Quality Of Intensity-modulated Radiation Therapy Plans Research Articles

Defining the dose-volume criteria for laryngeal sparing in locally advanced oropharyngeal cancer utilizing split-field IMRT, whole-field IMRT and VMAT.

PurposeTo determine the optimal dose‐volume constraint for laryngeal sparing using three commonly employed intensity modulated radiation therapy (IMRT) approaches in patients with oropharyngeal cancer treated to the bilateral neck.Materials and methodsThirty patients with stage II‐IVA oropharynx cancers received definitive radiotherapy with split‐field IMRT (SF‐IMRT) to the bilateral neck between 2008 and 2013. Each case was re‐planned using whole‐field IMRT (WF‐IMRT) and volumetric modulated arc therapy (VMAT) and plan quality metrics and dose to laryngeal structures was evaluated. Two larynx volumes were defined and compared on the current study: the Radiation Therapy Oncology Group (RTOG) larynx as defined per the RTOG 1016 protocol and the MDACC larynx defined as the components of the larynx bounded by the superior and inferior extent of the thyroid cartilage.ResultsTarget coverage, conformity, and heterogeneity indices were similar in all techniques. The RTOG larynx mean dose was lower with WF‐IMRT than SF‐IMRT (22.1 vs 25.8 Gy; P < 0.01). The MDACC larynx mean dose was 17.5 Gy ± 5.4 Gy with no differences between the 3 techniques. WF‐IMRT and VMAT plans were associated with lower mean doses to the supraglottic larynx (42.1 vs 41.2 vs 54.8 Gy; P < 0.01) and esophagus (18.1 vs 18.2 vs 36 Gy; P < 0.01).ConclusionsModern whole field techniques can provide effective laryngeal sparing in patients receiving radiotherapy to the bilateral neck for advanced oropharyngeal cancers.SummaryWe evaluated laryngeal dose in patients with locally advanced oropharyngeal cancer treated to the bilateral neck using split‐field IMRT (SF‐IMRT), whole‐field IMRT (WF‐IMRT) and volumetric arc therapy (VMAT). All three techniques provided good sparing of laryngeal structures and were able to achieve a mean larynx dose < 33 Gy. There were no significant differences in dose to target structures or non‐laryngeal organs at risk among techniques.

A Column Generation Approach Based on Region Growth

In intensity-modulated radiation therapy (IMRT), a network flow is adopted to solve the pricing problem of the generic column generation approach in order to obtain a deliverable aperture. However, excessive computation results from the direct use of a network flow. In addition, a decline in plan quality may result from the direct determination of the leaf position using the gradient information. To overcome these problems, a column generation approach based on region growth is proposed. The proposed method is designed to reduce the computational cost of solving the pricing problem and improve the IMRT plan quality. First, the gradients of the beamlets are obtained by an objective function constructed under the constraint conditions of the organs. Second, the gradients are transformed nonlinearly. Third, the positions of the continuous negative gradient regions in each row of the aperture are determined and stored. Fourth, these gradients are taken as a whole and added to the aperture network flow, which is solved as a shortest-path problem. Finally, the deliverable aperture is obtained and added to the treatment plan. To verify the effectiveness of the proposed method, experiments involving five five-field prostate cancer cases and five nine-field head and neck cancer cases were conducted. Compared with the generic column generation method, the dose distribution of the target is ensured by the proposed method, which also effectively protects organs at risk and reduces the running time. Specifically, in ten groups of comparative experiments, the normal tissue complication probability of the proposed method is reduced by up to 3.37%, and the maximum acceleration rate is 20.44%. According to the experimental results, the proposed method is more consistent with clinical requirements compared with the generic column generation method.

