verification of intensity-modulated radiotherapy (IMRT) treatments. Raw EPID images were deconvolved to accurate, high-resolution 2-D distributions of primary fluence using a scatter kernel composed of two elements: a Monte Carlo generated kernel describing dose deposition in the EPID phosphor, and an empirically derived kernel describing optical photon spreading. Relative fluence profiles measured with the EPID are in very good agreement with those measured with a diamond detector, and exhibit excellent spatial resolution required for IMRT verification. For dosimetric verification, the EPID-measured primary fluences are convolved with a Monte Carlo kernel describing dose deposition in a solid water phantom, and cross-calibrated with ion chamber measurements. Dose distributions measured using the EPID agree to within 2.1% with those measured with film for open fields of 2 x 2 cm2 and 10 x 10 cm2. Predictions of the EPID phantom scattering factors (SPE) based on our scatter kernels are within 1% of the SPE measured for open field sizes of up to 16 x 16 cm2. Pretreatment verifications of step-and-shoot IMRT treatments using the EPID are in good agreement with those performed with film, with a mean percent difference of 0.2 +/- 1.0% for three IMRT treatments (24 fields).     

Dosimetric accuracy of the ITP inverse treatment planning system

The dosimetric accuracy of the ITP system for intensity modulated radiation therapy was determined for 19 patient plans. Treatments were given with a Varian 2100C and a 120-leaf multileaf collimator using step and shoot delivery. Both absolute dose, determined by an ionization chamber, and relative dose, determined by film, were assessed. It was found that absolute dose agreement was within 0.1% +/- 1.5%, isodoses in the high dose-low gradient region were within 1.7% +/- 2%, and the distance to agreement for isodoses between 20% and 90% was 1.4 mm +/- 1 mm. This agreement is at least as good as that found for standard wedged fields in most treatment planning systems.     

Efficient schemes for robust IMRT treatment planning

We use robust optimization techniques to formulate an IMRT treatment planning problem in which the dose matrices are uncertain, due to both dose calculation errors and interfraction positional uncertainty of tumour and organs. When the uncertainty is taken into account, the original linear programming formulation becomes a second-order cone program. We describe a novel and efficient approach for solving this problem, and present results to compare the performance of our scheme with more conventional formulations that assume perfect knowledge of the dose matrix.     

Validation of intensity modulation on a commercial treatment planning system

For two years now, a study on intensity modulated radiotherapy (IMRT) has been in progress at the Antoine Lacassagne Hospital Center for Cancer Therapy (in Nice) in collaboration with the University of Nice-Sophia Antipolis. The kind of intensity modulation that was used is the "step and shoot" technique in which the modulated beam is created both by adding andjoining elementary fields. Before carrying out clinical tests, several problems regarding the production of modulated beams has to be mastered. The current developments of our study enable us to dosimetrically produce (in water phantom and in the PMMA phantom) complexmodulated whose segmentation was calculated by one commercial treatment planning system (TPS). Nevertheless, we showed and studied some critical discrepancies between standard clinical calculations and the calculations using field segmentation. We showed that with nonoptimal conditions of segmentation the discrepancies, which are due to the type of algorithm used, could bring about significant errors inside the field of up to 10% of maximum dose. Another point of our study is the quantification and resolution of differences between measurements and calculations due to the internal segmentation of calculated modulated fields and their realization on Linac. Once again, in none optimal conditions of segmentation and inside the field we obtained discrepancies up to 20% of maximum dose between calculations using field segmentation and measurements. That was mainly due to the tongue and groove effect and penumbra phenomena. This study allows us to show that the discrepancies between segmentation calculations and standard clinical calculations should be solved by the use of penumbra models during segmentation calculations. We will introduce both the study and its near-future perspectives.     

