Number Of Intensity Levels Research Articles

Does image normalization and intensity resolution impact texture classification?

Image texture is a very important component in many types of images, including medical images. Medical images are often corrupted by noise and affected by artifacts. Some of the texture-based features that should describe the structure of the tissue under examination may also reflect, for example, the uneven sensitivity of the scanner within the tissue region. This in turn may lead to an inappropriate description of the tissue or incorrect classification. To limit these phenomena, the analyzed regions of interest are normalized. In texture analysis methods, image intensity normalization is usually followed by a reduction in the number of levels coding the intensity. The aim of this work was to analyze the impact of different image normalization methods and the number of intensity levels on texture classification, taking into account noise and artifacts related to uneven background brightness distribution. Analyses were performed on four sets of images: modified Brodatz textures, kidney images obtained by means of dynamic contrast-enhanced magnetic resonance imaging, shoulder images acquired as T2-weighted magnetic resonance images and CT heart and thorax images. The results will be of use for choosing a particular method of image normalization, based on the types of noise and distortion present in the images.

The stability of imaging biomarkers in radiomics: a framework for evaluation

This paper studies the sensitivity of a range of image texture parameters used in radiomics to: (i) the number of intensity levels, (ii) the method of quantisation to select the intensity levels and (iii) the use of an intensity threshold. 43 commonly used texture features were studied for the gross target volume outlined on the CT component of PET/CT scans of 50 patients with non-small cell lung carcinoma (NSCLC). All cases were quantised for all values between 4 and 128 intensity levels using four commonly used quantisation methods. All results were analysed with and without a threshold range of −200 HU to 300 HU. Cases were ranked for each texture feature and for all quantisation methods with the Spearman’s rank correlation coefficient determined to evaluate stability. Results showed large fluctuations in ranking, particularly for low numbers of levels, differences between quantisation methods and with the use of a threshold, with values Spearman’s Rank Correlation for many parameters below 0.2. Our results demonstrated the sensitivity of radiomics features to the parameters used during analysis and highlight the risk of low reproducibility comparing studies with slightly different parameters. In terms of the lung cancer CT datasets, this study supports the use of 128 intensity levels, the same uniform quantiser applied to all scans and thresholding of the data. It also supports several of the features recommended in the literature for such studies such as skewness and kurtosis. A recommended framework is presented for curation of the data analysis process to ensure stability of results.

Using high-resolution binary layers and a low-resolution multibit backlight for a layered light-field display

We propose a structure for a layered light-field display composed of high-resolution binary layers and a low-resolution, multibit backlight. This structure aims to increase the upper bound of the spatial frequency while also reducing the total number of bits for the display. The increased layer resolution increases the upper bound of the spatial frequency, meaning that the display can reproduce an object with a large amount of pop-out more clearly than can a conventional light-field display. In contrast, limiting the layers’ transmittance to binary (on/off) levels reduces the total number of bits for the display, thus maintaining the high efficiency of light-field representation. The low-resolution backlight, whose pixels can take multibit values, compensates the number of intensity levels, which would otherwise be quite limited with only the binary layers. Through analytical and experimental results, we show that a display based on the proposed structure can reproduce a light field with high quality and high efficiency as a result of combining the high-resolution binary layers and low-resolution backlight.

Dosimetric Evaluation of Conventional Multileaf Collimator Based Intensity Modulated Radiotherapy Delivery Techniques; A Treatment Planning Study

The purpose of the study was to analyze the effects of the number of intensity levels on treatment planning outcome of static IMRT method with dynamic IMRT method and also to investigate the integral dose to non-target tissues in both the methods. The IMRT planning was carried out using Eclipse treatment planning system with millennium 120 multileaf collimator (Varian Clinac- 2100 DHX). Five cases each of head and neck, cervix and esophagus cancer were selected for this study. For each case, planning was carried out using both delivery methods. Further for the static IMRT, different numbers of intensity levels ranging from 5 to 20 were studied. The optimization values were kept common for both the techniques and only the leaf motion calculation was varied. The parameters associated with the Dose volume histograms were examined for a more quantitative comparison. The integral doses (0.5 Gy to 30 Gy) of Non-target tissues were also calculated for both techniques. Analyses were performed using a t test to determine difference in any of the parameters examined. For three sites studied, there were no significant changes observed between static IMRT (above 10 intensity levels) and dynamic IMRT method. However there were significant differences observed with 5 intensity level static IMRT plans compared to dynamic IMRT plans. There were no significant changes observed in normal tissue dose values between static IMRT plans and dynamic IMRT plans. The total number of monitor unit was more for dynamic IMRT plans compared to static IMRT plans for all three sites. The integral doses from 0.5 Gy to 30 Gy were analyzed and no significant changes were observed between static IMRT and dynamic IMRT plans.

