Intensity-modulated Radiotherapy Tomotherapy Research Articles

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

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

Treatment plan comparison between helical tomotherapy and MLC-based IMRT using radiobiological measures

The rapid implementation of advanced treatment planning and delivery technologies for radiation therapy has brought new challenges in evaluating the most effective treatment modality. Intensity-modulated radiotherapy (IMRT) using multi-leaf collimators (MLC) and helical tomotherapy (HT) are becoming popular modes of treatment delivery and their application and effectiveness continues to be investigated. Presently, there are several treatment planning systems (TPS) that can generate and optimize IMRT plans based on user-defined objective functions for the internal target volume (ITV) and organs at risk (OAR). However, the radiobiological parameters of the different tumours and normal tissues are typically not taken into account during dose prescription and optimization of a treatment plan or during plan evaluation. The suitability of a treatment plan is typically decided based on dosimetric criteria such as dose–volume histograms (DVH), maximum, minimum, mean and standard deviation of the dose distribution. For a more comprehensive treatment plan evaluation, the biologically effective uniform dose is applied together with the complication-free tumour control probability (P+). Its utilization is demonstrated using three clinical cases that were planned with two different forms of IMRT. In this study, three different cancer types at different anatomical sites were investigated: head and neck, lung and prostate cancers. For each cancer type, a linac MLC-based step-and-shoot IMRT plan and a HT plan were developed. The MLC-based IMRT treatment plans were developed on the Philips treatment-planning platform, using the Pinnacle 7.6 software release. For the tomotherapy HiArt plans, the dedicated tomotherapy treatment planning station was used, running version 2.1.2. By using as the common prescription point of the treatment plans and plotting the tissue response probabilities versus for a range of prescription doses, a number of plan trials can be compared based on radiobiological measures. The applied plan evaluation method shows that in the head and neck cancer case the HT treatment gives better results than MLC-based IMRT in terms of expected clinical outcome (P+ of 62.2% and 46.0%, to the ITV of 72.3 Gy and 70.7 Gy, respectively). In the lung cancer and prostate cancer cases, the MLC-based IMRT plans are better over the clinically useful dose prescription range. For the lung cancer case, the HT and MLC-based IMRT plans give a P+ of 66.9% and 72.9%, to the ITV of 64.0 Gy and 66.9 Gy, respectively. Similarly, for the prostate cancer case, the two radiation modalities give a P+ of 68.7% and 72.2%, to the ITV of 86.0 Gy and 85.9 Gy, respectively. If a higher risk of complications (higher than 5%) could be allowed, the complication-free tumour control could increase by over 40%, 2% and 30% compared to the initial dose prescription for the three cancer cases, respectively. Both MLC-based IMRT and HT can encompass the often-large ITV required while they minimize the volume of the organs at risk receiving high doses. Radiobiological evaluation of treatment plans may provide an improved correlation of the delivered treatment with the clinical outcome by taking into account the dose–response characteristics of the irradiated targets and normal tissues. There may exist clinical cases, which may look dosimetrically similar but in radiobiological terms may be quite different. In such situations, traditional dose-based evaluation tools can be complemented by the use of diagrams to effectively evaluate and compare treatment plans.

Tomotherapy and Other Innovative IMRT Delivery Systems

Fixed-field treatments, delivered using conventional clinical linear accelerators fitted with multileaf collimators, have rapidly become the standard form of intensity-modulated radiotherapy (IMRT). Several innovative nonstandard alternatives also exist, for which delivery and treatment planning systems are now commercially available. Three of these nonstandard IMRT approaches are reviewed here: tomotherapy, robotic linear accelerators (CyberKnife, Accuray Inc., Sunnyvale, CA), and standard linear accelerators modulated by jaws alone or by their jaws acting together with a tertiary beam-masking device. Rationales for the nonstandard IMRT approaches are discussed, and elements of their delivery system designs are briefly described. Differences between fixed-field IMRT dose distributions and the distributions that can be delivered by using the nonstandard technologies are outlined. Because conventional linear accelerators are finely honed machines, innovative design enhancement of one aspect of system performance often limits another facet of machine capability. Consequently the various delivery systems may prove optimal for different types of treatment, with specific machine designs excelling for disease sites with specific target volume and normal structure topologies. However it is likely that the delivery systems will be distinguished not just by the optimality of the dose distributions they deliver, but also by factors such as the efficiency of their treatment process, the integration of their onboard imaging systems into that process, and their ability to measure and minimize or compensate for target movement, including the effects of respiratory motion.

