Intensity modulated radiotherapy: advantages, limitations and future developments

Intensity modulated radiotherapy (IMRT) is widely used in clinical applications in developed countries, for the treatment of malignant and non-malignant diseases. This technique uses multiple radiation beams of non-uniform intensities. The beams are modulated to the required intensity maps for delivering highly conformal doses of radiation to the treatment targets, while sparing the adjacent normal tissue structures. This treatment technique has superior dosimetric advantages over 2-dimensional (2D) and conventional 3-dimensional conformal radiotherapy (3DCRT) treatments. It can potentially benefit the patient in three ways. First, by improving conformity with target dose it can reduce the probability of in-field recurrence. Second, by reducing irradiation of normal tissue it can minimise the degree of morbidity associated with treatment. Third, by facilitating escalation of dose it can improve local control. Early clinical results are promising, particularly in the treatment of nasopharyngeal carcinoma (NPC). However, as the IMRT is a sophisticated treatment involving high conformity and high precision, it has specific requirements. Therefore, tight tolerance levels for random and systematic errors, compared with conventional 2D and 3D treatments, must be applied in all treatment and pre-treatment procedures. For this reason, a large-scale routine clinical implementation of the treatment modality demands major resources and, in some cases, is impractical. This paper will provide an overview of the potential advantages of the IMRT, methods of treatment delivery, and equipment currently available for facilitating the treatment modality. It will also discuss the limitations of the equipment and the ongoing development work to improve the efficiency of the equipment and the treatment techniques and procedures

Impact of Multileaf Collimator Configuration Parameters on the Dosimetric Accuracy of 6-Mv Intensity-Modulated Radiation Therapy Treatment Plans

Purpose: To improve the dosimetric accuracy of intensity modulated radiation therapy (IMRT) dose distributions as calculated by the treatment planning system (TPS) by optimizing the parameters that govern multileaf collimator (MLC) transmission and rounded leaf offset. Methods: The MLC leaf transmission was optimized based on measurements made with ionization chambers and radiographic film. The rounded leaf offset table was optimized by measuring the radiation field edge as a function of leaf bank position with an ionization chamber in a water scanning tank and comparing the location to TPS equivalent dose calculations. Optimizations were validated by performing IMRT quality assurance (QA) tests on 19 gantry-static IMRT plans. Planar dose measurements were performed with film and a planar diode array and compared to TPS calculated dose distributions with default and optimized parameters. Results: Based on measurements, the leaf transmission factor was changed from a default value of 0.001 to 0.005. This optimization resulted in a statistically significant worsening of IMRT QA gamma index passing rate, because the currently used model is already slightly higher than the measured data originally used to commission the machine. The rounded leaf offset table had little room for improvement, with the average difference between the default and optimized offset values being -0.2 ± 0.7 mm. This reflects the excellent leaf position calibration protocol of physics staff. Conclusion: The hypothesis that TPS dosimetric accuracy of IMRT fields could be improved by optimizing the rounded leaf offset table and MLC transmission parameters was not supported by the results of this work

The Possibilities and Dosimetric Limitations of MLC-Based Intensity-Modulated Radiotherapy Delivery and Optimization Techniques

The use of intensity-modulated radiotherapy (IMRT) has increased extensively in the modern radiotherapy (RT) treatments over the past two decades. Radiation dose distributions can be delivered with higher conformality with IMRT when compared to the conventional 3D-conformal radiotherapy (3D-CRT). Higher conformality and target coverage increases the probability of tumour control and decreases the normal tissue complications. The primary goal of this work is to improve and evaluate the accuracy, efficiency and delivery techniques of RT treatments by using IMRT. This study evaluated the dosimetric limitations and possibilities of IMRT in small (treatments of head-and-neck, prostate and lung cancer) and large volumes (primitive neuroectodermal tumours). The dose coverage of target volumes and the sparing of critical organs were increased with IMRT when compared to 3D-CRT. The developed split field IMRT technique was found to be safe and accurate method in craniospinal irradiations. By using IMRT in simultaneous integrated boosting of biologically defined target volumes of localized prostate cancer high doses were achievable with only small increase in the treatment complexity. Biological plan optimization increased the probability of uncomplicated control on average by 28% when compared to standard IMRT delivery. Unfortunately IMRT carries also some drawbacks. In IMRT the beam modulation is realized by splitting a large radiation field to small apertures. The smaller the beam apertures are the larger the rebuild-up and rebuild-down effects are at the tissue interfaces. The limitations to use IMRT with small apertures in the treatments of small lung tumours were investigated with dosimetric film measurements. The results confirmed that the peripheral doses of the small lung tumours were decreased as the effective field size was decreased. The studied calculation algorithms were not able to model the dose deficiency of the tumours accurately. The use of small sliding window apertures of 2 mm and 4 mm decreased the tumour peripheral dose by 6% when compared to 3D-CRT treatment plan. A direct aperture based optimization (DABO) technique was examined as a solution to decrease the treatment complexity. The DABO IMRT technique was able to achieve treatment plans equivalent with the conventional IMRT fluence based optimization techniques in the concave head-and-neck target volumes. With DABO the effective field sizes were increased and the number of MUs was reduced with a factor of two. The optimality of a treatment plan and the therapeutic ratio can be further enhanced by using dose painting based on regional radiosensitivities imaged with functional imaging methods.Siirretty Doriast

Monte Carlo simulations for dosimetric verification in photon and electron beam radiotherapy