Evaluation of a commercial three-dimensional electron beam treatment planning system

We evaluated a commercial three-dimensional (3D) electron beam treatment planning system (CADPLAN V.2.7.9) using both experimentally measured and Monte Carlo calculated dose distributions to compare with those predicted by CADPLAN calculations. Tests were carried out at various field sizes and electron beam energies from 6 to 20 MeV. For a homogeneous water phantom the agreement between measured and CADPLAN calculated dose distributions is very good except at the phantom surface. CADPLAN is able to predict hot and cold spots caused by a simple 3D inhomogeneity but unable to predict dose distributions for a more complex geometry where CADPLAN underestimates dose changes caused by inhomogeneity. We discussed possible causes for the inaccuracy in the CADPLAN dose calculations. In addition, we have tested CADPLAN treatment monitor unit and electron cut-out factor calculations and found that CADPLAN predictions generally agree with manual calculations.     

Clinical use of a commercial Monte Carlo treatment planning system for electron beams

In 2002 we fully implemented clinically a commercial Monte Carlo based treatment planning system for electron beams. The software, developed by MDS Nordion (presently Nucletron), is based on Kawrakow's VMC++ algorithm. The Monte Carlo module is integrated with our Theraplan Plustrade mark treatment planning system. An extensive commissioning process preceded clinical implementation of this software. Using a single virtual 'machine' for each electron beam energy, we can now calculate very accurately the dose distributions and the number of MU for any arbitrary field shape and SSD. This new treatment planning capability has significantly impacted our clinical practice. Since we are more confident of the actual dose delivered to a patient, we now calculate accurate three-dimensional (3D) dose distributions for a greater variety of techniques and anatomical sites than we have in the past. We use the Monte Carlo module to calculate dose for head and neck, breast, chest wall and abdominal treatments with electron beams applied either solo or in conjunction with photons. In some cases patient treatment decisions have been changed, as compared to how such patients would have been treated in the past. In this paper, we present the planning procedure and some clinical examples.     

Monte Carlo-based treatment planning for a spoiler system with experimental validation using plane-parallel ionization chambers

A beam spoiler is often used to increase the build-up dose near the surface for treatment of superficial treatment areas. Photon-beam spoilers produce a large amount of contaminant electrons, conditions for which standard, commercial treatment-planning system dose-calculation algorithms are inadequate for producing accurate dose calculations. In this study, we implemented a Monte Carlo (MC) dose-calculation algorithm for this spoiler system. With and without a spoiler of 1 cm Lucite, depth doses and transverse profiles in the build-up region were measured for field sizes of 5 x 5 cm2 and 10 x 10 cm2 at the spoiler-to-surface distances (STSDs) of 6, 10 and 15 cm. An Attix chamber and a Markus chamber were used for depth doses, whereas a diode detector was used for transverse profiles. An MC simulation using BEAM/DOSXYZ was used to compare the calculated and the measured data. The MC calculations agreed with the Attix chamber measurements within 2% for all STSDs and field sizes, whereas the Markus data--even with corrections made-showed a discrepancy of about 3.5% with a maximum difference of 7.3% for a field size of 10 x 10 cm2 at an STSD of 6 cm. The MC treatment-planning system was successfully applied to a head-and-neck case using 6 MV photon beams with a beam spoiler.     

Accuracy of out-of-field dose calculations by a commercial treatment planning system

The dosimetric accuracy of treatment planning systems (TPSs) decreases for locations outside the treatment field borders. However, the true accuracy of specific TPSs for locations beyond the treatment field borders is not well documented. Our objective was to quantify the accuracy of out-of-field dose predicted by the commercially available Eclipse version 8.6 TPS (Varian Medical Systems, Palo Alto, CA) for a clinical treatment delivered on a Varian Clinac 2100. We calculated (in the TPS) and determined (with thermoluminescent dosimeters) doses at a total of 238 points of measurement (with distance from the field edge ranging from 3.75 to 11.25 cm). Our comparisons determined that the Eclipse TPS underestimated out-of-field doses by an average of 40% over the range of distances examined. As the distance from the treatment field increased, the TPS underestimated the dose with increasing magnitude--up to 55% at 11.25 cm from the treatment field border. These data confirm that accuracy beyond the treatment border is inadequate, and out-of-field data from TPSs should be used only with a clear understanding of this limitation. Studies that require accurate out-of-field dose should use other dose reconstruction methods, such as direct measurements or Monte Carlo calculations.     

