An Optimization-Based Approach to the Multiple Static Delivery Technique in Radiation Therapy

A multileaf collimator phantom for the quality assurance of radiation therapy planning systems and CT simulators

The build‐up dose in the megavoltage photon beams can be a limiting factor in intensity‐modulated radiation therapy (IMRT) treatments. Excessive surface dose can cause patient discomfort and treatment interruptions, while underdosing may lead to tumor repopulation and local failure. Dose in the build‐up region was investigated for IMRT delivery with solid brass compensator technique (compensator‐based IMRT) and compared with that of multileaf collimator (MLC)‐based IMRT. A Varian Trilogy linear accelerator equipped with an MLC was used for beam delivery. A special solid brass step‐wise compensator was designed and built for testing purposes. Two step‐and‐shoot MLC fields were programmed to produce a similar modulated step‐wise dose profile. The MLC and compensator dose profiles were measured and adjusted to match at the isocenter depth of 10 cm. Build‐up dose in the 1–5 mm depth range was measured with an ultrathin window, fixed volume parallel plate ionization chamber. Monte Carlo simulations were used to model the brass compensator and step‐and‐shoot MLC fields. The measured and simulated profiles for the two IMRT techniques were matched at the isocenter depth of 10 cm. Different component contributions to the shallow dose, including the MLC scatter, were quantified. Mean spectral energies for the open and filtered beams were calculated. The compensator and MLC profiles at 10 cm depth were matched better than ±1.5%. The build‐up dose was up to 7% lower for compensator IMRT compared to MLC IMRT due to beam hardening in the brass. Low‐energy electrons contribute 22% and 15% dose at 1 mm depth for compensator and MLC modalities, respectively. Compensator‐based IMRT delivers less dose in the build‐up region than MLC‐based IMRT does, even though a compensator is closer to the skin than the MLC.PACS number: 87.55.dk, 87.56.ng

Correlation of Integral Dose, White Blood Cell Counts, and Radiation Therapy Techniques for Head and Neck Cancer Patients Under Radiation Therapy

The hypofractionated radiotherapy modality was established to reduce treatment durations and enhance therapeutic efficiency, as compared to conventional fractionation treatment. However, this modality is challenging because of rigid dosimetric constraints. This study aimed to assess the impact of multi-leaf collimator (MLC) widths (10 mm and 5 mm) on plan quality during the treatment of prostate cancer. Additionally, this study aimed to investigate the impact of the MLC mode of energy on the Agility flattening filter (FF), MLC Agility-free flattening filter (FFF), and MLCi2 for patients receiving hypofractionated radiotherapy. Two radiotherapy techniques; Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Radiotherapy (VMAT), were used in this research. In the present study, computed tomography simulations of ten patients (six plans per patient) with localized prostate adenocarcinoma were analyzed. Various dosimetric parameters were assessed, including monitor units, treatment delivery times, conformity, and homogeneity indices. To evaluate the plan quality, dose-volume histograms (DVHs) were estimated for each technique. The results demonstrated that the determined dosimetric parameters of planning target volume (PTV)p (such as D mean, conformity, and homogeneity index) showed greater improvement with MLC Agility FF and MLC Agility FFF than with MLCi2. Additionally, the treatment delivery time was reduced in the MLC Agility FF (by 31%) and MLC Agility FFF (by 10.8%) groups compared to the MLCi2 group. It is concluded that for both the VMAT and IMRT techniques, the smaller width (5 mm) MLCs revealed better planning target volume coverage, improved the dosimetric parameters for PTV, reduced the treatment time, and met the constraints for OARs. It is therefore recommended to use 5 mm MLCs for hypofractionated prostate cancer treatment due to better target coverage and better protection of OARs.

Intensity modulated radiation therapy with multileaf collimators of different leaf widths: a comparison of achievable dose distributions