Dissertação para obtenção do Grau de Doutor em Engenharia BiomédicaOne of the primary requirements for successful radiotherapy treatments is the accurate calculation of dose distributions in the treatment planning process. Monte Carlo (MC) dose calculation algorithms are currently recognized as the most accurate method to meet this requirement and to increase even further dose accuracy. The improvements in computer processor technology and the development of variance reduction techniques for calculations have led to the recent implementation and use of MC algorithms for radiotherapy treatment planning at many clinical departments. The work conducting to the present thesis consists of several dosimetric studies which demonstrate the potential use of MC dose calculations as a robust tool of dose verification in two different fields of external radiotherapy: electron and photon beam radiotherapy. The first purpose of these studies is to evaluate dose distributions in challenging situations where conventional dose calculation algorithms have shown some limitations and it is very difficult to measure using typical clinical dosimetric procedures, namely in regions containing tissue inhomogeneities, such as air cavities and bones, and in superficial regions. A second goal of the present work is to use MC simulations to provide a detailed characterization of photon beams collimated by a multileaf collimator (MLC) in order to assess the dosimetric influences of these devices for the MC modeling of Intensity Modulated Radiotherapy (IMRT) plans. Detailed MC model of a Varian 2100 C/D linear accelerator and the Millenium MLC incorporated in the treatment head is accurately verified against measurements performed with ionization chambers and radiographic films. Finally, it is also an aim of this thesis to make a contribution for solving one of the current problems associated with the implementation and use of the MC method for radiotherapy treatment planning, namely the clinical impact of converting dose-to-medium to dose-to-water in treatment planning and dosimetric evaluation. For this purpose, prostate IMRT plans previously generated by a conventional dose algorithm are validated with the MC method using an alternative method, which involves the use of non-standard CT conversion ramps to create CT-based simulation phantoms.Fundação para a Ciência e Tecnologia; Centro de Física Nuclear da Universidade de Lisbo

Dose Conformation in Tumor Therapy with External Ionizing Radiation: Physical Possibilities and Limitations

The central problem in tumor irradiation is to deposit a high and spatially uniform dose in the tumor target volume while sparing the surrounding normal tissue as much as possible. The present work investigates how such an adaptation ("conformation") of the spatial dose distribution to arbitrarily shaped target volumes can be achieved, and where the physical limits lie. In particular, the specific possibilities of irradiation with different types of radiation are determined under these aspects, whereby a rough distinction is made between irradiation with charged and uncharged particles. Due to the different mechanisms of radiation-tissue interaction, a conformal dose distribution can be achieved with only one radiation field in the case of heavy charged particles; in the case of uncharged particles, several radiation fields from different directions are required. First, the possibilities and limits of dose conformation are evaluated theoretically. Analytical approximations for modeling dose distributions with uncharged and charged particles are developed. Within the framework of these approximations, the theory of the exponential Radon transform is used to determine the optimal parameters for obtaining a desired dose distribution. It is shown that for an infinite number of radiation fields in the plane, it is possible to adapt the high-dose region to arbitrarily shaped target volumes for both uncharged and charged particles. The dose in a small radiation-sensitive organ at risk in the immediate vicinity of the target volume can be reduced to small scatter contributions. In the case of charged particles, this is also possible for multiple organs at risk. Furthermore, the non-conformal "dose background" is always smaller for charged particles than for uncharged particles. In a more application-oriented chapter, an algorithm is developed for the optimization of dose distributions under practical boundary conditions, i.e. in three dimensions, with finitely many radiation fields and for finite resolutions of the beam shaping devices. To achieve optimal dose distributions, the use of fluence- and (in the case of charged particles) energy-modulated radiation fields is necessary. Especially in the case of uncharged particles, the technical prerequisites for this are not yet available in clinical practice. Therefore, newly developed approaches to fluence modulation for uncharged particles using a dynamically or quasi-dynamically driven "multileaf collimator" are presented. Furthermore, the first phantom experiment is described in which these generalized methods for achieving the best possible conformal dose distribution were realized with high-energy photons (15-MV bremsstrahlung spectrum). The high degree of practically achievable dose conformation is thus verified. Finally, a comparison of the optimized dose distributions achievable with photons and protons is performed for challenging clinical cases where conventional radiotherapy reaches its limits. The most important result is that irradiation with uncharged particles, and in particular with high-energy X-rays, can be optimized in such a way that, in all clinically relevant cases, tumor-conformal dose distributions can be achieved with relatively few (less than ten) radiation fields. The exposure of healthy tissue is naturally higher than for heavy charged particles. However, the tolerance dose values are not exceeded. Exceptions are the rare cases in which the target volume is surrounded on almost all sides by particularly radiation-sensitive risk organs. Only in these cases can a much better result be achieved with the technically more demanding heavy charged particle therapy

Tolerance limits and methodologies for IMRT measurement‐based verification QA: Recommendations of AAPM Task Group No. 218

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143649/1/mp12810_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/143649/2/mp12810.pd

Novel dual single sided silicon strip detector chip for radiotherapy verification

A novel dual single sided silicon strip detector (SSSSD) chip was designed to meet clinical requirements in radiotherapy verification. An available design from Micron Semiconductor Ltd. (BB7, 500 µ m thick) was the base of a two-dimensional detector adapted into a special configuration with the aim of uniforming and minimizing foreing materials around the active area (64 × 64 mm2). With this purpose, two independent BB7 SSSSDs were mounted in a perpendicular configuration, separated by a 500 µ m kapton dielectric film with the same dimensions as the silicon wafers, thus minimizing air gaps in between. This new configuration, called the dual SSSSD chip design, was mounted on kapton printed circuit board (PCB). Both silicon wafers were divided into 32 strips, 2 mm width each. The aim of developing this detector was to allow 2D dose measurements, improve spatial resolution and perform radiotherapy treatment verification faster than with a previous prototype. Characteristics and performance of the novel detector are presented