Effects of treatment setup variation on beam's eye view dosimetry for radiation therapy using the multileaf collimator vs. the cerrobend block

A validation framework to assess performance of commercial deformable image registration in lung radiotherapy

INTRODUCTION

Deformable image registration (DIR) can play an important role in the context of adaptive radiotherapy. The AAPM Task Group 132 (TG-132) has described several quantitative measures for DIR error assessment but they can only be accurately defined when there is a ground-truth present in high-contrast regions. This work aims to set out a framework to obtain optimal results for CT-CT lung DIR in clinical setting for a commercially available system by quantifying the DIR performance in both low- and high-contrast regions.

METHODS

Five publicly available thorax datasets were used to assess the DIR quality. A "Ghost fiducial" method was implemented by windowing the contrast in a new feature provided by Varian Velocity v4.1. Target registration error (TRE) of the landmarks and Dice-similarity coefficient of the tumour were calculated at three different contrast settings to assess the algorithm in high- and low-contrast scenarios.

RESULTS

For the original unedited dataset, higher resolution DIR methods showed best performance acceptable within the recommended limit according to TG-132, when actual displacements were less than 10 mm. The relation of the actual displacement of the landmarks and TRE shows the limited capacity of the algorithm to deal with movements larger than 10 mm.

CONCLUSION

This work found the performance of DIR methods and settings available in Varian Velocity v4.1 to be a function of contrast level as well as extent of motion. This highlights the need for multiple metrics to assess different aspects of DIR performance for various applications related to low-contrast and/or high-contrast regions.

Using patient-specific phantoms to evaluate deformable image registration algorithms for adaptive radiation therapy

The quality of adaptive treatment planning depends on the accuracy of its underlying deformable image registration (DIR). The purpose of this study is to evaluate the performance of two DIR algorithms, B‐spline‐based deformable multipass (DMP) and deformable demons (Demons), implemented in a commercial software package. Evaluations were conducted using both computational and physical deformable phantoms. Based on a finite element method (FEM), a total of 11 computational models were developed from a set of CT images acquired from four lung and one prostate cancer patients. FEM generated displacement vector fields (DVF) were used to construct the lung and prostate image phantoms. Based on a fast‐Fourier transform technique, image noise power spectrum was incorporated into the prostate image phantoms to create simulated CBCT images. The FEM‐DVF served as a gold standard for verification of the two registration algorithms performed on these phantoms. The registration algorithms were also evaluated at the homologous points quantified in the CT images of a physical lung phantom. The results indicated that the mean errors of the DMP algorithm were in the range of 1.0~3.1mm for the computational phantoms and 1.9 mm for the physical lung phantom. For the computational prostate phantoms, the corresponding mean error was 1.0–1.9 mm in the prostate, 1.9–2.4 mm in the rectum, and 1.8–2.1 mm over the entire patient body. Sinusoidal errors induced by B‐spline interpolations were observed in all the displacement profiles of the DMP registrations. Regions of large displacements were observed to have more registration errors. Patient‐specific FEM models have been developed to evaluate the DIR algorithms implemented in the commercial software package. It has been found that the accuracy of these algorithms is patient‐dependent and related to various factors including tissue deformation magnitudes and image intensity gradients across the regions of interest. This may suggest that DIR algorithms need to be verified for each registration instance when implementing adaptive radiation therapy.

PACS numbers: 87.10.Kn, 87.55.km, 87.55.Qr, 87.57.nj

The Dosimetric and Delivery Advantages of a New 160-leaf MLC

The purpose of this study was to conduct a measurement and treatment planning study on the dosimetric and delivery advantages of a new 160-leaf multileaf collimator (MLC). Recently, a new 160-leaf multileaf collimator (Siemens 160 MLC™) was introduced. The 160-MLC is a single focused design that consists of 160-leafs (80 pairs), each 95 mm thick with a projected leaf width of 5 mm at the machine isocenter. Compared to its double focused predecessors, the 82-leaf MLC (Siemens OPTIVIEW™ MLC) and 58-leaf MLC (Siemens 3-D MLC™), the 160-MLC has leaf widths of half the size. The most notable difference is the new slanted leaf design that replaced the tongue and groove system and allows for complete interdigitation. A systematic study that compared the dosimetric and delivery differences among the 160-MLC, 58-MLC, and divergent Cerrobend blocks was performed. Dosimetric conformity for each collimator type was determined by conforming each to circular targets of various diameters. The effective penumbra for each collimator type was calculated by conforming each, at various collimator angles, to a square stationary target. The quality of 3D conformal radiotherapy treatment (3D-CRT) plans and the quality intensity modulated radiation treatment (IMRT) plans were respectively compared with each collimator type. The 160-MLC was found to have improved dosimetric conformity over the 58-MLC. The divergent Cerrobend block showed marginal dosimetric conformity improvement over the 160-LMC. Overall, the 160-MLC had a 45% and 29% reduction in the 20/80 and 30/90 effective penumbra over the 58-MLC, respectively, while exhibiting only a slightly larger effective penumbra over the divergent Cerrobend block. Comparing 3D-CRT plans generated for small lesions of the head and neck, the V100 for the PTV of the plans generated with the Cerrobend blocks, the 58-MLC, and the 160-MLC were 97.78%, 92.51%, and 99.18%, respectively, while with regards to the OARs, the three produced similar DVHs. IMRT plans generated with the 160-MLC were found to significantly reduce the total delivered monitor units by up to 14.7% and the number of segments by as much as 10.7% compared to the 58-MLC. The average delivery time for the direct aperture optimized (DAO) IMRT plans generated with the 160-MLC was approximately 5 minutes. Overall, compared to the 58-MLC, the new 160-MLC was found to improve dosimetric conformity and IMRT delivery efficiency.