The aim of the study was to evaluate the impact of multileaf collimator (MLC) leaf width on treatment adaptation and delivery accuracy for concurrent treatment of the prostate and pelvic lymph nodes with intensity modulated radiation therapy (IMRT). Seventy-five kilovoltage cone beam CTs (KV-CBCT) from six patients were included for this retrospective study. For each patient, three different IMRT plans were created based on a planning CT using three different MLC leaf widths of 2.5, 5, and 10 mm, respectively. For each CBCT, the prostate displacement was determined by a dual image registration. Adaptive plans were created by shifting selected MLC leaf pairs to compensate for daily prostate movements. To evaluate the impact of MLC leaf width on the adaptive plan for each daily CBCT, three MLC shifted plans were created using three different leaf widths of MLCs (a total of 225 adaptive treatment plans). Selective dosimetric endpoints for the tumor volumes and organs at risk (OARs) were evaluated for these adaptive plans. Using the planning CT from a selected patient, MLC shifted plans for three hypothetical longitudinal shifts of 2, 4, and 8 mm were delivered on the three linear accelerators to test the deliverability of the shifted plans and to compare the dose accuracy of the shifted plans with the original IMRT plans. Adaptive plans from 2.5 and 5 mm MLCs had inadequate dose coverage to the prostate (D99 < 97%, or D(mean) < 99% of the planned dose) in 6%-8% of the fractions, while adaptive plans from 10 mm MLC led to inadequate dose coverage to the prostate in 25.3% of the fractions. The average V56Gy of the prostate over the six patients was improved by 6.4% (1.6%-32.7%) and 5.8% (1.5%-35.7%) with adaptive plans from 2.5 and 5 mm MLCs, respectively, when compared with adaptive plans from 10 mm MLC. Pelvic lymph nodes were well covered for all MLC adaptive plans, as small differences were observed for D99, D(mean), and V(50.4Gy). Similar OAR sparing could be achieved for the bladder and rectum with all three MLCs for treatment adaptation. The MLC shifted plans can be accurately delivered on all three linear accelerators with accuracy similar to their original IMRT plans, where gamma (3%∕3 mm) passing rates were 99.6%, 93.0%, and 92.1% for 2.5, 5, and 10 mm MLCs, respectively. The percentages of pixels with dose differences between the measurement and calculation being less than 3% of the maximum dose were 85.9%, 82.5%, and 70.5% for the original IMRT plans from the three MLCs, respectively. Dosimetric advantages associated with smaller MLC leaves were observed in terms of the coverage to the prostate, when the treatment was adapted to account for daily prostate movement for concurrent irradiation of the prostate and pelvic lymph nodes. The benefit of switching the MLC from 10 to 5 mm was significant (p ≪ 0.01); however, switching the MLC from 5 to 2.5 mm would not gain significant (p = 0.15) improvement. IMRT plans with smaller MLC leaf widths achieved more accurate dose delivery.

Radiation therapy (RT) is considered a standard treatment option for localized prostate cancer, with external beam RT (EBRT) being the most prevalent method used. EBRT evolved from 2-D conventional RT (2D-RT) to 3-D conformal RT (3D-CRT), and then to intensity-modulated RT (IMRT). In the 2D-RT era, radiation oncologists drew irradiation fields on simple roentgenographies, typically including two anterior-posterior fields with or without two lateral fields, where the radiation dose could be allowed within 70 Gy or less. In the 3D-CRT era, typically five to seven conformal irradiation fields shaped to fit the prostate with multileaf collimators, but without intensity modulation, were used, and the radiation dose could be escalated to 70–75 Gy without increasing toxicity. IMRT enabled further dose escalation; for example, to 74–86.4 Gy. IMRT requires high-quality control of multileaf collimators, which allow modulation of radiation intensity within the same field, and that can yield even a concave-shaped dose distribution that spares the rectum while covering the neurovascular bundles. Despite such high radiation doses administered in IMRT, late gastrointestinal toxicities of grade 3 or more were reported as just 0–3%.1 In the USA, 3D-CRT prevailed in the early 1990s, and IMRT began to replace it from around 2000. Sheets et al. reported a population-based study using Surveillance, Epidemiology and End Results Medicare-linked data from 2000 through 2009 for patients with non-metastatic prostate cancer. Use of IMRT versus conformal radiation therapy increased from 0.15% in 2000 to 95.9% in 2008. In propensity score-adjusted analyses (n = 12 976), men who received IMRT versus conformal radiation therapy were less likely to develop gastrointestinal morbidities, and also were less likely to receive additional cancer therapy.2 In Japan, the transition from 2D-RT through 3D-CRT to IMRT proceeded within a shorter span, or rather, all at once. Yoshioka et al. reported the change from 2D-RT to 3D-CRT in patients treated between 1998 and 2006, during which the actuarial 5-year grade 2–3 late rectal bleeding rate decreased from 23% to 7%.3 Meanwhile, around 2000, IMRT for prostate cancer began in Japan. Chiba Cancer Center, along with Kyoto University, should be recognized as pioneers in prostate IMRT in Japan.4 Kobayashi et al. reported long-term results of prostate IMRT at Chiba Cancer Center, including a successful biochemical control rate and a satisfactorily low toxicity rate.5 In 2008, the Japanese government began covering prostate IMRT under the national insurance system, and it is now rapidly becoming the prevalent therapy for prostate cancer in Japan. It should be noted that these pioneers contributed significantly to the current prevalence of IMRT in Japan. Another notable achievement of Kobayashi's study is their use of fiducial-marker guidance in the daily set-up for irradiation, which is now called image-guided RT. The authors imply that such a high-precision and high-dose IMRT should be accompanied by an accurate image-guided RT technique. This should be evaluated in the future through comparison with other image-guided RT techniques. None declared.

