MLC Leaf Positions Research Articles

SU‐FF‐J‐02: A Cone Beam CT—guided Online Plan Modification Technique to Correct Interfractional Anatomic Changes for Prostate Cancer IMRT Treatment

Purpose: To develop an online plan modification technique to compensate for the interfractional anatomic changes for prostate cancer IMRT treatment based on the daily cone beam CT (CBCT). Methods and Materials: Pre‐treatment CBCT were acquired after patients were set up using skin marks. Instead of moving the couch to rigidly align the target, or performing re‐planning, we modified the leaf sequence in the original IMRT plan to account for the interfractional target motion and deformation based on the daily CBCT. The MLC leaf positions for each subfield were automatically adjusted based on the position and shape change of target projection in the beam's‐eye‐view (BEV). Three prostate cases were adapted to evaluate the proposed method and the results were compared with those obtained with bony‐structure‐based correction (Strategy1), prostate‐based correction (Strategy2) and CBCT‐based re‐planning (Strategy3) strategies. Results: For case 1, there was a large prostate deformation. With our MLC modification method, the prostate coverage was improved to 94.0% from 85.3% with Strategy1 and 88.7% with Strategy2, the seminal vesicle (SV) coverage was improved to 93.9% from 60.7% with Strategy1 and 75.4% with Strategy2. For the other two cases, the prostate interfractional target deformation was insignificant, and the prostate coverage using our method was comparable to Strategy2, Strategy3 but was better than Strategy1. The SV coverage was significantly improved with our method compared with Strategy1 and Strategy2 and was similar to that with Strategy3. The DVHs for OARs with our method were similar to those with Strategy3. Conclusions: The proposed method is superior to the bony‐structure‐based and prostate‐based correction techniques, especially when large interfractional target deformation exists. Its dosimetric performance is similar to that of the re‐planning strategy, but with much higher efficiency. This method may be used clinically to compensate for the inter‐fractional target position and shape changes in prostate IMRT.

TU-FF-A3-04: Empirical Investigation of 3D Intrafraction Motion Management Using a Generalized Methodology for Tracking Translating, Rotating and Deforming Targets

Purpose: Real‐time tracking of tumor motion is a highly promising approach for intrafraction motion management in thoracic and abdominal cancerradiotherapy. We investigate the geometric and dosimetric impact of a generalized methodology for conformal and IMRT‐based radiation delivery to translating, rotating and deforming targets. Materials and Methods: The methodology is based on the concept of “relocation vectors”, which correlate instantaneous target position to each point on the treatment aperture(s), throughout the entire respiratory cycle. These vectors are determined during 4D treatment planning by combining instantaneous target position information, obtained from a real‐time position‐monitoring (RPM) system, with concurrently acquired 4DCT. The resulting plan comprises multiple “control points”. For each point, a set of MLC leaf positions is defined so as to conform to the instantaneous shape of the target and deliver the desired fluence. The methodology is, therefore, independent of the nature of target motion. During dose delivery, real‐time RPM data are used to update the relocation vectors so as to dynamically account for changes target shape/position relative to the shape/position observed in the planning stage. Initial studies were performed to demonstrate tracking of linear and elliptical motion. A laboratory system was designed and optical measurements of tracking accuracy and system latency were obtained. The methodology was subsequently tested on a moving lung phantom placed under a clinical linac, and dosimetric measurements were performed using a 2D ion‐chamber array. Results: Tracking accuracy (without using any predictive algorithm) was observed to be ∼1.25 mm for motion parallel to MLC leaf travel. For motion perpendicular to leaf travel, accuracy was significantly lower. Dosimetric measurements indicate that tracking achieves efficient dose delivery to the target and, simultaneously, significant dose reduction in surrounding regions. Conclusions: We have developed a robust, universal tracking methodology to manage 3D intrafraction motion. This work was partially supported by Varian.