Dosimetric characteristics of dual-layer multileaf collimation for small-field and intensity-modulated radiation therapy applications

The purpose of the present work was to measure the performance characteristics in the penumbra region and on the leaf‐end of an innovative dual‐layer micro multileaf collimator (DmMLC). The DmMLC consists of two orthogonal (upper and lower) layers of leaves; a standard MLC consists of one layer. The DmMLC provides unique performance characteristics in smoothing dose undulation, reducing leaf‐end transmission, and reducing MLC field dependence of the leaf stepping angle. Two standard MLCs (80‐leaf and 120‐leaf versions: Varian Medical Systems, Palo Alto, CA), a DmMLC (AccuKnife: Initia Medical Technology, Canton, MA), and a Cerrobend (Cerro Metal Products, Bellefonte, PA) block were used in performance studies involving a triangular field, a cross leaf‐end field, and a circular field. Measurements were made with 6‐MV X‐rays and extended dose range film at a depth of 5 cm in Solid Water (Gammex rmi, Middleton, WI) at a source–axis distance of 100 cm. The field penumbra width measured between the 20% and 80% isodose lines through the MLC‐80, MLC‐120, DmMLC, and Cerrobend block were 9.0, 5.0, 3.0, and 2.0 mm respectively. The dose undulation amplitude of the 50% isodose line was measured as 5.5, 2.0, and 0.5 mm for the MLC‐80, MLC‐120, and DmMLC respectively. The planar dose difference between the MLC‐80, MLC‐120, and DmMLC against Cerrobend block was measured as ranging at ±52.5%,±35.0%, and ±20.0% respectively. The leaf‐end transmission was measured at 22.4% in maximum and 15.4% in average when closing a single layer of the DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of the leaf stepping angle with the DmMLC ranged from 45 degrees to 90 degrees. The standard MLC leaf stepping angle ranged from 0 degrees to 90 degrees. In conclusion, the dose undulation, leaf‐end transmission, and MLC field dependence of the leaf stepping angle with the DmMLC were remarkably reduced as compared with those of the standard MLCs. And as compared with Cerrobend block, the DmMLC provided very comparable performance in field‐edge smoothing and in the shaping of complex fields.

PACS numbers: 87.56.Jk, 87.56.Nk, 87.56.Nj, 87.57.Nt

Patient-specific daily pretreatment setup protocol using electronic portal imaging for radiation therapy

The purpose of this study was to evaluate electronic portal imaging (EPI) as a means of identifying and correcting field displacement in patients with problematic external beam radiotherapy setups. Fourteen patients with problematic setups were identified for pretreatment daily EPI beam monitoring as part of a physician‐directed therapist intervention protocol. Pretreatment EPIs were used to realign fields as necessary to bring the setup within the physician‐prescribed tolerance level. For comparison, daily EPIs were available for 12 control patients who had no particular setup difficulties and for whom online beam realignment was not made. Anatomy‐matching software was used to measure setup variation along medial‐lateral, superior‐inferior, and anterior‐posterior axes. Online field realignment yielded a significant (p=0.001) improvement when comparing initial and final setup variations. The mean standard deviation of setup displacement averaged over three axes was reduced from 6.4 mm to 3.1 mm after realignment. The final variation of protocol patients was comparable to that of control patients. In conclusion, EPI provided effective means to perform online beam realignment in a group of difficult‐to‐position patients. This procedure resulted in a reduction in setup displacement that was statistically significant, clinically relevant, and approached that of a more typical patient group.

PACS number: 87.53.Oq

A dose-escalation trial with the adaptive radiotherapy process as a delivery system in localized prostate cancer: analysis of chronic toxicity

PURPOSE

To evaluate the validity of the chosen adaptive radiotherapy (ART) dose-volume constraints while testing the hypothesis that toxicity would not be greater at higher tumor dose levels.

MATERIALS AND METHODS

In the ART dose escalation/selection trial, treatment was initiated with a generic planning target volume (PTV) formed as a 1-cm expansion of the clinical target volume (CTV). After the first week of therapy, the patient was replanned with a patient-specific PTV, constructed with CT and electronic portal images obtained in the first 4 days of treatment. A new multileaf collimator beam aperture was used. A minimum dose prescribed to the patient-specific PTV, ranging 70.2-79.2 Gy, was determined on the basis of the following rectal and bladder constraints: <5% of the rectal wall has a dose >82 Gy, <30% of the rectal wall has a dose >75.6 Gy, <50% of the bladder volume has a dose >75.6 Gy, and the maximum bladder dose is 85 Gy. A conformal four-field and/or intensity-modulated radiotherapy (IMRT) technique was used. Independent reviewers scored toxicities. The worst toxicity score seen was used as per the Common Toxicity Criteria grade scale (version 2). We divided the patients into three separate groups: 70.2-72 Gy, >72-75.6 Gy, and >75.6-79.2 Gy. Toxicities in each group were quantified and compared by the Pearson chi-squared test to validate our dose escalation/selection model. Grades 0, 1, 2, and 3 were censored as none vs. each category and none vs. any.