Accurate multileaf collimator (MLC) leaf positioning plays an essential role in the effective implementation of intensity modulated radiation therapy (IMRT). This work evaluates the sensitivity of current patient‐specific IMRT quality assurance (QA) procedures to minor MLC leaf positioning errors. Random errors of up to 2 mm and systematic errors of ±1mm and ±2mm in MLC leaf positions were introduced into 8 clinical IMRT patient plans (totaling 53 fields). Planar dose distributions calculated with modified plans were compared to dose distributions measured with both radiochromic films and a diode matrix. The agreement between calculation and measurement was evaluated using both absolute distance‐to‐agreement (DTA) analysis and γ index with 2%/2mm and 3%/3mm criteria. It was found that both the radiochromic film and the diode matrix could only detect systematic errors on the order of 2 mm or above. The diode array had larger sensitivity than film due to its excellent detector response (such as small variation, linear response, etc.). No difference was found between DTA analysis and γ index in terms of the sensitivity to MLC positioning errors. Higher sensitivity was observed with 2%/2mm than with 3%/3mm in general. When using the diode array and 2%/2mm criterion, the IMRT QA procedure showed strongest sensitivity to MLC position errors and, at the same time, achieved clinically acceptable passing rates. More accurate dose calculation and measurement would further enhance the sensitivity of patient‐specific IMRT QA to MLC positioning errors. However, considering the significant dosimetric effect such MLC errors could cause, patient‐specific IMRT QA should be combined with a periodic MLC QA program in order to guarantee the accuracy of IMRT delivery.PACS numbers: 87.50.Gi, 87.52.Df, 87.52.Px, 87.53.Dq, 87.53.Tf, 87.53.Kn, 87.56.Fc

Introduction: During intensity modulated radiation therapy (IMRT) technique, theoretically, presence of flattening filter (FF) across the beamline of clinical linear accelerator (linac) is not essential to obtain uniform lateral profiles due to intensity modulation of photon beams by multileaf collimators (MLCs). The aim of this study was to investigate the dosimetrical properties of 18 MV photon Beam-Varian linac with and without FF. Materials and Methods: All dose measurements were performed on 18 MV, FF Mode-Varian 2100C/D linac. The FF and flattening filter free (FFF) modes of linac were modeled by MCNPX 2.4. code. The photon and contaminant electrons spectra, dose rate, present depth doses (PDD), lateral dose profiles, total and collimator scatter factors and out of field doses were calculated and compared with and without FF. Results: Removing the FF increased the photon and contaminant electron fluences by factors of 5.48 and 3.94 for a 5 × 5 cm2 field size, respectively. The surface dose increased up to 155%. The flatness of dose profile was disturbed and deteriorated with increase of field size. Despite the dependence of the total scattering factor on field size, the variation of collimator scattering factors was neglected. The out-of-field dose decreased about 81.5 % for a 5 × 5 cm2 field size. Conclusion: Removing FF from the linac’s head increases the dose rate and decreases the out-of-field dose, but the increased skin dose and deteriorated flatness of lateral dose profile are the main