Intensity-modulated radiotherapy (IMRT) for carcinoma of the maxillary sinus: A comparison of IMRT planning systems

The treatment of maxillary sinus carcinoma with forward planning can be technically difficult when the neck also requires radiotherapy. This difficulty arises because of the need to spare the contralateral face while treating the bilateral neck. There is considerable potential for error in clinical setup and treatment delivery. We evaluated intensity-modulated radiotherapy (IMRT) as an improvement on forward planning, and compared several inverse planning IMRT platforms. A composite dose-volume histogram (DVH) was generated from a complex forward planned case. We compared the results with those generated by sliding window fixed field dynamic multileaf collimator (MLC) IMRT, using sets of coplanar beams. All setups included an anterior posterior (AP) beam, and 3-, 5-, 7-, and 9-field configurations were evaluated. The dose prescription and objective function priorities were invariant. We also evaluated 2 commercial tomotherapy IMRT delivery platforms. DVH results from all of the IMRT approaches compared favorably with the forward plan. Results for the various inverse planning approaches varied considerably across platforms, despite an attempt to prescribe the therapy similarly. The improvement seen with the addition of beams in the fixed beam sliding window case was modest. IMRT is an effective means of delivering radiotherapy reliably in the complex setting of maxillary sinus carcinoma with neck irradiation. Differences in objective function definition and optimization algorithms can lead to unexpected differences in the final dose distribution, and our evaluation suggests that these factors are more significant than the beam arrangement or number of beams.

SU‐FF‐T‐457: Whole‐Body Dose for Helical Tomotherapy

Purpose: Tomotherapy is a form of intensity modulated radiation therapy (IMRT) that utilizes rotating fan beams modulated by a binary‐multileaf collimator. The radiation is delivered either serially or helically as the patient is moved in a craniocaudal sequence for tumor coverage. While Tomotherapy can deliver highly conformal dose distributions, it yields the lowest delivery efficiency (tumor dose per MU) of current IMRT‐delivery options. This relatively low efficiency has the potential for delivering high total‐body doses due to head leakage, so a quantitative evaluation of the whole‐body dose is warranted. Methods and Materials: We conducted this evaluation for a dedicated helical Tomotherapy delivery device (Hi‐Art System®, Tomotherapy Inc) and compared the results against the previously published serial Tomotherapy system (Corvus, NOMOS Corporation) and traditional IMRT whole‐body data. A typical head‐and‐neck treatment plan (2Gy per fraction, 6622MU) was prepared and delivered to a large water‐equivalent phantom. An ADCL‐calibrated large‐volume ionization chamber (A17 Exradin) was used to measure the low doses. The dose was measured at both 1.5cm (dmax) and at the center of the phantom. Results: From 10cm to 48cm from the inferior target edge (the most proximal serial tomotherapy point was at 10 cm), the helical tomotherapy dose was less than 0.5% of the target dose, and was between 20% and 30% of the serial tomotherapy leakage dose. This study showed that the whole‐body dose for the 70Gy is approximately 140mSv. This dose is less than the 560mSv for a 70Gy treatment as published by Followill, et al. (Int.J.Radiat.Oncol.Biol.Phys. 38, 667). Conclusions: This study indicates that the commercial helical Tomotherapy system provides less whole‐body dose than serial Tomotherapy or conventional IMRT. This is probably due to the internal linear accelerator shielding design and the use of 6 MV photons.This work was supported in part by funding from Tomotherapy, Inc.

Comparing step-and-shoot IMRT with dynamic helical tomotherapy IMRT plans for head-and-neck cancer

The goal of this planning study was to compare step-and-shoot intensity-modulated radiotherapy (IMRT) plans with helical dynamic IMRT plans for oropharynx patients on the basis of dose distribution. Five patients with oropharynx cancer had been previously treated by step-and-shoot IMRT at the University Medical Centre Utrecht, The Netherlands, applying five fields and approximately 60-90 segments. Inverse planning was carried out using Plato, version 2.6.2. For each patient, an inverse IMRT plan was also made using Tomotherapy Hi-Art System, version 2.0, and using the same targets and optimization goals. Statistical analysis was performed by a paired t test. All tomotherapy plans compared favorably with the step-and-shoot plans regarding sparing of the organs at risk and keeping an equivalent target dose homogeneity. Tomotherapy plans in particular realized sharper dose gradients compared with the step-and-shoot plans. The mean dose to all parotid glands (n = 10) decreased on average 6.5 Gy (range, -4 to 14; p = 0.002). The theoretical reduction in normal tissue complication probabilities in favor of the tomotherapy plans depended on the parotid normal tissue complication probability model used (range, -3% to 32%). Helical tomotherapy IMRT plans realized sharper dose gradients compared with the clinically applied step-and shoot plans. They are expected to be able to reduce the parotid normal tissue complication probability further, keeping a similar target dose homogeneity.