RESULTS

We analyzed patients with follow-up greater than 1 year. The mean duration of follow-up was 29 months (range, 12-46 months). We report on 280 patients, mean age 72 years (range, 51-87 years). Only 60 patients received adjuvant hormones. Mean pretreatment prostate-specific antigen level was 9.3 ng/mL (range, 0.6-120 ng/mL). Mean Gleason score was 6 (range, 3-9). The lowest dose level was given to 49 patients, the intermediate dose to 131 patients, and 100 patients received the highest dose escalation. One hundred eighty-one patients (65%) were treated to a prostate field only and 99 patients (35%) to prostate and seminal vesicles. Chronic genitourinary and/or gastrointestinal categories were incontinence, persistent urinary retention, increased urinary frequency/urgency, urethral stricture, hematuria, diarrhea, rectal pain, bleeding, ulcer, fistula, incontinence, and proctitis. Toxicity at the high dose level was not different from toxicity at the intermediate or lower dose levels. No significant difference was observed in any of the individual toxicity categories.

CONCLUSIONS

By applying the ART process--namely, developing a patient-specific PTV--to prostate cancer patients, significant dose escalation can be achieved without increases in genitourinary or gastrointestinal toxicity. Our data validate the rectal and bladder dose-volume constraints chosen for our three-dimensional conformal and IMRT prostrate radiotherapy planning.

Dosimetric evaluation and clinical application of virtual mini-multileaf collimator

One of the major concerns with multileaf collimators (MLC) is the jagged field edge that produces a larger penumbra compared with that produced by a Cerrobend block. The dosimetric undulation of the MLC can be minimized by replacing an existing MLC with a mini-MLC, an expensive replacement, or by software implementation, which essentially converts a regular MLC into a virtual mini-MLC. In this study, the dosimetry in the penumbra region of a virtual mini-MLC replacing the Cerrobend block is investigated for clinical applications. HD270, a software program implemented by Siemens (Concord, CA), combines the use of an MLC and a table translation perpendicular to the leaf plane to produce a smooth field edge, thus reducing isodose undulation. Three different step resolutions are available: 5 mm, 3 mm, and 2 mm. Using film dosimetry, the penumbra regions are studied at two different depths for clinical blocks and corresponding MLC setup, as well as HD270 with different resolutions for both 6-MV and 15-MV x-ray beams. The dose delivery time for HD270 on auto-sequencing mode is compared with the use of Cerrobend blocks. The clinical applications of HD270 in head-and-neck (head and neck) and prostate treatments are investigated. For single-field irradiation, the 80-20% penumbra widths for both the 45 degrees block and the circular block are reduced with HD270 compared with MLC for both 6 and 15 MV at different depths. At 2-mm resolution, the scalloping isodose lines (IDLs) with MLC completely disappear, although the penumbra is still larger than the Cerrobend block. On the other hand, the difference in dose undulations between 2-mm and 3-mm resolution is small. In the head and neck irradiation, the 80-20% widths with HD270 are 1 to 2 mm less than MLC, but they are still 2 mm wider than with a Cerrobend block. The 50% IDL is reduced by 2 mm with HD270 compared with MLC, which provides safety near spinal cord. Dose-volume histogram (DVH) calculations for the different shielding techniques indicate that the HD270 improves the spinal cord dose distribution significantly compared with MLC. A similar improvement in dose undulation is observed for the prostate case. In the dose region, >60% of the prescribed dose, there is approximately 10% less irradiated volume for the rectum when HD270 (3 mm resolution) is employed compared with MLC. The treatment time was compared with that from the Cerrobend block, and it was found that even at 3-mm resolution, there is a 20% reduction in treatment time in a head and neck treatment; with a 2-mm resolution, there is a 15% increase in time. The isodose undulation due to MLC can be significantly reduced with the HD270. Clinical application with HD270 for head and neck and prostate irradiation provides a smaller penumbra region compared with MLC, although it still gives a larger one compared with the Cerrobend block. In the clinical cases presented in this study, the 3-mm resolution is the most effective in improving the penumbra and delivery time. The HD270 implementation is a versatile and cost-effective solution for reducing MLC undulation.

A dosimetric comparison of various multileaf collimators

The dosimetric characteristics of three multileaf collimator (MLC) systems (Elekta, Siemens and Varian) having 10 mm leaf width are compared. A 6 MV photon beam was used from each unit for measurements. Film dosimetry was performed for the measurements and the analysis techniques were exactly duplicated in each system. Two of the collimators have rounded leaf ends (Elekta and Varian) and the third (Siemens) has a flat end that follows beam divergence. A scanning densitometer (Wellhöfer with 0.45 mm spot and 0.5 mm step size) was used for film analysis. The dosimetric characteristics studied include: penumbra width (80-20%) as a function of position of the leaf end in the field, inter- and intra-leaf radiation leakage, dose distribution of the tongue and groove, and isodose curves for stepped leaves forming 45 degrees angle beam edge. Results show that MLC designs with divergent and non-divergent leaves produce penumbra (80-20%) widths that are within 2.0 mm of each other. However, the distance of the collimator from the x-ray target plays an important role, and the smallest penumbra width was noted for the Varian MLC despite its rounded leaf-end design. Compared to the other systems, this collimator is positioned about 15 cm closer to the patient which affects the skin dose. The MLC with flat leaf end, although closer to the target, showed slightly poorer penumbra width. Inter-leaf leakage through the leaves is 1.3% for two of the collimators (Elekta and Varian) with the backup jaws and is nearly 1% for the third system (Siemens). The Siemens MLC produces reduced tongue-and-groove effect compared to the other two collimators (Elekta and Varian). The isodose undulation for a stepped edge is found to be significant for the collimator closest to the patient (Varian) and does not depend on the leaf-end shape. There is no perfect MLC system that can be recommended, rather each one has unique advantages and disadvantages that should be weighed with comfort, ease and cost effectiveness for clinical use.