SU-E-T-797: Effects of the Number of Intensity Levels in Step&shoot IMRT for Prostate Treatments

Purpose: Step&shoot IMRTTreatment planning requires the conversion of an optimized fluence into deliverable sequences of MLC segments. The objective of this paper is to study the effects of varying the intensity levels (IL's) number in prostate step&shoot IMRT treatments. Methods: Patient data with prostate cancer and IMRT treatment indication was selected. Treatment planning was done using a 6MV 7‐field arrangement (Primus linac, Optifocus MLC, 10mm leaf‐width) and Konrad v2.2 (Siemens) TPS. The treatment plan includes the simultaneous irradiation of the prostate to 82Gy and the seminal vesicles to 64Gy in 41 fractions. Four plans were generated using 5, 7, 10 and 15 IL's and normalized to deliver the same minimum dose to the prostate. Comparisons between plans were done in terms of total segment number, MU, treatment time and DVH for PTVs and OARs (rectum, bladder and femoral heads). Results: The total segment numbers were 44, 60, 84 and 116 from 5 to 15 IL's. The MU's total numbers decreased with the increase of the IL's (527, 522, 518 and 508). Increasing the number of IL's was accompanied by the treatment times raise (11%, 23% and 37% in relation to 5 IL's, 387seg) and resulted in increases of Prostate PTV dose homogeneity and conformity index (CI). The maximum dose (CI) was 87.7Gy (0.957) and 89.7Gy (0.836) for 15 and 5 IL respectively. No difference in minimum dose and CI was found for the seminal vesicle PTV. All plans showed insignificant OAR DVH differences. Conclusions: The increment in the IL's number resulted in an improvement of the Prostate PTV homogeneity and treatment time without any extra sparing to OAR. The benefit of a better uniform dose in the Prostate PTV should be analyzed for each treatment in order to set planning guidelines.

Robust and Geometric Invariant Watermarking Scheme using Block and Gray-Level Histograms

As one of the crucial problems in watermarking, robustness against geometric distortion and JPEG compression becomes more than challenging and problematic. In this paper, a robust and geometric invariant watermarking scheme using block histogram and intensity-level histograms is proposed. During watermark embedding procedure, we first compute the number of valid intensity-levels of the given image, and group the pixels into blocks of variable size such that each of which has the same number of intensity-levels. Thus the block histogram is generated for the given image. Then the intensity-level histogram of each block is modified according to the embedding bits of the watermark. During the extraction procedure, the block histogram and intensity-level histogram of each block in the watermarked image are generated in a similar manner and the watermark is decoded according to the distribution of the intensity-level histogram in each block. Experimental results show that the proposed scheme is highly robust against not only JPEG compression, but also geometric attacks and common signal processing such as median filtering. The comparison results also show that the proposed scheme outperforms the existing methods in terms of robustness.

Optimal planning parameters for simultaneous boost IMRT treatment of prostate cancer using a Beam Modulator™