MO‐D‐I‐618‐04: A Cost‐Effectiveness Model for New Radiation Oncology Technologies

Purpose: The additional equipment and personnel costs of supplying Intensity Modulated Radiation Therapy (IMRT) technology have caused many to question if the marginal gains in patient health‐related quality of life are worth the additional cost. A novel IMRT technology, helical tomotherapy, provides the opportunity to study cost and effectiveness for patients. Method and Materials: This methodological study proposes to evaluate the cost and effectiveness of treating conventional radiotherapy versus tomotherapy IMRT patients prospectively, among several institutions. The cost of treating patients varies between institutions, depending on personnel, equipment and overhead costs; however, the nature and quality of services provided are expected to be consistent. Results: The methodology study tracks cost information at a single institution, and simultaneously as the median from multiple institutions. Effectiveness measures include both standard quality adjusted life year instruments completed by patients and performance status measures completed by institutional personnel. In addition, disease specific effectiveness measures are accommodated in the study. Each participating institution uses the same effectiveness measures to track patients with similar disease. Conclusion: The resulting cost and effectiveness data is available to investigators at any point during the study, immediately upon the completion of a trial, or when statistical acceptability is achieved.

Fast radiographic film calibration procedure for helical tomotherapy intensity modulated radiation therapy dose verification

Film dosimetry offers an advantageous in-phantom planar dose verification tool in terms of spatial resolution and ease of handling for quality assurance (QA) of intensity modulated radiation therapy (IMRT) plans. A critical step in the success of such a technique is that the film calibration be appropriately conducted. This paper presents a fast and efficient film calibration method for a helical tomotherapy unit using a single sheet of film. Considering the unique un-flattened cone shaped profile from a helical tomotherapy beam, a custom leaf control file (sinogram) was created, to produce a valley shaped intensity pattern. There are eleven intensity steps in the valley pattern, representing varying dose values from 38 to 265 cGy. This dose range covers the most commonly prescribed doses in fractionated IMRT treatments. An ion chamber in a solid water phantom was used to measure the dose in each of the eleven steps. For daily film calibration the whole procedure, including film exposure, processing, digitization and analysis, can be completed within 15 min, making it practical to use this technique routinely. This method is applicable to film calibration on a helical tomotherapy unit and is particularly useful in IMRT planar dose verification due to its efficiency and reproducibility. In this work, we characterized the dose response of the KODAK EDR2 ready-pack film which was used to develop the step valley dose maps and the IMRT QA planar doses. A comparison between the step valley technique and multifilm based calibration showed that both calibration methods agreed with less than 0.4% deviation in the clinically useful dose ranges.

Refining target coverage and normal tissue avoidance with helical tomotherapy vs linac-based IMRT for oropharyngeal cancer

Refining target coverage and normal tissue avoidance with helical tomotherapy vs linac-based IMRT for oropharyngeal cancer

Refining target coverage and normal tissue avoidance with helical tomotherapy vs linac-based IMRT for oropharyngeal cancer