Verification of dynamic intensity-modulated beam deliveries in canine subjects

Our objective in this work was to assess the precision and degree of accuracy with which intensity modulated radiation therapy (IMRT) can deliver highly localized dose distributions to tumors near critical structures using the dynamic sliding window technique. Measurements of dose distribution were performed both in vivo and in vitro using a combination of dosimeters [thermoluminescent dosimeters (TLD's), films, and diodes]. In vivo measurements were performed in two groups of purpose-bred dogs: one receiving four-field three-dimensional (3D) conformal treatment and the other receiving IMRT. The algorithms used in the inverse planning process included the Macro Pencil Beam (MPB) model and Projections onto Convex Sets (POCS). Single beam measurements were performed in phantoms to verify the accuracy of monitor unit settings required for delivering the desired doses. The composite doses from the delivery of the seven beam intensity modulated plans were measured in phantoms and cadavers, Biological end points (spinal cord toxicity and neurologic deficits due to irradiation) were evaluated at the end of one year to determine the spatial accuracy of the IMRT treatments over a fractionated course in live subjects. Results in single beam measurements were used at first to improve the dose calculation and translation algorithms. Results of the measurements for the delivery of all seven beams in phantoms confirmed that the system was capable of accurate spatial and dosimetric IMRT delivery. The in vivo results showed dramatic differences between control and IMRT-treated dogs, with the IMRT group showing no adverse effects and the control animals showing severe spinal cord injuries due to irradiation. The measurements presented in this paper have helped to verify the successful and accurate delivery of IMRT in a clinically related model using the University of Washington Medical Center (UWMC) system.

The dosimetric consequences of inter-fractional patient movement on three classes of intensity-modulated delivery techniques in breast radiotherapy

BACKGROUND AND PURPOSE

A comparison between three classes of intensity-modulated delivery techniques was undertaken to examine the dosimetric consequences of using a multileaf collimator (MLC) reshaped on each imaged fraction as opposed to compensators designed on the first day of treatment potentially giving a treatment technique whose accuracy is thus degraded by movement.

MATERIALS AND METHODS

The effects of inter-fractional patient movement for a cohort of six breast patients were studied. Five treatment techniques were evaluated, two using a compensator, two using multiple static fields (MSF) and one using a dynamic multileaf collimator (DMLC). The compensated techniques consisted of (i) the use of compensators designed on day 1 only and used each fraction thereafter and (ii) the use of a compensator redesigned for each imaged fraction. The two MSF techniques were (i) a four-field-component design and (ii) a method where the fluence interval between the MLC field components was set so they were equivalent to the compensator ('quantized' MSF-MLC). The final technique investigated was the DMLC. Plans were produced for each of the five methods and a paired t-test was used to assess the reduction in the breast volume outside the dose range 95-105% between sets of pairs of techniques. An on-line correction strategy was simulated to determine the number of treatments that required intervention. The action levels were calculated using the difference between the volume outside the dose range 95-105% calculated for treatments where the DMLC was designed on day 1 only and for each imaged fraction. Differences of greater than 2%, greater than 5% and greater than 10% were investigated.

RESULTS

Thirty-five plans were evaluated for each technique. Results showed that a statistically significant mean reduction in the volume of the breast outside the dose range 95-105% could be achieved if the compensators were designed on each imaged fraction rather than on day 1 only (P=0.0045). When the comparison was made between the 'quantized' MSF-MLC and the technique where the compensators were designed on day 1 only, a statistically significant mean reduction in the volume of the breast tissue outside the dose range 95-105% was not achieved (P=0.21). Comparison of the DMLC technique to the technique where the compensators were designed on day 1 only results in a statistically significant mean reduction in the volume outside the dose range 95-105% (P=0.024). This corresponds to a mean reduction in the volume outside 95-105% dose of 1.94%. The 2% action level showed the greatest reduction in the volume outside 95-105% dose and intervention was only required in approximately one-third of the treatments investigated.

CONCLUSIONS

Redesigning MSFs for each imaged fraction did not provide a statistically significant mean reduction in the volume outside the dose range 95-105%. However, using the DMLC technique creates a statistically significant mean reduction in the volume outside the dose range 95-105%.

Adverse impact of multileaf collimator field shaping on lens dose in children with acute leukemia receiving cranial irradiation

PURPOSE

This study was designed to investigate the impact of multileaf collimator (MLC) on lens dose in children with leukemia undergoing cranial irradiation.

METHODS AND MATERIALS

This is a prospective study utilizing three common cranial irradiation techniques. Technique A uses a half-beam, nondivergent radiation field. Technique B has the anterior divergent field edge at the lateral bony canthus. Technique C is similar to B, but with a field collimator angle. Thermoluminescent dosimeter (TLD) lens dose measurements were obtained in children and phantom with all three techniques.

RESULTS

Seventeen children were studied. Lens dose measurements were obtained in 14 children with technique A using MLC and blocks. In 7 of 14 children, dose measurements were obtained with MLC only. One child was treated with technique B and 2 children were treated with C, with MLC +/- blocks. In all 3 techniques, with MLC alone, the lens dose increased by 64%, 119%, and 72%, respectively. Similar results were obtained in phantom.

CONCLUSION

This study demonstrates that independent of irradiation technique, additional custom blocking is required to maximally protect the lens with MLC shaped fields. This is due to the lack of conformity between MLC and the desired field edge at the lateral bony canthus.