The Dosimetric Effects of Different Multileaf Collimator Widths on Physical Dose Distributions

Introduction: Geometric changes in the multileaf collimator (MLC) led to dosimetric considerations in intensity-modulated radiation therapy (IMRT) due to the number and size of the pixels in the intensity map, which are determined by the MLC leaf width. In this study, we evaluated the dosimetric effects of different MLC widths on physical dose distributions for IMRT plans. Materials and Methods: Forty-two IMRT plans based on different MLC devices were generated and analyzed to study the effect of MLC width on plan quality. Results: Improvements in IMRT plan quality using 0.4 cm leaf width in comparison with 1 cm leaf width were evaluated. The 0.4 cm leaf-based plans resulted in significantly higher Dmean, D98%, D95% , D5%, and V95 (58.86 Gy, 95.11%, 96.57%, 104%, and 97.92%, respectively) compared to the 1 cm leaf plans (58.66 Gy, 92.56%, 94.56%, 104.14%, and 95.72%, respectively). Conformation number (CN) for planning target volume in 0.4 cm leaf plans was significantly higher than the 1 cm leaf plans (0.74 vs. 0.67; P<0.05). In addition, the 0.4 cm leaf plans significantly improved dose homogeneity compared to the 1 cm leaf plans (1.08 vs. 1.10; P<0.05). We found that 0.4 cm leaf width significantly decreased the integral dose to normal tissue compared to the 1 cm leaf width (from 56.09 to 49.46 Gy.Kg P<0.05). Conclusion: No significant clinical differences were observed between the two plans for a serially functioning tissue, while the differences in mean doses were statistically significant for parallel functioning normal tissues.

Is it possible for knowledge-based planning to improve intensity modulated radiation therapy plan quality for planners with different planning experiences in left-sided breast cancer patients?

BackgroundKnowledge-based planning (KBP) is a promising technique that can improve plan quality and increase planning efficiency. However, no attempts have been made to extend the domain of KBP for planners with different planning experiences so far. The purpose of this study was to quantify the potential gains for planners with different planning experiences after implementing KBP in intensity modulated radiation therapy (IMRT) plans for left-sided breast cancer patients.MethodsThe model libraries were populated with 80 expert clinical plans from treated patients who previously received left-sided breast-conserving surgery and IMRT with simultaneously integrated boost. The libraries were created on the RapidPlanTM. 6 planners with different planning experiences (2 beginner planners, 2 junior planners and 2 senior planners) generated manual and KBP optimized plans for additional 10 patients, similar to those included in the model libraries. The plan qualities were compared between manual and KBP plans.ResultsAll plans were capable of achieving the prescription requirement. There were almost no statistically significant differences in terms of the planning target volume (PTV) coverage and dose conformality. It was demonstrated that the doses for most of organs-at-risk (OARs) were on average lower or equal in KBP plans compared to manual plans except for the senior planners, where the very small differences were not statistically significant. KBP data showed a systematic trend to have superior dose sparing at most parameters for the heart and ipsilateral lung. The observed decrease in the doses to these OARs could be achieved, particularly for the beginner and junior planners. Many differences were statistically significant.ConclusionsIt is feasible to generate acceptable IMRT plans after implementing KBP for left-sided breast cancer. KBP helps to effectively improve the quality of IMRT plans against the benchmark of manual plans for less experienced planners without any manual intervention. KBP showed promise for homogenizing the plan quality by transferring planning expertise from more experienced to less experienced planners.

The Effect of Beams' Orientations on the Intensity-Modulated Radiation Therapy Plan Quality

Purpose: The work aims at studying forty Intensity-modulated radiation therapy (IMRT) plans designed using KonRad system and evaluating each of them under different conditions to study the effect of beams' orientations on the plan quality. Methods: Clinical step-and-shoot IMRT treatment plans were designed for twenty patients, suffering from different types of non-CNS solid tumors, to be delivery on a Siemens Oncor accelerator with multi-leaf collimators MLCs (82 leaf). To ensure that the similarity or difference between the plans was due to effect of beams' orientations, the same optimization constraints were applied for each plan and all other parameters were kept constant. Results: The analysis of performance was based on isodose distributions, Dose Volume Histograms (DVHs) for Planning Target Volume (PTV), the relevant Organs at Risk (OARs) as well as several physical indices like mean dose (Dmean), maximum dose (Dmax), 95% dose (D95), integral dose, volume of tumor receiving 2 Gy and 5 Gy, total number of segments and monitor units (MUs). Homogeneity index and conformation number were two other evaluation parameters that were considered in this study. Conclusion: All coplanar CP and non-coplanar NC techniques result in clinically acceptable plans, with comparable target doses and dose to critical organs within prescribed dose constraints. For different organs at risk, the coplanar IMRT, the greatest advantage of non-coplanar IMRT was the marked reduction of general trend was that non-coplanar plans achieved the lowest values, while coplanar plans showed the highest. Significant differences (P<0.05) exist only between the homogeneity index HI (1.092 and 1.088) and Monitor Unit MU (357.79 and 341.80) of CP and NC plans respectively. Compared with the integral dose (p=0.007).