Objective To determine the optimum energy and beam arrangement for prostate intensity-modulated radiation therapy (IMRT) delivery using an Elekta Beam Modulator™ linear accelerator, in order to inform decisions when commissioning IMRT for prostate cancer. Methods CMS XiO was used to create IMRT plans for a prostate patient. Arrangements with 3, 5, 7, 9 and 11 equally spaced fields, containing both a direct anterior and a direct posterior beam were used, with both 6 MV and 10 MV photons. The effects of varying the maximum number of iterations, leaf increment, number of intensity levels and minimum segment size were investigated. Treatment plans were compared using isodose distributions, conformity indices for targets and critical structures, target dose homogeneity, body dose and plan complexity. Results Target dose conformity and homogeneity and sparing of critical structures improved with an increasing number of beams, although any improvements were small for plans containing more than five fields. Set-ups containing a direct posterior field provided superior conformality around the rectum to anterior beam arrangements. Mean non-target dose and total number of monitor units were higher with 6 MV for all beam arrangements. The dose distribution resulting from seven 6 MV beams was considered clinically equivalent to that with five 10 MV beams. Conclusion Methods have been developed to plan IMRT treatments using XiO for delivery with a Beam Modulator™ that fulfil demanding dose criteria, using many different set-ups. This study suggests that 6 MV photons can produce prostate IMRT plans that are comparable to those using 10 MV. Work is ongoing to develop a complete class solution.

Investigation of Simple IMRT Delivery Techniques for Non-Small Cell Lung Cancer Patients with Respiratory Motion Using 4DCT

Techniques for generating simplified IMRT treatment plans for treating non-small cell lung cancer (NSCLC) patients with respiratory motion were investigated. To estimate and account for respiratory motion, 4-dimensional computed tomography (4DCT) datasets from 5 patients were used to design 5-field 6-MV ungated step-and-shoot intensity modulated radiotherapy (IMRT) plans delivering a dose of 66 Gy to the planning target volume (PTV). For each patient, 2 plans were generated using the mean intensity and the maximum intensity of 10 CT datasets from different breathing phases. The plans also utilized different margins around the clinical target volume/internal target volume (CTV/ITV) to account for tumor motion. To reduce the treatment time and ensure accurate dose delivery to moving targets, the number of intensity levels was minimized while maintaining dose coverage to PTV and minimizing dose to organs at risk (OARs). Dose-volume histograms (DVHs), dosimetric metrics, and outcome probabilities were evaluated for all plans. Plans using the averaged CT image dataset were inferior, requiring larger margins around the PTV, with a maximum of 1.5 cm, to ensure coverage of the tumor, and therefore increased the dose to OARs located in proximity of the tumor. The plans based on superimposed CT image datasets achieved full coverage of the tumor, while allowing tight margins around the PTV and minimizing the dose to OARs. A small number of intensity-levels (3 to 5), resulting in IMRT plans with a total of 13 to 30 segments, were sufficient for homogeneous PTV coverage, without affecting the sparing of OARs. In conclusion, a technique involving treatment planning with the superimposed CT scans of all respiratory phases, and the application of IMRT with only a small number of segments was feasible despite significant tumor motion; however, greater patient numbers are needed to support the statistical significance of the results presented in this work.

Comparison of the KonRad IMRT and XiO treatment planning systems

Reducing the treatment time for IMRT patients is highly desirable. The objective of this work was to evaluate the new clinical Siemens KonRad inverse treatment planning system (TPS) and compare it to the CMS XiO TPS with special emphasis on the segmentation efficiency. For head and neck, liver and prostate cancer patients, step‐and‐shoot IMRT plans were designed using both CMS XiO and Siemens KonRad TPS. Number, direction and energy of beams used were the same in the plans from both systems for each treatment site. The plans were optimized to achieve the same clinical objectives concerning dose to the target volume and to the relevant organs‐at‐risk (OARs). The number of intensity levels were minimized until the clinical objectives could not be achieved anymore. Dose‐volume histograms (DVHs), mean and maximum doses were compared, as well as the number of beam segments and monitor units (MUs). The beams of each plan were delivered individually on a MapCheck device to verify the agreement between calculations and measurements to be less than 3%–3 mm distance‐to‐agreement. Plans optimized with KonRad resulted in fewer segments and lower number of MUs and therefore reduced delivery time on average by 28% or 3.6 min, while achieving similar dose distributions. CMS XiO plans exhibited a slightly steeper dose fall‐off outside the target volumes; however the difference was not clinically significant. DVHs to OARs were comparable. All calculated dose distributions passed the 3%–3 mm verification check.PACS numbers: 87.55.D

SU-GG-T-68: The Impact of Intensity Levels and Number of Segments On Skullbase IMRT Plan Quality and Efficiency