Guide to clinical use of electronic portal imaging

The Electronic Portal Imaging Device (EPID) provides localization quality images and computer-aided analysis, which should in principal, replace portal film imaging. Modern EPIDs deliver superior image quality and an array of analysis tools that improve clinical decision making. It has been demonstrated that the EPID can be a powerful tool in the reduction of treatment setup errors and the quality assurance and verification of complex treatments. However, in many radiation therapy clinics EPID technology is not in routine clinical use. This low utilization suggests that the capability and potential of the technology alone do not guarantee its full adoption. This paper addresses basic considerations required to facilitate clinical implementation of the EPID technology and gives specific examples of successful implementations.

A practical method for the calculation of multileaf collimator shaped fields output factors

Output factors of multileaf-collimator (MLC) shaped radiation fields were measured for a commercial linear accelerator whose MLC leaves form parts of the upper collimator system. The approach of taking into account the reduced phantom scatter due to the MLC shaping on the output factor has previously been shown to be inadequate for this type of machine because of the effect of the MLC leaves on the collimator factor [Palta et al., Med. Phys. 23, 1219-1224(1996)]. In this article, we present two forms of the collimator factor that give satisfactory agreement with measured values of the output factors of MLC-shaped fields. The present method should be directly applicable to other linacs of similar MLC configuration. For clinical treatment planning, we believe the method is practical and accurate enough to be satisfactory. The equation for calculating the output factor requires only peak scatter and output factors of the machine. These are normally measured during machine commissioning.

The roles of multileaf collimators and micro-multileaf collimators in conformal and conventional nasopharyngeal carcinoma radiotherapy treatments

The purpose of this work is to study the efficacy and limitations of using standard multileaf collimators (MLCs) and micro-multileaf collimators (mMLCs) in the treatment of nasopharyngeal carcinoma (NPC) by conventional and conformal radiotherapy techniques. The penumbra characteristics of MLC, mMLC, and customized block collimated beams are measured with respect to leaf edge angle, beam energy, treatment depth, and field size and compared with those generated by a commercial three-dimensional planning computer system. Upon verification of the planning system, it is used to evaluate the treatment plans generated with these beam shapers for conventional and conformal NPC treatments. The effective penumbra of a MLC beam is strongly influenced by its edge angle, leaf width, and treatment depth. The suitability of standard MLCs in conventional NPC treatments is determined mainly by the edge angle to be used. For conformal NPC treatments involving six or more fields, dose volume histograms comparable to those of customized beam blocks are obtained with a standard MLC. The mMLC does not have the same restrictions as those on standard MLC but is limited to phase II treatment by its small usable field size. Both standard MLCs and mMLCs can be used to replace customized divergent beam blocks in both conventional and conformal NPC treatments. However, a MLC, due to its larger effective penumbra, may be unsuitable for use in cases when the tumor volumes extend very close to the critical normal structures. A mMLC, on the other hand, is limited by its small maximum field size and can only be used for collimating the facial portals in the second phase treatment.

Verification of a pencil beam based treatment planning system: output factors for open photon beams shaped with MLC or blocks

The accuracy of monitor unit calculations from a pencil beam based, three-dimensional treatment planning system (3D TPS) has been evaluated for open irregularly shaped photon fields. The dose per monitor unit was measured in water and in air for x-ray beam qualities from 6 to 15 MV. The fields were shaped either with a multileaf collimator (MLC) or with customized alloy blocks. Calculations from the 3D TPS were compared with measurements. The agreement between calculated and measured dose per monitor unit depended on field size and the amount of blocking and was within 3% for the MLC-shaped fields. The deviation could be traced to limitations in head scatter modelling for the MLC. For fields shaped with alloy blocks, the dose per monitor unit was calculated to be within 1.6% of measured values for all fields studied. The measured and calculated relative phantom scatter for fields with the same equivalent field size were identical for MLC and alloy shaped fields. These results indicate that the accuracy in the TPS calculations for open irregular fields, shaped with MLC or blocks, is satisfactory for clinical situations.

The single-isocentre treatment of head and neck cancer: time gain using MLC and automatic set-up

PURPOSE

In this manuscript, we studied the difference in the treatment time required to execute a single-isocentre three-field irradiation of the head and neck, using either tray-mounted cerrobend blocks or a multileaf collimator (MLC) for field shaping and automatic set-up.

MATERIALS AND METHODS

A total of twenty consecutive, unselected patients (16 males, four females), were eligible for this study because the dose they were to received was 44 Gy (2 Gy/fraction) to the head, neck and supraclavicular regions. Patients were randomly allocated to one of two treatment groups. The first group (n = 11) was treated on a Philips SL-75 linear accelerator (SL-75), using 5 MV photons and tray-mounted cerrobend blocks. The second group (n = 9) was treated on a Philips SL-25 linear accelerator (SL-25-MLC), using 6 MV photons and a MLC. Patients of the second group were treated using the automatic set-up facility of the SL-25-MLC, without entering the treatment room between consecutive fields.

RESULTS

Overall treatment time was significantly shorter on the SL-25-MLC than on the SL-75 (P < 0.0001). The difference in total treatment-execution time was in the range of 157 s per treatment session. The largest difference was observed in the set-up time. There was an average of a 125 s time gain per treatment day (P < 0.0001) in favour of the SL-25-MLC.

CONCLUSIONS

Compared to tray-mounted cerrobend blocks, a MLC and automatic set-up results in a significant time advantage when a single isocentre technique is used to treat head and neck cancer.

Commissioning of a micro multi-leaf collimator and planning system for stereotactic radiosurgery

PURPOSE

A computer controlled micro multi-leaf collimator, m3 mMLC, has been commissioned for conformal, fixed-field radiosurgery applications. Measurements were made to characterise the basic dosimetric properties of the m3, such as leaf transmission, leakage and beam penumbra. In addition, the geometric and dosimetric accuracy of the m3 was verified when used in conjunction with a BrainSCAN v3.5 stereotactic planning system.