Intelligence-guided beam angle optimization in treatment planning of intensity-modulated radiation therapy

An intelligence guided approach based on fuzzy inference system (FIS) was proposed to automate beam angle optimization in treatment planning of intensity-modulated radiation therapy (IMRT). The model of FIS is built on inference rules in describing the relationship between dose quality of IMRT plan and irradiated region of anatomical structure. Dose quality of IMRT plan is quantified by the difference between calculated and constraint doses of the anatomical structures in an IMRT plan. Irradiated region of anatomical structure is characterized by the metric, covered region of interest, which is the region of an anatomical structure under radiation field while beam’s eye-view is conform to target volume. Initially, an IMRT plan is created with a single beam. The dose difference is calculated for the input of FIS and the output of FIS is obtained with processing of fuzzy inference. Later, a set of candidate beams is generated for replacing the current beam. This process continues until no candidate beams is found. Then the next beam is added to the IMRT plan and optimized in the same way as the previous beam. The new beam keeps adding to the IMRT plan until the allowed beam number is reached. Two spinal cases were investigated in this study. The preliminary results show that dose quality of IMRT plans achieved by this approach is better than those achieved by the default approach with equally spaced beam setting. It is effective to find the optimal beam combination of IMRT plan with the intelligence-guided approach.

SU-F-T-355: Evaluation of Knowledge-Based Planning Model for the Cervical Cancer Radiotherapy

Purpose: The Varian RapidPlan™ is a commercial knowledge-based optimization process which uses a set of clinically used treatment plans to train a model that can predict individualized dose-volume objectives. The purpose of this study is to evaluate the performance of RapidPlan to generate intensity modulated radiation therapy (IMRT) plans for cervical cancer. Methods: Totally 70 IMRT plans for cervical cancer with varying clinical and physiological indications were enrolled in this study. These patients were all previously treated in our institution. There were two prescription levels usually used in our institution: 45Gy/25 fractions and 50.4Gy/28 fractions. 50 of these plans were selected to train the RapidPlan model for predicting dose-volume constraints. After model training, this model was validated with 10 plans from training pool(internal validation) and additional other 20 new plans(external validation). All plans used for the validation were re-optimized with the original beam configuration and the generated priorities from RapidPlan were manually adjusted to ensure that re-optimized DVH located in the range of the model prediction. DVH quantitative analysis was performed to compare the RapidPlan generated and the original manual optimized plans. Results: For all the validation cases, RapidPlan based plans (RapidPlan) showed similar or superior results compared to the manual optimized ones. RapidPlan increased the result of D98% and homogeneity in both two validations. For organs at risk, the RapidPlan decreased mean doses of bladder by 1.25Gy/1.13Gy (internal/external validation) on average, with p=0.12/p<0.01. The mean dose of rectum and bowel were also decreased by an average of 2.64Gy/0.83Gy and 0.66Gy/1.05Gy,with p<0.01/ p<0.01and p=0.04/<0.01 for the internal/external validation, respectively. Conclusion: The RapidPlan model based cervical cancer plans shows ability to systematically improve the IMRT plan quality. It suggests that RapidPlan has great potential to make the treatment planning process more efficient.

An Automated Treatment Plan Quality Control Tool for Intensity-Modulated Radiation Therapy Using a Voxel-Weighting Factor-Based Re-Optimization Algorithm.