Purpose: The aim of this study was to investigate the impact of the number of intensity levels and segments, on plan quality and efficiency, when planning treatment for skullbase lesions. Method and materials: Five previously treated cases of skullbase meningioma were re‐planned using inversed planned IMRT to an escalated dose of 60Gy/30♯. 3 IMRT plans were calculated for each case, using 5, 10 and 15 intensity levels. These plans were then compared, using physical (PTV Dmax, V100, V95, OAR Dmax, conformity index CI) and radiobiological parameters (Equivalent Uniform Dose EUD, Integral dose ID). Plan efficiency was measured as a function of ‘beam on’ time. Results: The optimal plan generated was a 7–9 coplanar beam arrangement in all cases. Doses to OAR were within clinical tolerance levels. There was no statistically significant difference between OAR sparing or PTV coverage based on the intensity level used to generate the plans. (p>0.05) The average V90%, V95% and V100% across all plans was 96.2%, 93.3% and 84.6% respectively. The average EUD and CI were 62Gy and 1.3 respectively. Brainstem Dmax was limited to an average of 47.4Gy (range 37.9Gy to 52.7Gy), Optics Dmax to an average of 48.9Gy (range 40Gy to 51.8Gy) and ID was limited to an average of 10.2Gy (range 3.4 to 27 Gy). On average, the numbers of segments for a 5, 10 and 15 level plan were 85, 171 and 238 respectively. Beam on time per fraction increased from an average of 17mins for a 5 intensity level plan to 50 minutes for a 15 intensity level plan. Discussion: When using IMRT for skullbase lesions, our department has adopted a standard intensity level of 5 for treatment planning. This will result in maximal plan efficiency without compromising plan quality. Conflict of interest: Research sponsored by Siemens Oncology Solutions.

WE‐C‐M100F‐10: Evaluation of the Beam Segmentation Algorithm in the KonRad IMRT Treatment Planning System

Purpose: Reducing the treatment time for IMRT patients is highly desirable. For step‐and‐shoot IMRT plans, the delivery time of a treatment fraction is determined by the number of beam segments, and to a lesser extent by the total number of monitor units (MUs). The objective of this work was to study the segmentation efficiency of the new clinical Siemens KonRad inverse treatment planning system (TPS) and compare it to the CMS XiO TPS. Method and Materials: For head and neck, liver and prostate cancer patients, step‐and‐shoot IMRT plans were designed using both CMS XiO and Siemens KonRad. Number, direction and energy of beams were the same in both systems. The plans were optimized to achieve the same clinical objectives concerning dose to the target volume and to the relevant organs‐at‐risk. The number of intensity levels were minimized until the clinical objectives could not be achieved anymore. DVHs, EUD, mean‐ and maximum‐doses were compared, as well as the number of beam segments and MUs. Finally the beams of each plan were delivered individually on a MapCheck device to verify the agreement between calculations and measurements to be less than 3%–3 mm distance‐to‐agreement. Results: Plans optimized with KonRad resulted in fewer segments and lower number of MUs and therefore reduced delivery time, while achieving similar dose distributions. CMS XiO plans exhibited a slightly steeper dose fall‐off outside the target volumes, however the difference was not clinically significant. DVHs to organs‐at‐risk were comparable. All calculated dose distributions passed the 3%–3 mm dose verification check. Conclusion: Both commercial systems produce clinical similar plans. The sequencer of the KonRad system is more efficient, reducing the number of beam segments and therefore delivery time on average by more than 20%.

Dosimetric and QA aspects of Konrad inverse planning system for commissioning intensity-modulated radiation therapy