The use of intensity modulated radiation therapy (IMRT) in the treatment of head and neck (H&N) cancer is expanding worldwide. As radiation dose to selected normal structures (ie, parotid glands) is reduced, dose to adjacent regions (ie, anterior oral cavity) may be increased. Ultimate H&N cancer outcome and patient quality of life (QOL) is complex and can be influenced by many aspects of the radiation dose profile. This study was carried out to compare the capacity of helical tomotherapy versus linac-based IMRT to achieve target dose conformality and normal tissue sparing in oropharynx cancer patients. Twenty oropharyngeal cancer patients treated with curative intent using 7 field H&N IMRT between 2001–2003 were selected for study. Treatment planning and delivery was performed using the PinnacleTM planning system. In general, doses of 66–70 Gy at 2.1–2.2 Gy per fraction were delivered to primary planning target volume (PTV) targets, with 50–60 Gy to subclinical PTV targets. Dose constraints were employed for various normal H&N tissue structures such as parotid gland (20 Gy mean dose), spinal cord and anterior oral cavity (35 Gy maximum dose). Comprehensive H&N tomotherapy plans were designed for these 20 patients using identical prescription parameters. For conventional IMRT design, a 7 field arrangement of equally spaced beams was employed, whereas for helical tomotherapy, over 1000 projections per treatment were used (51 projections per rotation with a minimum of 20 rotations). Resultant treatment plans were then rigorously compared and contrasted. All 40 oropharynx cancer treatment plans analyzed (20 conventional IMRT and 20 helical tomotherapy) met primary treatment objectives regarding dose specification. Specifically, the resultant Equivalent Uniform Dose (EUD) calculations for high and low dose PTV were ≥the prescription dose. In general, helical tomotherapy plans offered favorable dose distributions over that achieved with 7 field IMRT plans as manifested by increased EUD to PTV targets, as well as reduced mean dose to normal tissue avoidance structures. The EUD analysis revealed that tomotherapy plans provided up to 4% increase to high dose PTV over that achieved with 7 field IMRT plans. Simultaneously, mean dose delivered to normal tissue avoidance structures such as contralateral parotid gland, spinal cord and oral cavity was reduced up to 50% with tomotherapy plans. The daily patient treatment time with helical tomotherapy is essentially identical to that with conventional IMRT. However, to ensure comparable delivery times, a jaw opening of 2.5 cm was chosen which yields a 2-fold increase (from 5 to 10 mm) in the conventional 80–20% penumbra in the cephalo-caudal dimension. The overall analysis of H&N treatment plans identifies several potential advantages for helical tomotherapy over that achieved with conventional IMRT. The improved dose distributions are largely explained by the fact that tomotherapy offers a significantly larger number of projections than available with conventional MLC-based IMRT. Nevertheless, the ultimate clinical significance of these dose distribution refinements remains ill defined. Intuitively, it would seem that higher conformality of dose to specified targets, and dose reduction to normal tissue avoidance structures may be of value. To further evaluation of this question, a prospective clinical trial is underway to examine normal tissue functional outcome and QOL in H&N cancer patients receiving IMRT with conventional and tomotherapy techniques. In addition to direct measurement of salivary, auditory, voice and swallow function, validated health outcome surveys are obtained to assess patient perception regarding voice, swallow, hearing and overall H&N function before and following treatment. These data should provide further insight regarding the potential value of refining dose distribution in H&N cancer patients receiving highly conformal treatment using either conventional IMRT or helical tomotherapy

The evolution of quality assurance for intensity- modulated radiation therapy (IMRT): sequential tomotherapy

Purpose To identify the pertinent issues to be addressed in successfully implementing IMRT using sequential tomotherapy into clinical reality and presenting the maturation of quality assurance (QA) programs for both the delivery system and patient treatments that allow routine clinical use of the system. Methods and materials Initially, a cubic phantom containing silver halide film was exposed to the entire treatment before patient treatment. The processed films were digitized with a laser densitometer and the dose distributions were compared with that generated by the planning system. Later, software that calculates the dose delivered to any phantom employing the intensity patterns developed in the inverse planning system for an individual patient was implemented for point checks of dose. A measurement phantom for use with this software was developed and evaluated on a large number of patients. Invasive fixation was used for all cranial patients initially. To use sequential tomotherapy for other sites and larger targets, noninvasive immobilization systems using two types of thermoplastic masks for cranial targets and reusable, evacuated body cradles were evaluated for positional accuracy and suitability for use with port films for patient QA. Results The program for equipment validation is divided into daily, weekly, and monthly programs that add only small amounts of time to routine QA programs. For the first 15 patients treated with this modality, the maximum dose measured on the film was within 5% of that predicted by the planning computer. The prescription isodose line was measured in the anteroposterior and lateral dimensions and the average discrepancy between measured and predicted was less than 2 mm. For an isodose line between 50% and 70% of the prescribed dose, the agreement was better than 3 mm. Success with the volume QA program was followed by a point check QA program that reduced the time required for individual patient QA from days to hours. Phantom measurements compared with computer predictions for 588 data points resulted in only 8% being outside a ±5% criterion. These cases were identified and allow a further reduction in the frequency of tests. Thermoplastic mask materials have adequate restraint characteristics for use with the system and port films on 21 patients resulted in one standard deviation = 1.3 mm. Body cradles are less accurate and require more frequent port films. A QA system that reduces the frequency of port films was developed. Conclusion The evolution of sequential tomotherapy in our department has been from a maximum of 3 cranial patients per day with invasive fixation to 60 patients per day for treatment of cranial, head-and-neck, and prostate tumors using different immobilization techniques. With proper preparation and refinement of tools used in commissioning and validation, sequential tomotherapy IMRT can become a routine clinical treatment modality.