Intensity-modulated radiation therapy (IMRT) currently plays an important role in radiotherapy, but its treatment plan quality can vary significantly among institutions and planners. Treatment plan quality control (QC) is a necessary component for individual clinics to ensure that patients receive treatments with high therapeutic gain ratios. The voxel-weighting factor-based plan re-optimization mechanism has been proved able to explore a larger Pareto surface (solution domain) and therefore increase the possibility of finding an optimal treatment plan. In this study, we incorporated additional modules into an in-house developed voxel weighting factor-based re-optimization algorithm, which was enhanced as a highly automated and accurate IMRT plan QC tool (TPS-QC tool). After importing an under-assessment plan, the TPS-QC tool was able to generate a QC report within 2 minutes. This QC report contains the plan quality determination as well as information supporting the determination. Finally, the IMRT plan quality can be controlled by approving quality-passed plans and replacing quality-failed plans using the TPS-QC tool. The feasibility and accuracy of the proposed TPS-QC tool were evaluated using 25 clinically approved cervical cancer patient IMRT plans and 5 manually created poor-quality IMRT plans. The results showed high consistency between the QC report quality determinations and the actual plan quality. In the 25 clinically approved cases that the TPS-QC tool identified as passed, a greater difference could be observed for dosimetric endpoints for organs at risk (OAR) than for planning target volume (PTV), implying that better dose sparing could be achieved in OAR than in PTV. In addition, the dose-volume histogram (DVH) curves of the TPS-QC tool re-optimized plans satisfied the dosimetric criteria more frequently than did the under-assessment plans. In addition, the criteria for unsatisfied dosimetric endpoints in the 5 poor-quality plans could typically be satisfied when the TPS-QC tool generated re-optimized plans without sacrificing other dosimetric endpoints. In addition to its feasibility and accuracy, the proposed TPS-QC tool is also user-friendly and easy to operate, both of which are necessary characteristics for clinical use.

Comparison of Planning Quality and Efficiency Between Conventional and Knowledge-Based Algorithms in Nasopharyngeal Cancer Patients Using Intensity Modulated Radiation Therapy

Intensity modulated radiation therapy (IMRT) is being widely used for its capability of achieving highly conformal dose and improving locoregional control. However, treatment plan quality and planning time are institute- and planner-dependent, and there is no standardized tool to recognize an optimal plan. A knowledge-based algorithm (KBA) can generate automated constraints to assist optimization and produce planner-independent, high-quality plans within a short time. This report compares the efficiency and quality of IMRT plans generated with and without KBA-assisted IMRT (KBA_IMRT) for nasopharyngeal carcinoma (NPC) patients. Seventy-nine NPC patients treated with radical IMRT at our institution between the years of 2013 and 2014 were used to configure the KBA_IMRT system. Ten consecutive NPC cases indicated for radical radiation therapy were then recruited to evaluate its performance. For these 10 cases, both the IMRT plan quality and planning time with and without using KBA_IMRT were compared using paired t test. Ten consecutive NPC patients treated with radical intent between October 2014 and January 2015 were recruited into the study. Their stage distributions were stage I (1), II (0), III (7), IVA (1), and IVB (1). The treatment gave 70 Gy to PTV1 (for gross tumor volume [GTV]), 63 Gy to PTV2 (for adjacent high-risk structures and bilateral retropharyngeal, levels IB, II, III, VA cervical lymph nodes), and 52.5 Gy to PTV3 (for prophylactic nodal irradiation), over 35 fractions. The results were shown in Table 1. There were no significant differences in dose coverage to GTVP (primary tumor), GTVN (involved nodes), and PTV1. The conformity index to PTV1 was comparable but the homogeneity index with KBA_IMRT was slightly worse. KBA_IMRT achieved a better sparing to brainstem (54.0 Gy vs 56.4 Gy, P=.027) and parotid glands (left: 36.2 Gy vs 43.4 Gy; right: 40.3 Gy vs 45.0 Gy, P<.05), but gave a higher dose to spinal cord (45.6 Gy vs 43.9 Gy, P=.037). The planning time was reduced almost by half using KBA_IMRT (144.5 minutes vs 261.3 minutes, P<.001). This study demonstrates that KBA_IMRT can significantly improve planning efficiency and produce quality IMRT plans for NPC patients. Further fine-tuning of KBA_IMRT parameters and other clinical applications including adaptive radiation therapy are being tested in our center.Poster Viewing Abstracts 2795; Table 1Manual planKBA_IMRTP valueGTVP (min dose)67.6 Gy66.7 Gy.071GTVN (min dose)70.2 Gy70.0 Gy.556% PTV1 < 100% dose2.7%5.4%.192Conformity index PTV11.1501.134.079Homogeneity index PTV10.1220.196.005*Brainstem (max dose)56.4 Gy54.0 Gy.027*Spinal cord (max dose)43.9 Gy45.6 Gy.037*Chiasm (max dose)54.3 Gy54.2 Gy.65Lt Parotid Gland (mean dose)43.4 Gy36.2 Gy.001*Rt Parotid Gland (mean dose)45.0 Gy40.3 Gy.039*Planning time261.3 minutes144.5 minutes<.001**P<.05 was considered statistically significant Open table in a new tab