MATERIALS AND METHODS

The m3 was detachably mounted to a Varian Clinac 2100C accelerator delivering 6 MV X-rays. Leaf transmission, leakage, penumbra and multiple, conformal fixed field dose distributions were measured using calibrated film in solid water. Beam data were collected using a diamond detector in a scanning water tank and planned dose distributions were verified using LiF TLDs and film. A small, shaped phantom was also constructed to confirm field shaping accuracy using portal images.

RESULTS

Mean transmission through the closed multi-leaves was 1.9 +/- 0.1% and leakage between leaves was 2.8 +/- 0.15%. Between opposing leaves abutting along the central beam-axis transmission was approximately 15 +/- 3%, but was reduced to a mean of 4.5 +/- 0.6% by moving the abutmen position 4.5 cm off-axis. Beam penumbrae were effectively constant as a function of increasing square field size and asymmetric fields and was seen to vary non-linearly when shaped to diagonal, straight edges. TMR, OAR and relative output beam data measurements of circular m3 fields were comparable to conventional, circular stereotactic collimators. Multiple, conformal field dose distributions were calculated with good spatial and dosimetric accuracy, with the planned 90% isodose curves agreeing with measurements to within 1-2 mm and to +/- 3% at isocentre. Portal films agreed with planned beams eye-view field shaping to within 1 mm.

CONCLUSIONS

The m3 micro multi-leaf collimator is a stable, high precision field-shaping device suitable for small-field, radiosurgery applications. Dose distributions can be accurately calculated by a planning system using only a few beam data parameters.

Tomotherapy

Tomotherapy is delivery of intensity-modulated, rotational radiation therapy using a fan-beam delivery. The NOMOS (Sewickley, PA) Peacock system is an example of sequential (or serial) tomotherapy that uses a fast-moving, actuator-driven multileaf collimator attached to a conventional C-arm gantry to modulate the beam intensity. In helical tomotherapy, the patient is continuously translated through a ring gantry as the fan beam rotates. The beam delivery geometry is similar to that of helical computed tomography (CT) and requires the use of slip rings to transmit power and data. A ring gantry provides a stable and accurate platform to perform tomographic verification using an unmodulated megavoltage beam. Moreover, megavoltage tomograms have adequate tissue contrast and resolution to provide setup verification. Assuming only translational and rotational offset errors, it is also possible to determine the offsets directly from tomographic projections, avoiding the time-consuming image reconstruction operation. The offsets can be used to modify the leaf delivery pattern to match the beam to the patient's anatomy on each day of a course of treatment. If tomographic representations of the patient are generated, this information can also be used to perform dose reconstruction. In this way, the actual dose distribution delivered can be superimposed onto the tomographic representation of the patient obtained at the time of treatment. The results can be compared with the planned isodose on the planning CT. This comparison may be used as an accurate basis for adaptive radiotherapy whereby the optimized delivery is modified before subsequent fractions. The verification afforded tomotherapy allows more precise conformal therapy. It also enables conformal avoidance radiotherapy, the complement to conformal therapy, for cases in which the tumor volume is ill-defined, but the locations of sensitive structures are adequately determined. A clinical tomotherapy unit is under construction at the University of Wisconsin.

An integrated system for the production of field shaping devices in radiotherapy

A system has been developed in our department that simplifies the production processes of field shaping devices in radiotherapy, integrating an image grabbing and processing facility at a radiotherapy simulator and an automated block cutter. The data acquisition subsystem captures images, processes and corrects them for pincushion distortions, creates a composite radiograph, records user defined contours of blocks and exports data to the block cutter controller. A robotic subsystem drives and controls the polystyrene cutting unit. The system has been experimentally evaluated. Errors in contour definition were found to be less than 1 mm for a broad range of gantry angles and not exceeding 1.5 mm for those gantry orientations that present maximum magnetic field related image intensifier distortion, while the automated block cutter is capable of cutting out contours in polystyrene with an accuracy comparable to that of commercially available systems. The system is expected to contribute to the overall improvement of radiotherapy processes, particularly in low budget radiotherapy departments, introducing improvements in accuracy and efficiency at minimum costs.

Using static MLC fields to replace partial transmission cerrobend blocks in treatment planning of rectal carcinoma cases

A treatment planning technique has been developed for using static multileaf collimators to replace partial transmission blocks for treating rectal or cervix carcinoma. The static MLC fields were used to replace the partial transmission block of the anterior-posterior pelvis field for the so-called "thunderbird" technique. Treatment plans were developed and evaluated on a commercial three-dimensional treatment planning system (FOCUS, Computerized, Medical Systems, St. Louis, MO). The result of the treatment plan comparison indicates that the static MLC fields are capable of achieving the same target, inguinal and pelvic dose distribution as the partial transmission cerrobend blocked fields. The MLC fields are easy to modify particularly for the match line adjustments. In conclusion, it is efficient and effective to use static MLC fields to replace partial transmission blocks in the "thunderbird" technique for treating rectal or cervix carcinoma.

Update on the treatment of prostate cancer with external beam irradiation

BACKGROUND

We review the recent changes in the radiotherapeutic management of clinically localized prostate cancer, including the implementation of three-dimensional (3-D) conformal radiation therapy (3DCRT), biochemical disease-free survival (bNED control) using conventional and 3DCRT techniques, and the morbidity of these treatment strategies.