SU‐FF‐T‐449: Verification of Multi‐Leaf Collimator Position Accuracy Using a Commercial EPID and Vendor Supplied Portal Dosimetry Software

Purpose: To develop and implement an efficient QA routine capable of verifying multi‐leaf collimator position accuracy using a commercial EPID and vendor supplied portal dosimetry software. With proper implementation, this routine will provide a rapid technique for detecting minor leaf position misalignments which could result in significant deviation from planned dose distributions for Dynamic delivered IMRT. Method and Materials: A common dynamic MLC leaf position verification test, the Picket Fence, was utilized. This test consists of a series of five continuous 1.0 mm strips, with a separation of 2.0 cm between strips, delivered in dynamic mode. Testing was performed on a Varian EPID aS500 (0.78mm pixel resolution) at 100 cm SSD. A baseline integrated image of the picket fence MLC file was captured. A second picket fence MLC file, with intentional single leaf discrepancies of ±0.2, ±0.3, and ±0.5 mm, was also imaged. Integrated EPID images were first visually inspected for deviations in absolute leaf position and then analyzed with the Varian portal dosimetry software. Within the software, a profile was taken to determine peak intensity between strips, which should have a 2.0 cm separation. Additionally, the profile tool was used to determine peak intensity changes along the length of the 1.0mm strips. These peak intensity changes along the strips can be correlated to leaf positioning errors. Results: All single leaf deviations greater than ±0.3mm were visually recognized and profile analysis confirms that leaf errors of this magnitude produce approximately a 10% change in peak intensity along the 1.0mm strips. Profile analysis did not detect the ±0.2mm error. Conclusion: As noted in relevant literature, leaf position accuracy is of paramount importance for dynamic MLC IMRT delivery. Once a baseline image is validated, these tests can be conducted daily and evaluated in a matter of minutes.

Monte Carlo based IMRT dose verification using MLC log files and R/V outputs

Conventional IMRT dose verification using film and ion chamber measurements is useful but limited with respect to the actual dose distribution received by the patient. The Monte Carlo simulation has been introduced as an independent dose verification tool for IMRT using the patient CT data and MLC leaf sequence files, which validates the dose calculation accuracy but not the plan delivery accuracy. In this work, we propose a Monte Carlo based IMRT dose verification method that reconstructs the patient dose distribution using the patient CT, actual beam data based on the information from the record and verify system (R/V), and the MLC log files obtained during dose delivery that record the MLC leaf positions and MUs delivered. Comparing the Monte Carlo dose calculation with the original IMRT plan using these data simultaneously validates the accuracy of both the IMRT dose calculation and beam delivery. Such log file based Monte Carlo simulations are expected to be employed as a useful and efficient IMRT QA modality to validate the dose delivered to the patient. We have run Monte Carlo simulations for eight IMRT prostate plans using this method and the results for the target dose were consistent with the original CORVUS treatment plans to within 3.0% and 2.0% with and without heterogeneity corrections in the dose calculation. However, significant dose deviations in nearby critical structures have been observed. The results showed that up to 9.0% of the bladder dose and up to 38.0% of the rectum dose, to which leaf position errors were found to contribute <2%, were underestimated by the CORVUS treatment planning system. The concept of average leaf position error has been defined to analyze MLC leaf position errors for an IMRT plan. A linear correlation between the target dose error and the average position error has been found based on log file based Monte Carlo simulations, showing that an average position error of 0.2 mm can result in a target dose error of about 1.0%.

MO‐D‐224A‐09: Validation of Synchronized Dynamic Dose Reconstruction Using Film Dosimetry

Purpose: To validate a method of retrospective dose reconstruction that uses real‐time intra‐treatment patient motion data that is synchronized with MLC leaf positions during IMRT treatments. Method and Materials: IMRT fields from an IRB‐approved prostate protocol were delivered to a water‐equivalent phantom on a programmable translation stage. Kodak XV film was placed at 5 cm depth with an SSD of 95cm and marked to register it with the Monte Carlo (MC) calculation grid. Motion was synchronized with beam delivery by using the target signal to trigger motion. Film measurements were repeated for each beam while the phantom was stationary, and while moving with both idealized and clinically measured motion profiles. MLC leaf positions and fluence state for each beam were obtained from the Varian DynaLog files. MC dose accumulation was performed which incorporated the real‐time phantom motion, DynaLog files and beam state. Films were digitized and compared to the results of the MC calculations. Results: Film measurements in stationary phantoms were measured three times on two machines. The measured dose distributions were compared and showed an average difference of 0.38 +/− 1.53 cGy. The average difference between MC and film measurements of the moving phantom was 0.44 +/− 3.4 cGy and was independent of the motion profile. Measured dose patterns, for both stationary and moving phantoms, were generally well reproduced by MC dose accumulation, including tongue‐and‐groove and motion related features. Doses in moving and static phantoms were compared, for both films and simulations. The measured dose deviations due to motion were well‐characterized by the MC dose accumulation method and not significantly different when a static phantom was compared to MC calculation. Conclusion: Real‐time motion and machine data may be used to reconstruct the dose delivered to the target volume, and may serve as a basis for dynamic refinement of treatments.