Implementation and utility of a daily ultrasound-based localization system with intensity-modulated radiotherapy for prostate cancer

Purpose : To evaluate the clinical feasibility of daily computer-assisted transabdominal ultrasonography for target position verification in the setting of intensity-modulated radiotherapy (IMRT) for prostate cancer. Methods and Materials : Twenty-three patients with clinically localized prostate cancer were treated using a sequential tomotherapy IMRT technique (Peacock) and daily computer-assisted transabdominal ultrasonography (BAT) for target localization. Patients were instructed to maintain a full bladder and were placed in the supine position using triangulation tattoos and a leg immobilizer to minimize pelvic rotation. The BAT ultrasound system is docked to the treatment collimator and electronically imports the CT simulation target contours and isocenter. The system is able to use the machine isocenter as a reference point to overlay the corresponding CT contours onto the ultrasound images captured in the transverse and sagittal planes. A touch screen menu is used to maneuver the CT contours in three dimensions such that they match the ultrasound images. The system then displays the three-dimensional couch shifts required to produce field alignment. Data were prospectively collected to measure the frequency by which useful ultrasound images were obtained, the amount of time required for localization/setup, and the direction/magnitude of the positional adjustments. Results : Of the 23 patients, the BAT ultrasound system produced images of sufficient quality to perform the overlay of the CT contours in 19 patients such that positional verification could be reliably performed. Poor image quality was associated with patient inability to maintain a full bladder, large body habitus, or other anatomic constraints. Of the 19 assessable patients, a total of 185 treatment alignments were performed (mean 8.8/patient). For all cases, the average time required for the daily ultrasound imaging and positional adjustments was 11.9 min. After the initial 5 cases, the user experience skills improved such that the time required for image verification/positional adjustments decreased to a mean of 5.6 min. The average right-left, AP, and cranial-caudal adjustment was 2.6 ± 2.1 mm, 4.7 ± 2.7 mm, and 4.2 ± 2.8 mm, respectively. Positional adjustments >10 mm were infrequent and related primarily to misidentification of the target structures on the ultrasound image, patient movement, or improper registration of the triangulation tattoos. Conclusion : Daily computer-assisted BAT ultrasound positional verification of the prostate can be successfully performed through the acquisition of high-quality images in most patients with only a modest increase in treatment setup time. Positional data obtained with this system resulted in clinically meaningful adjustments in daily setup for sequential IMRT that would not be otherwise apparent from other verification modalities.

Room shielding for intensity-modulated radiation therapy treatment facilities

Purpose: The traditional assumptions used in room-shielding calculations are reassessed for intensity-modulated radiation therapy (IMRT). IMRT makes relatively inefficient use of monitor units (MUs) when compared to conventional radiation therapy, affecting the assumptions used in room-shielding calculations. For the same single-fraction tumor dose delivered, the total number of MUs for IMRT is much greater than for a conventional treatment. Therefore, the exposure contribution from the linear accelerator head leakage will be significantly greater than with conventional treatments. Methods and Materials: We propose a shielding calculation model that decouples the concepts of workload, MUs, and target dose when determining primary and secondary barrier thicknesses. The workload for primary barrier calculations for conventional multileaf collimator (MLC) IMRT treatments is determined according to patient tumor doses. The same calculation for accelerator-based serial tomotherapy IMRT requires scaling by the average number of treatment slices. However, rotational therapy yields a small use factor that compensates for this increase. We further define a series of efficiency factors to account for the small field sizes employed in IMRT. For secondary barrier calculations, the patient-scattered radiation is assumed to be the same for all IMRT modalities as for conventional therapy. The accelerator head leakage contribution is proportional to the number of MUs. Knowledge of the average number of MUs per patient is required to estimate the head leakage contribution. We used a 6-MV linear accelerator photon beam to guide the development of this technique and to evaluate the adequacy of conventional barriers for IMRT. Average weekly IMRT workload estimates were made based on our experience with 180 serial tomotherapy patients and published data for both “step and shoot” and dynamic MLC delivered treatments. Results: We found that conventional primary barriers are adequate for both dynamic MLC and serial tomotherapy IMRT. However, the excessive head leakage produced by these modalities requires an increase in secondary barrier shielding. Conclusion: When designing shielding for an IMRT facility, increases in accelerator head leakage must be taken into account for secondary shielding. Adequacy of secondary shielding will depend on the IMRT patient load. For conventional facilities that are being assessed for IMRT therapy, existing primary barriers will typically prove adequate.