METHODS

The components of 3DCRT are discussed, including patient immobilization, 3-D treatment planning, multileaf collimation, and electronic portal imaging. bNED control rates from institutions using conventional and 3DCRT techniques are compared. The gastrointestinal (GI) and genitourinary (GU) morbidity from prospective trials using conventional doses of radiation are compared to data from 3DCRT series. bNED control rates stratified by pretreatment prostate-specific antigen (PSA) are compared between surgical and radiation series.

RESULTS

bNED control rates (3-5 years) for patients treated with conventional and 3DCRT techniques ranged from 44-70% and 30-72% with pretreatment PSA levels 4-10 and 10-20, respectively. Although direct comparisons are difficult between treatment modalities, no difference in bNED control stratified by pretreatment PSA was observed between surgical and radiation patients.

CONCLUSIONS

Patients with clinically localized prostate cancer treated with 3DCRT demonstrate durable bNED control at 5 years. Conformal radiation techniques, multileaf collimation, electronic portal imaging, and patient immobilization have reduced acute and chronic GI and GU morbidity while allowing safe dose escalation in an effort to further improve local control and overall survival.

Beam characteristics of a retrofitted double-focused multileaf collimator

Multileaf collimators (MLCs) are generally believed to be convenient and cost-effective tools for intensity modulation and conformal therapy. They are becoming a standard feature on new accelerators; however, the older units can be retrofitted with modern MLCs. Before such a unit can be clinically used, the beam characteristics must be verified. In this study the beam characteristics of a Siemens double-focused MLC retrofitted to an MD2 linear accelerator are presented. The head leakage along with inter- and intra-leaf radiation transmission were measured using film. The collimator (Sc), phantom (Sp), total (Scp) scatter factors, central axis depth dose, beam profiles for off-axis ratios, penumbra, and surface dose were evaluated for square, rectangular, and irregularly shaped fields. The maximum head leakage was estimated to be < 0.05% in any plane at a distance of 1 m and maximum transmission through the MLC leaves was estimated to be < 1.4% and < 1.1% for the 10 MV and 6 MV beams, respectively. The maximum differences between pre- and post-MLC installation data for the Sc and Scp were < or = 0.7% and < or = 1.4%, respectively. Similarly, the percent depth dose data for all fields and both beam energies were within 1.5% of the original data. The beam profiles measured at various depths were also in agreement with those of the pre-MLC installation data. The measured beam penumbra (20%-80%) showed a range of 7.8 mm-11.0 mm for the 6 MV and 8.4 mm-11.1 mm for the 10 MV beams from smallest to largest fields. These ranges differ by less than a millimeter from those of the old data. The surface dose measurements were slightly lower than the conventional jaw values suggesting that MLC does not produce significant electron contamination. It is concluded that the retrofitted MLC maintains the integrity of the original beam and may provide a cost-effective conformal therapy.

Geometric and dosimetric analysis of multileaf collimation conformity

BACKGROUND AND PURPOSE

There is concern over the stepped edges of multileaf collimator (MLC) fields for target coverage compared with those of cerrobend. Despite recent reports dispelling this concern, users are still cautious when using MLCs for small fields. Leaf orientation can be a problem if one is required to orient the leaves along an axis not ideal for conformity (such as dynamic or universal wedge cases). In this study we examined the dependence of MLC field conformity on field size and elongation.

MATERIALS AND METHODS

We examined circles of varying diameter and ellipses of varying eccentricity with leaves oriented along the major and minor axes of ellipses. The tests were both geometric, comparing areas of overblocking or underblocking (leaves inside and outside the field), and dosimetric, using radiographic films at depth in the beam's eye view plane.

RESULTS

For the geometric comparison there is a rapid increase in non-conformity, defined as the percentage of overblocking or underblocking area, as the circle diameter decreases. For ellipses, when the leaves move along one axis direction, the conformity does not depend on the diameter of the same axis, but instead improves as the dimension of the axis in the non-leaf motion direction increases. The best conformity is achieved when the maximum number of leaves is used to shape the field. When the dosimetry is analyzed, the predictability of these trends decreases due to the impact of undulations (scatter), leaf inaccuracies and dosimetric uncertainties.

CONCLUSIONS

We recommend that for small round fields MLC should be used with caution and that for ellipses the direction of leaf movement should be aligned with the minor axis whenever possible. Though these experiments are for idealized geometries, the observations can be applied to clinical fields. An MLC with a thinner leaf width could be beneficial for small round fields.

A comparison of multileaf-collimator and alloy-block field shaping

PURPOSE

The purpose of this report was to compare the dose distribution at a field edge defined with divergent alloy blocks to the distribution obtained with a multileaf collimator (MLC). The comparison is made for simple block replacement situations.

METHODS AND MATERIALS

A tertiary multileaf collimator mounted on a linear accelerator operating at 6 MV was compared to divergent alloy blocks positioned at the level of the blocking tray. The leaves of the MLC were positioned to give maximum stepping (leaf displacement equals leaf width), and the blocking produced the same field shape. Three different treatment plans were compared: single field, opposed fields, and a four-field "box." Dose distributions were determined using radiographic film scanned with a laser densitometer with a 0.45-mm spot size. One experiment was repeated using radiochromic film with reduced energy dependence. Dose distributions were examined on the isocenter plane, and on planes displaced by 1.0 and 2.5 cm. The effect of daily setup variations was also studied by comparing a single fraction treatment with a fractionated treatment consisting of 15 fields slightly displaced relative to each other. The magnitude of these displacements was determined using available literature on treatment reproducibility.