SU‐FF‐T‐126: Clinical Experience Using Dynalog Files for Verification of IMRT Delivery

Purpose: Validation of IMRT field delivery and the dynamic leaf movement during treatment is an important part of the IMRT QA procedure. In the past this was done using film during treatment. In this presentation, we summarize our experience using the dynalog files to validate the delivered fluence pattern of individual IMRT fields for each patient. Method and Materials: The DynaLog files were generated during IMRT delivery on Varian Trilogy and 21EX linear accelerators. They represent the actual leaf movement, as MLC leaf positions were recorded, during delivery, every 50 ms. Argus IMRT QA software was used to interrogate the dynalog files after delivery for evaluation. The delivered and planned fluence pattern can be displayed for comparison as well as fluence difference and Gamma function values. Maximum and average fluence difference and Gamma function values can be employed for quantitative analysis, as well as actual versus planned leaf positions and speeds. The pass/fail status of each IMRT field can be set based on pre‐determined parameters. Results: We have employed the Argus IMRT QA software in analyzing the fluence patterns and leaf motions for all IMRT fields used for patient treatment. Several hundred fluence patterns were analyzed on 100 patients treated so far. The average and maximum gamma function value of each field was analyzed to determine appropriate limits for the pass/fail criteria in accordance with our clinical data. Conclusion: Verification of delivered fluence patterns is an important part of the IMRT QA process. It assures the accuracy of IMRT delivery on a daily basis. With the aid of Argus IMRT QA software, validation of all the fields used for IMRT treatment can be performed within minutes for qualitative and quantitative analysis. With the proper choice of limits for test parameters, this approach further ensures the quality of IMRT delivery.

SU‐EE‐A2‐04: A Method of Online MLC Aperture Adjustment for Treatment of Patients with Set Up Variations

Purpose: To investigate MLC aperture adjustments to compensate patient setup variations, replacing couch shift methods for precision treatment delivery. Method and Materials: Patient setup variations can be described by 3‐D translational shifts. A scheme of adjusting MLC apertures to compensate for translational displacements of the patient has been developed. Patient shift information, such as provided by commercial image matching software, establishes the aperture shift vector. The projection of this vector into and orthogonal to the BEV plane was used to determine the displacement vector and divergence for the aperture. Modified beam apertures were generated and MLC leaf positions were determined through a polynomial interpolation.Dosimetric plan comparisons were made within Pinnacle 7.6c. Static field and segmented IMRT patient plans were investigated for pelvic as well as head and neck sites. Arbitrary shift vectors ranging from 3 mm up to 30 mm have been investigated. Non‐integral MLC leaf width shifts parallel to the leaf bank face to characterize leaf width effects. Results: Conformal plans show dose variations from the original plan of up to 3% in the pelvis for shifts of up to 30 mm. Dose coverage of the PTV was maintained except in the superior‐inferior direction, where coverage in the last 3 mm of target fell as low as 92% of the prescribed dose. Results for prostate and oropharynx IMRT plans showed little increase in maximum critical structure doses, and small increases in mean doses. DVH's for IMRT plans confirmed minimal impact on critical structure doses. Conclusion: An alternative method of on‐line adaptive treatment delivery has been explored which eliminates the need to adjust the patient position. This can potentially increase treatment accuracy and efficiency through minimizing patient disturbances and reducing the time between imaging and treatment.

SU‐FF‐T‐420: The Impact of Random and Systematic Errors of MLC Leaves On Head‐And‐Neck IMRT Plans

Purpose: To investigate dosimetric effect of the random and systematic errors of MLC leaf positions on the IMRT plans for patients with head and neck cancer. Method and Materials: For six clinical IMRT plans, random errors (from −2 to 2 mm) and systematic errors (±0.5 mm and ±1 mm) MLC positioning errors were introduced into the each segment for these plans, simulating the mechanical uncertainties and potential miscalibration of the MLC. The altered MLC segments for each plan were imported to a commercial treatment planning system to evaluate dosimetric changes of the plan. The altered plans were compared to the original plans, based on DVHs and defined endpoint doses. Results: With the up‐to‐2 mm random errors in MLC positions, the dose changes to the 95% of the tumor volumes were (−1.247± 1.153)%. For serial structures, the dose changes to the 0.1 cc of the brainstem and spinal cord were (0.230±0.794)% and (−0.340±1.254)%, respectively. The dose changes to the 50% of the parotids varied from patient to patient, from 0.536% to 11.333%, (1.641±4.274)% and (4.910±5.322)% overall. With systematic errors in MLC positions up to 1 mm, the dose changes to the 95% of the tumor volumes were (−0.078±1.048)%. The dose changes to the 0.1 cc of the brainstem and spinal cord for all patients except one were (0.591±1.058)%. The dose changes to the 50% of the parotids overall were (7.782±3.111)%, beyond 5% limit. Conclusion: The dosimetric changes introduced by the random errors for each leaf within 2mm were not significant compared to the original plans. The systematic error up to 1mm for each leaf did not significantly changed the target dose and the maximum doses to the serial structures while the doses changes to the parotid could be significant. Conflict of Interest: This project is partly funded by SIEMENS.