Automatically-generated rectal dose constraints in intensity-modulated radiation therapy for prostate cancer

The dose constraint during prostate intensity-modulated radiation therapy (IMRT) optimization should be patient-specific for better rectum sparing. The aims of this study are to suggest a novel method for automatically generating a patient-specific dose constraint by using an experience-based dose volume histogram (DVH) of the rectum and to evaluate the potential of such a dose constraint qualitatively. The normal tissue complication probabilities (NTCPs) of the rectum with respect to V%ratio in our study were divided into three groups, where V%ratio was defined as the percent ratio of the rectal volume overlapping the planning target volume (PTV) to the rectal volume: (1) the rectal NTCPs in the previous study (clinical data), (2) those statistically generated by using the standard normal distribution (calculated data), and (3) those generated by combining the calculated data and the clinical data (mixed data). In the calculated data, a random number whose mean value was on the fitted curve described in the clinical data and whose standard deviation was 1% was generated by using the ‘randn’ function in the MATLAB program and was used. For each group, we validated whether the probability density function (PDF) of the rectal NTCP could be automatically generated with the density estimation method by using a Gaussian kernel. The results revealed that the rectal NTCP probability increased in proportion to V%ratio, that the predictive rectal NTCP was patient-specific, and that the starting point of IMRT optimization for the given patient might be different. The PDF of the rectal NTCP was obtained automatically for each group except that the smoothness of the probability distribution increased with increasing number of data and with increasing window width. We showed that during the prostate IMRT optimization, the patient-specific dose constraints could be automatically generated and that our method could reduce the IMRT optimization time as well as maintain the IMRT plan quality.

Impact of dose calculation accuracy during optimization on lung IMRT plan quality