RESULTS

For a single field plan, maximum stepping of an MLC-defined edge produces an obvious undulating dose pattern compared to an alloy block edge. At the isocenter plane, this pattern is unchanged when parallel opposed fields are used. However, blurring occurs for both MLC and block edges when planes displaced from the isocenter are examined. The gradient for the block edge is 8%/mm for opposed fields and a plane 2.5 cm from the isocenter, compared to 15%/mm for the isocenter plane. Adding two additional fields does not change the dose pattern in the isocenter plane, but does reduce the gradient across the steepest portion of the penumbra to 8%/mm, and shifts the isodose line with the most pronounced stepping to higher values (from 50 to 80%). Introducing daily setup variations results in a reduction of the sharp dose gradient along the sides of a single field, and around the periphery of the beam at the isocenter plane of opposed fields. Smaller changes are found for edges already blurred by other factors. Radiochromic film was generally noisier than radiographic film, but comparison of the two films did not show a significant difference, indicating that the energy dependence of the radiographic film was not a problem.

CONCLUSIONS

The obvious dose stepping seen on a portal image of a single field with MLC shaping is shown to be partially erased by the addition of other fields, and for planes away from the isocenter. However, the effects of daily setup variations must be included to more effectively blur dose stepping along the external envelope of a single field or near the isocenter plane of opposed fields. This result conflicts with attempts to improve immobilization.

Execution of a single-isocenter three-field technique, using a multileaf collimator or tray-mounted cerrobend blocks: effect on treatment time

PURPOSE

In this article, we studied the total treatment time of a single-isocenter three-field irradiation of breast and axilla, using either tray-mounted cerrobend blocks, or a multileaf collimator (MLC) for field shaping.

METHODS AND MATERIALS

A total of 20 female, unselected patients were given 50 Gy (2 Gy/fraction) on breast and 46 Gy on axilla and supraclavicular region (2 Gy/fraction). Patients were randomized between two different treatment groups. The first group (n = 10) was treated on a Philips SL-75 linear accelerator (SL-75), using 5 MV photons with tray-mounted cerrobend blocks. The second group (n = 10) was treated on a Philips SL-25 linear accelerator, using 6 MV photons and a MLC (SL-25-MLC).

RESULTS

Although the beam-on time on the SL-25-MLC was significantly higher (p < 0.0001) compared to the SL-75, overall treatment time was significantly shorter using a MLC instead of tray-mounted cerrobend blocks (p < 0.0001). The difference in total treatment time was in the range of 100 s per patient per day. The main difference between the two accelerators was observed when setup of the second and third field was done using the automatic setup facility of the SL-25-MLC (avoids entering the treatment room). A mean time gain of 124 s per treatment session was observed using automatic setup. Considering the yearly number of patients receiving this treatment, a total time gain equivalent to 16.15 8-h workdays was calculated.

CONCLUSIONS

Compared to a technique using tray-mounted cerrobend blocks in the single-isocenter three-field irradiation of a breast and axilla, a MLC combined with automatic field setup provides a significant time advantage, by reducing the number of manipulations inside the treatment room.

The optimum intensities for multiple static multileaf collimator field compensation

A method of determining the optimum beam intensities for compensation using multiple static multileaf collimator fields is presented. In this method a histogram of the number of beam pixels against beam intensity is generated for the intensity-modulated beam (IMB). The intensity of each beam to be used is chosen to minimize the mean square deviation between each bin in the histogram and the closest beam intensity. This method has been applied to sample IMBs possessing one maximum and two maxima. For both cases, the use of uniform beam intensity increments is shown to be close to optimal. In the case with two maxima, the efficacy of irradiating both peaks simultaneously, rather than separately, has been studied and shown to be of potential benefit. The optimum intensities for an IMB for breast radiotherapy are also presented.

The cumulative verification image analysis tool for offline evaluation of portal images

PURPOSE

Daily portal images acquired using electronic portal imaging devices contain important information about the setup variation of the individual patient. The data can be used to evaluate the treatment and to derive correction for the individual patient. The large volume of images also require software tools for efficient analysis. This article describes the approach of cumulative verification image analysis (CVIA) specifically designed as an offline tool to extract quantitative information from daily portal images.

METHODS AND MATERIALS

The user interface, image and graphics display, and algorithms of the CVIA tool have been implemented in ANSCI C using the X Window graphics standards. The tool consists of three major components: (a) definition of treatment geometry and anatomical information; (b) registration of portal images with a reference image to determine setup variation; and (c) quantitative analysis of all setup variation measurements. The CVIA tool is not automated. User interaction is required and preferred. Successful alignment of anatomies on portal images at present remains mostly dependent on clinical judgment. Predefined templates of block shapes and anatomies are used for image registration to enhance efficiency, taking advantage of the fact that much of the tool's operation is repeated in the analysis of daily portal images.

RESULTS

The CVIA tool is portable and has been implemented on workstations with different operating systems. Analysis of 20 sequential daily portal images can be completed in less than 1 h. The temporal information is used to characterize setup variation in terms of its systematic, random and time-dependent components. The cumulative information is used to derive block overlap isofrequency distributions (BOIDs), which quantify the effective coverage of the prescribed treatment area throughout the course of treatment. Finally, a set of software utilities is available to facilitate feedback of the information for treatment plan recalculation and to test various decision strategies for treatment adjustment.

CONCLUSIONS

The CVIA tool provides comprehensive analysis of daily images acquired with electronic portal imaging devices. Its offline approach allows characterization of the nature of setup variation for the individual patient that would have been difficult to deduce using only a few daily or weekly portal images. Distribution of the tool will help establish an important database of setup variation from many clinics. The information derived from CVIA can also serve as the foundation to integrate treatment verification, treatment planning, and treatment delivery.