MO‐D‐224A‐03: Dependence of Planar IMRT QA On MLC Positional Inaccuracies

Purpose: This work aims to investigate the sensitivity of IMRT QA done by means of planar dosimetry to MLC positional inaccuracies. Also we propose an accurate method for measurement of MLC positioning errors using the 2D diode array. Methods: A method to measure the MLC position errors by using 2D‐arrary of 455 diodes (MapCheck, Sun Nuclear) is developed. Our method utilizes the fact that each diode's signal will be most sensitive to the MLC position error when its center coincides with the edge of a MLC leaf where a small deviation from the accurate position produces sharp increase or decrease in the diode output. We designed various multi‐segmented test patterns based on this principle that can evaluate the MLC deviations with sub‐mm accuracy at multiple MLC banks simultaneously. By using this information as a deviation histogram, we created deliberately erroneous IMRT fields with varying standard deviations of MLC leaf position error. MapCHECK planar dose analysis is performed and the sensitivity of the IMRT QA procedure to MLC position inaccuracy using MapCHECK device is evaluated. Results: Our results indicate that for SIEMENS Primus and MD machines, the MLC positional errors show a standard deviation of about +/− 0.7mm. Right after the MLC calibration, this deviation might reduce to 0.55mm. It has been found that a single leaf position can vary by as much as 1mm between two consecutive measurements. Fields with less number of segments that are generated by Direct Aperture Optimization are found to be less sensitive to these errors by measurements with deliberately modified fields with random MLC inaccuracies. Conclusion: A method to accurately quantify MLC position is proposed and used to obtain a distribution of leaf position errors for many leaves at multiple banks. The sensitivity of planar dosimeter to the MLC positioning errors is investigated.

TH‐D‐224C‐06: An Optimal MLC Quality Assurance Procedure Using EPID

Purpose: IMRT QA is extremely time consuming and labor intensive, while IMRT delivery is complex. The purpose of this work is to develop an automated and quantitative IMRT QA process utilizing an EPID. One of the aspects of this is testing of MLC leaf position accuracy. The Memorial‐Sloan‐Kettering strip test was modified to provide an automated EPID‐based process.Method and Materials: To provide a baseline fluence map, Kodak XV film was irradiated to obtain an open field 2D beam profile, which was fitted with a Gaussian and used in the image correction routine. We used a strip test consisting of seven adjacent segments with an intentional gap of nominally 0.5 mm. The gap was intentionally varied for the positions located at regularly placed hexagonal regions in order to create a set of calibration curves. A fitting technique using a Lorentzian type function was developed to determine the dose‐peak‐height, dose‐peak‐position and FWHM. Results: The standard deviation of dose‐peak‐height at each abutment for traditional Memorial‐Sloan‐Kettering pattern was of the same order of magnitude as that of the average over the entire image. In contrast, in our pattern a difference of an order of magnitude was observed, which indicated the reducing of cross‐talk effect from adjacent leaves. The leaf positioning error can be identified by using standard calibration curves of dose‐peak‐height vs. gap. The coordinate of dose‐peak‐position is found to be an extremely sensitive indicator of leaf error with sub‐pixel standard deviation. Conclusions: This procedure is able to identify MLC positional errors less than 0.5 mm by using a fitting technique and by reducing the cross‐talk effect. Conflict of Interest: Research sponsored by Varian Corporation.