Anal canal squamous cell carcinoma (SCC) is managed definitively with chemoradiation, reserving surgery for salvage. The dosage of radiation has varied from 30 Gy to in excess of 60 Gy. RTOG 0529 established intensity modulated radiation therapy (IMRT) as standard of care for anal canal SCC with doses of 50.4 to 54 Gy. We sought to use the National Cancer Database to examine trends in dose selection and radiation technique over time. We queried the National Cancer Database from 2004 to 2015 for cases of anal cancer stage groups 1 to 3, treated with definitive doses of radiation with chemotherapy. Dose escalation was defined as >54 Gy. Univariable and multivariable analyses were performed to identify factors predictive of dose, IMRT, and overall survival. Propensity-adjusted Cox proportional hazard ratios for survival were used to account for indication bias. We identified 7792 patients meeting the eligibility criteria, with 4269 treated to doses of 45 to 54 Gy and 3163 treated to doses >54 Gy. Patients who were older, had government or private insurance, IMRT treatment, treatment at an academic center, or more recent years were less likely to get dose escalation. The use of dose escalation decreased over time, from 50% in 2005 to 30% in 2015. IMRT use increased over time from 2% to 63%. On multivariable analysis with propensity score included it was found that increased age, higher comorbidity score, lower income, shorter distance to facility, and male sex were predictive of decreased overall survival. In addition, escalated dose was associated with a lower survival (hazard ratio: 1.10, 95% confidence interval: 1.01-1.20, P=0.03). The results of this analysis show a steady increase in the use of IMRT, with corresponding decrease in dose escalation. These findings correlate with the results of RTOG 0529 establishing IMRT as standard of care for anal SCC, using doses of 50.4 to 54 Gy.

Objective To investigate the dosimetric effect of multi-leaf collimator (MLC)transmission on intensity-modulated radiation therapy (IMRT).Methods MLC transmission through theleaves and rounded ends were measured with ion chamber for Varian Trilogy linear accelerator with the X-ray of 6 MV and 10 MV.The intraleaf and interleaf transmission were also measured with the electronic portal imaging device of aS1000.10 tumor patients treated with IMRT were used to evaluate the MLC parameters modeling in the Eclipse treatment planning system.Results The average transmissions of Millennium MLC were 1.6％ and 1.8％ for 6 MV and 10 MV X-rays.The transmission increased with the field size and depth of measurement.The transmissions at off axis position were a little lower than those on the central axis.The intraleaf transmissions measured with aS1000 were 0.8％-1.2％ and 1.1％-1.6％,the interleaf transmission were 1.3％-1.9％ and 1.6％-2.5％ for 6 MV and 10 MV X-rays respectively.Modeling with the measured MLC parameters,the Eclipse treatment planning system could calculate the dose distribution accurately.The γ pass rate at 3％ 3 mm was above 95％,except for two patients with nasopharyngeal cancer with the pass rate of 93.6％ and 94.5％.Conclusions The transmissions through the MLC leaves and the leaf ends contribute to the dose throughout the target significantly for IMRT.MLC transmission varied with the field size,depth of measurement and off axis position. Key words: Multi-leaf collimator; Transmission; Intensity-modulated radiotherapy

Since multileaf collimator (MLC) was introduced to replace Cerrobend blocks, its use has even further evolved for intensity modulated radiation therapy (IMRT). MLC use for IMRT often requires small beamlets (even smaller than Icm) to conform the radiation distribution to physical target volume (PTV). For computer treatment planning for IMRT, analytical algorithms have been used. For an example, the finite size pencil beam dose modeling (FSPB) was used in the CORWS IMRT treatment planning system. The requirement of implementing the FSPB dose modeling in the system includes output measurements (from 2 cm /spl times/ 2 cm to 40 cm /spl times/ 40 cm) and geometrical penumbra measurements. The field sizes used for penumbra measurements are 4 cm /spl times/ 4cm and 20 cm /spl times/ 20 cm at 10 and 20 cm depths. For a MIMiC, field sizes of 2 cm /spl times/ 4 cm and 2 cm /spl times/ 20 cm are used for 1 cm mode. For 2 cm mode, 4 cm /spl times/ 4 cm and 4 cm /spl times/ 20 cm are used. Although the FSPB modeling algorithm is fast and simple to use for dose calculation, the algorithm including other analytical algorithms has not been verified with significant measurement data, especially for small beams. Most of verifications were performed with Monte Carlo calculations. Due to the lack of lateral electronic equilibrium in small beams, it is nearly impossible to measure TAR/TMR even with micro-ionization chambers. Therefore, the authors used the near tissue-equivalent film (GAFChromic film) to measure the percent-depth dose (PDD) of 6 MV photon beam. To measure the PDD, the IMRT solid water phantom (NOMOS) was used, which had 0.5 cm sliced water slabs and clamps to prevent any air gap between the film and water slabs.