The intensity-modulated radiation therapy (IMRT) planning is performed using the Konrad inverse treatment planning system and the delivery of the treatment by using Siemens Oncor Impression Plus linear accelerator (step and shoot), which has been commissioned recently. The basic beam data required for commissioning the system were generate. The quality assurance of relative and absolute dose distribution was carried out before clinical implementation. The salient features of Konrad planning system, like dependence of grid size on dose volume histogram (DVH), number of intensity levels and step size in sequencer, are studied quantitatively and qualitatively.To verify whether the planned dose [from treatment planning system (TPS)] and delivered dose are the same, the absolute dose at a point is determined using CC01 ion chamber and the axial plane dose distribution is carried out using Kodak EDR2 in conjunction with OmniPro IMRT Phantom and OmniPro IMRT software from Scanditronix Wellhofer. To obtain the optimum combination in leaf sequencer module, parameters like number of intensity levels, step size are analyzed. The difference between pixel values of optimum fluence profile and the fluence profile obtained for various combinations of number of intensity levels and step size is compared and plotted. The calculations of the volume of any RT structure in the dose volume histogram are compared using grid sizes 3 mm and 4 mm. The measured and planned dose at a point showed good agreement (<3%) except for a few cases wherein the chamber was placed in a relatively high dose gradient region. The axial plane dose distribution using film dosimetry shows excellent agreement (correlation coefficient >0.97) in all the cases. In the leaf sequencer module, the combination of number of intensity level 7 with step size of 3 is the optimal solution for obtaining deliverable segments. The RT structure volume calculation is found to be more accurate with grid size of 3 mm for clinical use.Thus a study regarding various aspects of commissioning of the Konrad inverse planning system for IMRT has been presented, which has been implemented in our clinic.

SU‐FF‐T‐106: A Greedy Segment Generating Algorithm for IMRT

Purpose: Time is one of the factors that affect IMRT. Decreasing the number of segment is necessary. Knowing the fact that the most optimal number of segments can be reached is a help to estimate the plan's optimization level on time factor. Also an ideal optimal number of segments can help to overcome MLC delivery constraint without increasing the segment number too much. Therefore, greedy algorithm is used to find an optimal number of unconstraint segments. Method and Materials: For each IMRT intensity map, an objective function is defined as intensity level multiplied by the area which is the number of grids. Then two‐ dimension search problem turns into one dimension search problem so that greedy strategy can be used. The search space turns into O(N), which N is the number of intensity level. By sorting the objective function values, the intensity level satisfying that volume with maximum dose is delivered first (MDDF). Iteratively, an optimal segments sequence is generated. Given maximum step size, with the intensity map from IMRT plan being generated by CMS Xio plan system, this number of segments is compared with that generated by slide window algorithm. Results: To deliver the same intensity map dose, MDDF algorithm gives the smallest number of segments compared to slide window algorithm from CMS Xio plan system given maximum step size. Conclusion: The optimal delivery segments sequence generated by using MDDF algorithm can be an estimate of the IMRT delivery optimization level and the sequence can be a start for embedding MLC hardware constraint by divide‐and‐conquer strategy.

TU-C-T-6C-01: Introduction: “How much complexity is necessary for IMRT?

IMRT treatment planning and delivery systems have developed rapidly over the past 5 years. Every manufacturer of linear accelerators now offers IMRT delivery systems using multileaf collimators and some sort of automated delivery scheme. Most treatment planning systems now offer “inverse” planning algorithms that optimize fluence and dose distributions given a set of dose constraints for tumors and normal tissues. For some accelerator manufacturers, the output of the planning system is a set of multileaf shapes, or “segments” with associated intensities, which combine to give a variable intensity distribution for each beam angle. This delivery system is referred to as static multileaf IMRT (or SMLC). For others, the intensity distribution is continuous and the leaves move with variable speed across the field with the beam remaining on. This is called dynamic multileaf IMRT, or DMLC. SMLC tends to be somewhat time consuming relative to conventional delivery, as there is overhead due to communication between the MLC controller and the accelerator. For DMLC, very little time is lost relative to conventional beam delivery times. Most IMRT planning systems have a mechanism for controlling the complexity of the resulting plan, which is determined by the number of beam angles and the number of intensity levels specified in the plan. Highly complex plans are those that end up with large numbers of beam segments and small monitor units per segment, translating into rapid dose gradients within the patient. These are presumably desirable when a tumor is immediately adjacent to a very sensitive normal tissue and the dose falloff must be rapid outside the tumor border. On the other hand, dose delivery errors are expected to be greater with highly complex plans, since the dosimetry of small fields and the linearity of the dose at small numbers of monitor units, are rather uncertain. This fact drives us to limit the complexity as much as reasonably possible. Since DMLC does not have a significant time penalty for high complexity, there is a tendency to plan highly complex IMRT for these delivery systems. For SMLC, the constraints of treatment time tend to drive the planner to choose less complex solutions. The question to be addressed in this Symposium is how much complexity is necessary for IMRT. Clearly, the answer is site-dependent, but even at a given site, the answer is not clear. Today's speakers will describe the incentives for simplicity (time and dose accuracy), methods of controlling the complexity and the tradeoffs between complexity and conformality.