早期鼻咽癌Rapid Arc和IMRT计划的剂量学对比

目的:分析和比较早期NPC患者IMRT和Rapid Arc两种治疗技术的剂量学差别。方法:选取10例早期NPC病例分别对10例病例做IMRT和Rapid Arc计划,肿瘤计划靶区分为PTVnx、PTV60。PTVnx处方剂量为70 Gy,PTV60为60 Gy。比较两种方法的DVH图,等剂量分布和治疗时间。结果:两种不同计划中的靶区分布,PTVnx分布差异不大,但是RapidArc计划中的PTV60高剂量区明显多于IMRT计划。对其它正常组织来说,脊髓、脑干、腮腺、颞叶、视神经有着显著性差异。结论:对于早期NPC病人,Rapid Arc技术在剂量学上比IMRT技术有其更多的优势,不仅可以缩短治疗时间,而且其它一些正常组织也可减少受照剂量。     

Incorporating organ movements in IMRT treatment planning for prostate cancer: minimizing uncertainties in the inverse planning process

We investigate an off-line strategy to incorporate inter fraction organ movements in IMRT treatment planning. Nowadays, imaging modalities located in the treatment room allow for several CT scans of a patient during the course of treatment. These multiple CT scans can be used to estimate a probability distribution of possible patient geometries. This probability distribution can subsequently be used to calculate the expectation value of the delivered dose distribution. In order to incorporate organ movements into the treatment planning process, it was suggested that inverse planning could be based on that probability distribution of patient geometries instead of a single snapshot. However, it was shown that a straightforward optimization of the expectation value of the dose may be insufficient since the expected dose distribution is related to several uncertainties: first, this probability distribution has to be estimated from only a few images. And second, the distribution is only sparsely sampled over the treatment course due to a finite number of fractions. In order to obtain a robust treatment plan these uncertainties should be considered and minimized in the inverse planning process. In the current paper, we calculate a 3D variance distribution in addition to the expectation value of the dose distribution which are simultaneously optimized. The variance is used as a surrogate to quantify the associated risks of a treatment plan. The feasibility of this approach is demonstrated for clinical data of prostate patients. Different scenarios of dose expectation values and corresponding variances are discussed.     

Dosimetric evaluation of MRI-based treatment planning for prostate cancer

The purpose of this study is to evaluate the dosimetric accuracy of MRI-based treatment planning for prostate cancer using a commercial radiotherapy treatment planning system. Three-dimensional conformal plans for 15 prostate patients were generated using the AcQPlan system. For each patient, dose distributions were calculated using patient CT data with and without heterogeneity correction, and using patient MRI data without heterogeneity correction. MR images were post-processed using the gradient distortion correction (GDC) software. The distortion corrected MR images were fused to the corresponding CT for each patient for target and structure delineation. The femoral heads were delineated based on CT. Other anatomic structures relevant to the treatment (i.e., prostate, seminal vesicles, lymph notes, rectum and bladder) were delineated based on MRI. The external contours were drawn separately on CT and MRI. The same internal contours were used in the dose calculation using CT- and MRI-based geometries by directly transferring them between MRI and CT as needed. Treatment plans were evaluated based on maximum dose, isodose distributions and dose-volume histograms. The results confirm previous investigations that there is no clinically significant dose difference between CT-based prostate plans with and without heterogeneity correction. The difference in the target dose between CT- and MRI-based plans using homogeneous geometry was within 2.5%. Our results suggest that MRI-based treatment planning is suitable for radiotherapy of prostate cancer.