Purpose: In this study we describe quality assurance (QA) results of 356 modulated beam using MatriXX (IBA Dosimetry, Sweden). Intensity modulated radiotherapy(IMRT) using segmental multileaf collimator(MLC) is currently applied for different type of cancers at our institution. Dosimetric QA of all modulated beam is done as a part of pre treatment QA protocol. Method and Materials: XiO (version 4.34.02) treatment planning system is used to generate optimized IMRT plans. Oncor Expression linear accelerator, equipped with Optifocus MLC, is used to deliver the IMRT treatment. MartiXX, 2D ionization chamber array, consists of 1020 air‐vented plane parallel cylindrical ionization chambers arrange in a 24×24 cm2 area. Each chamber has 0.45 cm diameter, 0.5 cm height & center to center distance is 0.76 cm. The device runs with two separate counters to avoid dead time. Resolution of the device is 0.76 cm and can be interpolated down to 0.1 cm. Results: We have analyzed all beams using gamma, distance to agreement (DTA), profile comparisons, measured dose (relative/absolute), visual comparison & coefficient of correlation. We have found gamma index & DTA is pass in more than 97 % beam with a 3% & 3 mm passing criteria and coefficient of correlation was 0.987 (SD 0.047). Conclusion: We have found MatriXX to be energy & dose rate independent. The minimum read out time is 20 ms, it allows us to measure and analyze dynamic processes like dynamic IMRT, start‐up process of the linear accelerator, build‐up process of a virtual wedge. The disadvantage of MartiXX for IMRT QA is the limited resolution, resulting in limited sensitivity to MLC failures. MartiXX is a useful device for IMRT pre treatment QA as it is time saving, efficient, easy to use and it can be used for both relative and absolute dose measurements.

Amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) continue to be investigated as treatment verification tools, with a particular focus on intensity modulated radiation therapy (IMRT). This verification could be accomplished through a comparison of measured portal images to predicted portal dose images. A general fluence determination tailored to portal dose image prediction would be a great asset in order to model the complex modulation of IMRT. A proposed physics-based parameter fluence model was commissioned by matching predicted EPID images to corresponding measured EPID images of multileaf collimator (MLC) defined fields. The two-source fluence model was composed of a focal Gaussian and an extrafocal Gaussian-like source. Specific aspects of the MLC and secondary collimators were also modeled (e.g., jaw and MLC transmission factors, MLC rounded leaf tips, tongue and groove effect, interleaf leakage, and leaf offsets). Several unique aspects of the model were developed based on the results of detailed Monte Carlo simulations of the linear accelerator including (1) use of a non-Gaussian extrafocal fluence source function, (2) separate energy spectra used for focal and extrafocal fluence, and (3) different off-axis energy spectra softening used for focal and extrafocal fluences. The predicted energy fluence was then convolved with Monte Carlo generated, EPID-specific dose kernels to convert incident fluence to dose delivered to the EPID. Measured EPID data were obtained with an a-Si EPID for various MLC-defined fields (from 1 x 1 to 20 x 20 cm2) over a range of source-to-detector distances. These measured profiles were used to determine the fluence model parameters in a process analogous to the commissioning of a treatment planning system. The resulting model was tested on 20 clinical IMRT plans, including ten prostate and ten oropharyngeal cases. The model predicted the open-field profiles within 2%, 2 mm, while a mean of 96.6% of pixels over all IMRT fields was in agreement with the 2%, 3 mm criteria. This model demonstrates accuracy commensurate to existing methods for IMRT pretreatment verification with portal dose image prediction of complex clinical examples (< 2%, 3 mm).

Intensity‐modulated radiotherapy (IMRT) is now common in many therapy centers. Quality assurance (QA) procedures for a linear accelerator (linac) and multi‐leaf collimator (MLC) designed for conventional static fields do not address the unique dosimetric issues pertinent to IMRT planning and delivery. IMRT portals are composed of many irregular, small, off‐center, and abutting field segments throughout the target volume, each delivering only a few MU. In these regards, emphasis should be placed on beam stability for small MU and leaf position accuracy with gantry rotation. At the same time, the target volume, shielded by the MLC for much of the treatment, sees a larger transmitted dose, especially for MLC with rounded leaf ends, The impact of delivery technique, segmented (step and shoot) or dynamic (sliding window), on the required leaf position tolerance is not fully appreciated and will be addressed. While the mechanics and software of the various linacs and MLC distinguish them, there are many aspects of QA in common. This course will discuss: (1) the specific QA related to the MLC design; (2) additional QA for linear accelerators pertinent to the small MU and small field sizes used in IMRT; (3) tools and procedures often used to perform these QA tasks; (4) specific QA issues for different IMRT delivery methods.Educational Objectives:1. Understand different IMRT delivery methods and their specific QA issues.2. Understand effect of QA on the IMRT delivery accuracy.3. Understand the importance of proper commissioning of the planning system to avoid confusion with dose delivery issues.