Planning quality and delivery efficiency of sMLC delivered IMRT treatment of oropharyngeal cancers evaluated by RTOG H-0022 dosimetric criteria

The time required to deliver intensity-modulated radiation therapy (IMRT) treatments can be significantly longer than conventional treatments, especially for the segmented multileaf collimator (sMLC) delivery system with a large record and verification (R&V) overhead. In this work, we evaluate the impact of the number of intensity-modulated beams (IMBs) and the number of intensity levels (ILs) on the quality and delivery efficiency of IMRT plans, generated by the Corvus planning system for sMLC delivery on a Siemens LINAC with the Lantis R&V system. Detailed studies were performed for three image data sets of previously treated oropharyngeal patients. Treatment plans for patient 1 were developed using 5, 7, 9, or 15 evenly spaced axial IMBs as well as one with 7 axial IMBs whose directions were user-selected, each using ILs of 3, 5, 10, or 20. For patients 2 and 3, plans with 15 IMBs and 20 ILs were not attempted. A total of 42 plans were developed using three oropharyngeal cancer CT image data sets. Plan quality was evaluated by assessing compliance with the Radiation Therapy Oncology Group (RTOG) H-0022 protocol criteria and the physician's clinical judgment. Plan efficiency was accessed by the number of segments of each plan. We found that for our treatment-planning and delivery system, an IMRT plan that uses a moderate number of IMBs and ILs, such as 7 or 9 IMBs with 3 or 5 ILs, would appear to be the optimal approach when both quality of the plan and delivery efficiency are considered. Based on this study, we have routinely used 9 IMBs with 3 ILs or 7 IMBs with 5 ILs for head and neck patients. A retrospective comparison indicates that delivery efficiency is improved on the order of 30% compared to plans generated with 9 IMBs with 5 ILs. PACS number: 87.53.Tf

773 poster Number of intensity levels and fluence smoothing in step and shoot IMRT for sinonasal carcinomas

773 poster Number of intensity levels and fluence smoothing in step and shoot IMRT for sinonasal carcinomas

Planning quality and delivery efficiency of sMLC delivered IMRT treatment of oropharyngeal cancers evaluated by RTOG H‐0022 dosimetric criteria

The time required to deliver intensity‐modulated radiation therapy (IMRT) treatments can be significantly longer than conventional treatments, especially for the segmented multileaf collimator (sMLC) delivery system with a large record and verification (R&V) overhead. In this work, we evaluate the impact of the number of intensity‐modulated beams (IMBs) and the number of intensity levels (ILs) on the quality and delivery efficiency of IMRT plans, generated by the Corvus planning system for sMLC delivery on a Siemens LINAC with the Lantis R&V system. Detailed studies were performed for three image data sets of previously treated oropharyngeal patients. Treatment plans for patient 1 were developed using 5, 7, 9, or 15 evenly spaced axial IMBs as well as one with 7 axial IMBs whose directions were user‐selected, each using ILs of 3, 5, 10, or 20. For patients 2 and 3, plans with 15 IMBs and 20 ILs were not attempted. A total of 42 plans were developed using three oropharyngeal cancer CT image data sets. Plan quality was evaluated by assessing compliance with the Radiation Therapy Oncology Group (RTOG) H‐0022 protocol criteria and the physician's clinical judgment. Plan efficiency was accessed by the number of segments of each plan. We found that for our treatment‐planning and delivery system, an IMRT plan that uses a moderate number of IMBs and ILs, such as 7 or 9 IMBs with 3 or 5 ILs, would appear to be the optimal approach when both quality of the plan and delivery efficiency are considered. Based on this study, we have routinely used 9 IMBs with 3 ILs or 7 IMBs with 5 ILs for head and neck patients. A retrospective comparison indicates that delivery efficiency is improved on the order of 30% compared to plans generated with 9 IMBs with 5 ILs.PACS number: 87.53.Tf