WE-D-224A-05: Developing a Comprehensive Patient-Specific QA Procedure for IMRT

Purpose: To develop a comprehensive patient‐specific IMRTquality assurance (QA) procedure that verifies both dose calculation and dose delivery. Method and Materials:IMRT plan delivery is well described by actual gantry angles, collimator angles, jaw settings, MUs, and couch angles, which are output from Record and Verify system (R/V), as well as actual MLC leaf positions and fractional MUs recorded in MLC log files. MLC log files are used to rebuild leaf sequence files for actual leaf motion. The rebuilt leaf sequence files and the R/V output are used in the Monte Carlo simulation with patient CT to verify both dose calculation and dose delivery before treatment. Ion chamber or film measurement can be used to further investigate other causes when large errors are encountered. Post‐treatment QA can be performed with EPID that records the intensity maps behind the patient which include the dosimetric and patient setup information during treatment. The intensity maps above and behind the patient are re‐constructed using phase space data and compared with the measured EPIDimages to verify the dose delivered in the patient. Results: This method was tested with ion chamber measurement for four real prostate plans and the agreement was within 2%. A linear correlation between average leaf position error and target dose error was found. Average leaf position error of 0.2mm resulted in about 1% error in target dose. Eight IMRT prostate plans were used in this study and the errors caused by dose calculation and delivery did not cause significant errors in the dose delivered (<2.0%). Conclusions: We are developing a comprehensive patient‐specific IMRT QA procedure utilizing the Monte Carlo simulation,MLC log files, R/V output and EPID. This method can be used to verify both dose calculation and dose delivery for pre‐ and post‐treatment IMRT QA.

SU-FF-T-94: Clinical Evaluation of Direct Machine Parameter Optimization Algorithm for Head and Neck IMRT Treatment

Purpose: Because many critical structures are in close proximity to target volumes, cancers of the head and neck (H&N) are often suited for treatment with IMRT. However, the time required to generate and deliver a clinically acceptable IMRT plan can be significantly longer than a conventional plan. This study evaluated a new inverse planning algorithm, DMPO (direct machine parameter optimization), with attention to parameter settings, plan quality and treatment efficiency for H&N cancers. Method and Materials: The Pinnacle treatment planning system version 7.4 was used. The DMPO allows users to limit the number of total MLC segments (N) for treatment. After a user-defined number of iterations (n) for pencil beam optimization, the DMPO generates MLC segments for each field for dose calculations using a convolution algorithm. Both the MLC leaf positions and the weight of each segment are then optimized until cost tolerance or iteration number is reached. Treatment plans generated using DMPO were compared with H&N cases that were previously treated using an older version (6.2). The plan quality was compared using cost functions and DVHs of target volumes and critical structures. The total monitor units and MLC segments for treatment were compared for different combinations of n and N. Results: The DMPO provided plans of DVHs similar to clinical cases with significantly less planning time. More importantly, the total MU and MLC segments for treatment delivery were reduced by 40% to 50%. Cost functions changed only slightly on n and N and total MU increased as n increased, but was independent of N. Our preliminary data indicated a combination of n=10–15 with 10 segments per field appeared to be optimal for most H&N cases. Conclusion: The DMPO algorithm generated more efficient plans while providing equal or better quality than the previous plans for IMRT treatment.

SU‐FF‐T‐155: Monte Carlo Dosimetric Verification for IMRT QA Using MLC Log Files and EPID

Purpose: EPID is a useful tool for pre‐treatment patient setup and target localization and post‐treatment dosimetry verification. However, EPID images are significantly affected by photon attenuation and scattering in the patient and the detector and therefore cannot be used directly for dose reconstruction. This work investigates a Monte Carlo dosimetric verification method based on MLC log files and the electronic portal imaging device (EPID) for IMRT quality assurance (QA). Method and Materials: We have developed Monte Carlo based software to derive accurate intensity‐modulated fluence maps behind the patient using MLC log files, which take into account the accelerator head geometry, the MLC leaf movement accuracy and the patient attenuation and scatter. Patient initial simulation and pre‐treatment CT data were used to simulate the patient anatomy for interfraction dose comparison. The phase space data behind the patient were used in the EPID response simulation and compared with measured EPID images to investigate patient setup accuracy and dose reconstruction uncertainty. Results: Ten previously treated prostate IMRT plans and patient CT data are included in this study. The MLC leaf position accuracy of the dynalog files from a Varian 21Ex accelerator is verified to within 1mm. The dose distributions based on the leaf sequences from the treatment planning system and the fluence maps rebuilt from the dynalog files are consistent to within 2%, validating our software implementation. The primary photons and scatters are recorded separately behind the patient for different applications as described above. Energy spectrum, fluence distribution and angular distribution are derived to facilitate dose calculation in the EPID. Conclusion: We have developed a log file based Monte Carlo method to generate phase space and fluence maps for pre‐treatment patient setup and post‐treatment dose verification with EPID. This work is implemented as part of our IMRT QA procedure.