The purpose of this study was to evaluate the effect of dose calculation accuracy and the use of an intermediate dose calculation step during the optimization of intensity‐modulated radiation therapy (IMRT) planning on the final plan quality for lung cancer patients. This study included replanning for 11 randomly selected free‐breathing lung IMRT plans. The original plans were optimized using a fast pencil beam convolution algorithm. After optimization, the final dose calculation was performed using the analytical anisotropic algorithm (AAA). The Varian Treatment Planning System (TPS) Eclipse v11, includes an option to perform intermediate dose calculation during optimization using the AAA. The new plans were created using this intermediate dose calculation during optimization with the same planning objectives and dose constraints as in the original plan. Differences in dosimetric parameters for the planning target volume (PTV) dose coverage, organs‐at‐risk (OARs) dose sparing, and the number of monitor units (MU) between the original and new plans were analyzed. Statistical significance was determined with a p‐value of less than 0.05. All plans were normalized to cover 95% of the PTV with the prescription dose. Compared with the original plans, the PTV in the new plans had on average a lower maximum dose (69.45 vs. 71.96 Gy, p=0.005), a better homogeneity index (HI) (0.08 vs. 0.12, p=0.002), and a better conformity index (CI) (0.69 vs. 0.59, p=0.003). In the new plans, lung sparing was increased as the volumes receiving 5, 10, and 30 Gy were reduced when compared to the original plans (40.39% vs. 42.73%, p=0.005; 28.93% vs. 30.40%, p=0.001; 14.11% vs. 14.84%, p=0.031). The volume receiving 20 Gy was not significantly lower (19.60% vs. 20.38%, p=0.052). Further, the mean dose to the lung was reduced in the new plans (11.55 vs. 12.12 Gy, p=0.024). For the esophagus, the mean dose, the maximum dose, and the volumes receiving 20 and 60 Gy were lower in the new plans than in the original plans (17.91 vs. 19.24 Gy, p=0.004; 57.32 vs. 59.81 Gy, p=0.020; 39.34% vs. 41.59%, p=0.097; 12.56% vs. 15.35%, p=0.101). For the heart, the mean dose, the maximum dose, and the volume receiving 40 Gy were also lower in new plans (11.07 vs. 12.04 Gy, p=0.007; 56.41 vs. 57.7 Gy, p=0.027; 7.16% vs. 9.37%, p=0.012). The maximum dose to the spinal cord in the new plans was significantly lower than in the original IMRT plans (29.1 vs. 31.39 Gy, p=0.014). Difference in MU between the IMRT plans was not significant (1216.90 vs. 1198.91, p=0.328). In comparison to the original plans, the number of iterations needed to meet the optimization objectives in the new plans was reduced by a factor of 2 (2–3 vs. 5–6 iterations). Further, optimization was 30% faster corresponding to an average time savings of 10–15 min for the reoptimized plans. Accuracy of the dose calculation algorithm during optimization has an impact on planning efficiency, as well as on the final plan dosimetric quality. For lung IMRT treatment planning, utilizing the intermediate dose calculation during optimization is feasible for dose homogeneity improvement of the PTV and for improvement of optimization efficiency.PACS numbers: 87.55.D‐, 87.55.de, 87.55.dk

SU-E-T-720: Maintaining Prostate IMRT Plan Quality Using the Fraction of Rectum Overlapping the PTV

Purpose: It is clinically meaningful to generate the optimal intensity‐modulated radiation therapy (IMRT) treatment plans within the restricted time with equally good quality for different patients, which is planner‐dependent. We developed a planner‐independent method for a clinical decision of prostate IMRT plan using the rectal complication probability with the volume of rectum overlapping the PTV. Methods: Eighteen prostate IMRT plans were retrospectively analyzed. The prescription dose in the all the planning target volumes (PTVs) was normalized with 76 Gy in 2 Gy fractions. We found a correlation between the fraction of rectum overlapping the PTV and the rectal normal tissue complication probability (NTCP), which is the predictable NTCP curve, which means the lower bound of the rectal NTCP. The plans beyond the predictable NTCP criteria were generated again with seven 10 MV beams from the Varian Clinac iX linear accelerator. The beams were arranged coplanar at: 0, 50, 100, 140, 220, 260, and 310. IMRT plans were generated using direct machine parameter optimization in the Pinnacle3. Results: All the regenerated plans came to be included in the predictable NTCP criteria. In our patient data, two plans were re‐planned: the NTCP for patient 7 was changed from 10.9 to 8.8 and the other NTCP for patient 14 was from 15.6 to 12.9. The rectal NTCP is clinically more meaningful than the mean dose of rectum. Conclusion: This model can predict the rectum NTCP before IMRT planning, to control the IMRT plan quality during planning. Hence we can maintain prostate IMRT plan quality using the fraction of rectum overlapping the PTV.