A scheme for broadening the scope of IMRT optimization

IMRT planning usually pre-determine fixed field or rotational delivery, neither are truly optimal in terms of dosimetry and delivery efficiency. The number of beams or arcs and their orientations are also pre-selected. Inverse planning is limited to finding the intensities for the selected beams. This study is to test a new scheme for broadening the scope of optimization to include the selection of the delivery scheme (rotational, gantry-fixed, or hybrid), the number and orientation of the beams and arcs. Angular preference is quantified using Angular Cost Function (ACF). A beam angle is more preferential when it contributes more to the objective function used for optimization. Instead of simply selecting the angles with higher values, we choose to fit the ACF with “delivery sectors”. The basic idea is illustrated in Figure 1. The ACF for an oropharyngeal cancer is approximated with delivery sectors (rectangles). For this case, we have two delivery sectors (at the bottom) with angular range suitable for arc delivery. The remaining delivery sectors can be delivered with a fixed beam at 355o and a beam at 180o. With this basic scheme, all the user needs to specify are the objectives of the treatment. We have tested the new scheme with clinical examples. For the case of Figure 1, we compared the ACF guided choice of two fixed fields in combination with two arcs with 5 equal-spaced fields each with 7 intensity levels. Figures 2 a) b) c) and d) show the DVH comparison of the hybrid technique (dashed) and fixed-field technique (solid) for the target, spinal cord, right and left parotid. The hybrid technique improved dose uniformity for the target by 1.3% and reduced 30% dose volume to the right parotid by 11%. The dose to the spinal cord was also significantly lower. A scheme of broadening the scope of optimization for IMRT has been developed and tested. The scheme automates the selection of the delivery scheme (rotational, gantry fixed, or hybrid), gantry angles, the number of intensity levels, and the number of arcs and their angular ranges. The ACF reveals the inherent set of freedoms for satisfying the objectives, and the new scheme provides a way to maximally use the available freedom with efficient delivery. View Large Image Figure ViewerDownload (PPT)

Intensity modulated arc deliveries approximated by a large number of fixed gantry position sliding window dynamic multileaf collimator fields.

The intensity modulated arc has been proposed as an alternative to tomotherapy. Treatment planing systems more typically model the conventional step and shoot or sliding window dynamic multileaf collimator (DMLC) deliveries, and may not support intensity modulated arc therapy (IMAT). As well, another potential drawback to this technique is that increasing the number of intensity levels required to achieve certain dose distributions necessitates increasing the number of gantry passes, as may occur if the desired dose distribution is complex (e.g., concave or bifurcated), potentially increasing the overall treatment time. A technique is presented here for the delivery of tomotherapy like dose distributions in a single gantry pass by the use of a large number of fields modulated by a sliding window DMLC technique from fixed equally spaced gantry positions. This serves as a good approximation to either IMAT or tomotherapy deliveries. The planning of these fields is achieved using iterative filtered back projection. Measured results of deliveries of varying degrees of complexity on a homogeneous phantom are compared to desired distributions.

A quality and efficiency analysis of the IMFAST segmentation algorithm in head and neck "step & shoot" IMRT treatments.

The performance of segmentation algorithms used in IMFAST for "step & shoot" IMRT treatment delivery is evaluated for three head and neck clinical treatments of different optimization objectives. The segmentation uses the intensity maps generated by the in-house TPS PLANUNC using the index-dose minimization algorithm. The dose optimization objectives include PTV dose uniformity and dose volume histogram-specified critical structure sparing. The optimized continuous intensity maps were truncated into five and ten intensity levels and exported to IMFAST for MLC segments optimization. The MLC segments were imported back to PLUNC for dose optimization quality calculation. The five basic segmentation algorithms included in IMFAST were evaluated alone and in combination with either tongue and groove/match line correction or fluence correction or both. Two criteria were used in the evaluation: treatment efficiency represented by the total number of MLC segments and optimization quality represented by a clinically relevant optimization quality factor. We found that the treatment efficiency depends first on the number of intensity levels used in the intensity map and second the segmentation technique used. The standard optimal segmentation with fluence correction is a consistent good performer for all treatment plans studied. All segmentation techniques evaluated produced treatments with similar dose optimization quality values, especially when ten-level intensity maps are used.