A novel image analysis approach for verifying MLC leaf positions in electronic portal images

A novel image analysis approach for verifying MLC leaf positions in electronic portal images

Automatic detection of single MLC leaf positions with corrections for penumbral effects and portal imager dose rate characteristics

A method has been developed to determine the individual settings of the MLC leaves from the image of the beam portal. The portal image is analysed by a two-step approach: (i) The general orientation of the collimator leaves is determined. (ii) Next, a one-dimensional edge operator (generalized Laplacian) is applied in a scanline fashion to extract an array holding the image coordinates of the field edge. The prescribed position of each leaf is then compared with the position of the steepest gradient behind that leaf as determined from the portal image. To obtain adequate precision in the determination of the leaf positions, the dose rate dependency of the electronic image detector, the increased transmission through the leaf tips and flanges, and the properties of the edge detector used for image processing were investigated. From these studies, a set of correction terms were obtained that are used to correctly calculate the position of the field edge from the prescribed MLC leaf settings. If not corrected, a leaf position-dependent discrepancy in the range -0.5 to 1.0 mm will arise. The results from tests of this method, applying the correction terms, show that the leaf positions can be determined with a precision better than 0.1 mm (one standard deviation). The reproducibility of the leaf positions was found to be better than 0.1 mm.

Dosimetric evaluation of the conformation of the multileaf collimator to irregularly shaped fields

The goal of this study was to evaluate the dosimetric characteristics of geometric MLC prescription strategies and compare them to those of conventional shielding block. Circular fields, square fields, and 12 irregular fields for patients with cancer of the head and neck, lung, and pelvis were included in this study. All fields were shaped using the MLC and conventional blocks. A geometric criterion was defined as the amount of area discrepancy between the MLC and the prescription outline. The "least area discrepancy" (LAD) of the MLC conformation was searched by selecting the collimator angle, meanwhile keeping a preselected position along the width of the leaf into the prescribed field. Five LAD conventions were studied. These included the LAD-0, LAD-1/3, LAD-1/2, and LAD-2/3 that inserted the leaves at the 0, 1/3, 1/2, and 2/3 of the leaf end into the prescription field, respectively. In addition, the LAD optimization was applied to the transecting (TRN) approach for leaf conformation that prescribed an equal area of overblocking and underblocking under each leaf. Film dosimetry was performed in a 20 cm polystyrene phantom at 10 cm depth 100 cm from source to axis distance (SAD) for both 6 and 18 MV photons with each of the above MLC conformations and conventional blocks. The field penumbra width, defined as the mean of the separation between the 20% and 80% isodose lines along the normal of the prescription field edge, was calculated using both the MLC and conventional block film dosimetry and compared. In a similar way, the d20 is defined as the mean separation between the 20% isodose line and the prescription field edge, and the d80 is defined as the mean separation between the 80% isodose line and the prescription field edge. The field penumbra width for all MLC conventions was approximately 2 mm larger than that of the conventional block. However, there was a larger variation of the separation distribution in the penumbra region of the irregular fields for the MLC, which had a standard deviation of 1 mm (a factor of 5 larger than the conventional block). The dosimetry for the circular fields showed that the LAD-TRN, LAD-1/2, and LAD-1/3 approximated the conventional blocking well in terms of d20 and d80; however, no single convention produced the best conformation for both measures. The dosimetric result of the patient treatment fields was similar for all sites. The LAD-1/3, LAD-1/2, and LAD-TRN strategies conformed to within 1 to 1.5 mm of the d80 of the conventional block for both 6 MV and 18 MV, respectively. The LAD-1/2 and LAD-TRN conformations were virtually identical, although it is proven analytically that the LAD-1/2 convention has the least overall area discrepancy of all conventions. The five MLC conformation conventions resulted in similar dosimetric penumbrae for all field shapes studied. The LAD-1/3, LAD-TRN, and LAD-1/2 produced the more favorable approximation to conventional block. The field penumbra width, although useful for evaluating irregular field shapes, could not describe the large local variations in the penumbra along the field edge for the MLC. These local variations could be of clinical concern when they appear near vital organs. However, the variation in a local region can potentially be reduced by minimizing the jaggedness of the leaf steps in that local region. The dosimetric results were useful as guidelines for the clinicians in the evaluation and adjustment of MLC leaf positions.