A Comprehensive Comparison of IMRT and VMAT Plan Quality for Prostate Cancer Treatment

We performed a comprehensive comparative study of the plan quality between volumetric-modulated arc therapy (VMAT) and intensity-modulated radiation therapy (IMRT) for the treatment of prostate cancer. Eleven patients with prostate cancer treated at our institution were randomly selected for this study. For each patient, a VMAT plan and a series of IMRT plans using an increasing number of beams (8, 12, 16, 20, and 24 beams) were examined. All plans were generated using our in-house-developed automatic inverse planning (AIP) algorithm. An existing eight-beam clinical IMRT plan, which was used to treat the patient, was used as the reference plan. For each patient, all AIP-generated plans were optimized to achieve the same level of planning target volume (PTV) coverage as the reference plan. Plan quality was evaluated by measuring mean dose to and dose-volume statistics of the organs at risk, especially the rectum, from each type of plan. For the same PTV coverage, the AIP-generated VMAT plans had significantly better plan quality in terms of rectum sparing than the eight-beam clinical and AIP-generated IMRT plans (p < 0.0001). However, the differences between the IMRT and VMAT plans in all the dosimetric indices decreased as the number of beams used in IMRT increased. IMRT plan quality was similar or superior to that of VMAT when the number of beams in IMRT was increased to a certain number, which ranged from 12 to 24 for the set of patients studied. The superior VMAT plan quality resulted in approximately 30% more monitor units than the eight-beam IMRT plans, but the delivery time was still less than 3 min. Considering the superior plan quality as well as the delivery efficiency of VMAT compared with that of IMRT, VMAT may be the preferred modality for treating prostate cancer.

Influence of photon beam energy on IMRT plan quality for radiotherapy of prostate cancer

Summary Background Intensity-modulated radiation therapy (IMRT) has been widely used for prostate cancer treatments. 6MV photon beams were found to be an effective energy choice for most IMRT cases. The use of high-energy photons raise concerns about increased leakage and secondary neutron dose for the patients. Aim In this work, the effect of beam energy on the quality of IMRT plans for prostate radiotherapy was systematically studied for competing IMRT plans optimized for delivery with either 6 or 10MV beams. Materials and Methods A cohort of 20 prostate cases was selected for this study. All patients received full-course IMRT treatments to a dose of 79.2Gy to PTV in 44 fractions. For all of the cases we developed treatment plans using 6 MV and 10MV intensity-modulated beams with identical dose volume constraints. Results Percentage of doses received by the percentage volume of PTV was higher for 6MV photons compared to 10MV photons for 12 patients, less than or equal to 1% for 6 patients and 2.6%, 3.6% for the remaining 2 patients irrespective of the PTV volume. Percentage doses received by 15% of bladder volume were higher for 10 MV photons. Percentage doses received by 15% of rectum volume were also higher for 10 MV photons. Conclusions Since there is no greater advantage from 10MV photons as compared with 6MV photons in large volume pelvic IMRT dosimetry and also 10MV photons lie on the threshold energy border for the induction of photo neutrons from the accelerator components, we recommend the use of 6MV photons for IMRT of prostate cancer to achieve better results in tumour control and acceptable probability of complication rate.

The Effect of Beam Energy on the Quality of IMRT Plans for Prostate Conformal Radiotherapy

Three dimensional conformal radiation therapy (3DCRT) for prostate cancer is most commonly delivered with high-energy photons, typically in the range of 10-21 MV. With the advent of Intensity Modulated Radiation Therapy (IMRT), an increase in the number of monitor units (MU) relative to 3DCRT has lead to a concern about secondary malignancies. This risk becomes more relevant at higher photon energies where there is a greater neutron contribution. Subsequently, the majority of IMRT prostate treatments being delivered today are with 6-10 MV photons where neutron production is negligible. However, the absolute risk is small [Hall, E. J. Intensity Modulated Radiation Therapy, Protons, and the Risk of Second Cancers. Int J Radiat Oncol Bio Phys 65, 1-7 (2006); Kry, F. S., Salehpour, M., Followill, D. S., Stovall, M., Kuban, D. A., White, R. A., and Rosen, I. I. The Calculated Risk of Fatal Secondary Malignancies From Intensity Modulated Radiation Therapy. Int J Radiat Oncol Bio Phys 62, 1195-1203 (2005).] and therefore it has been suggested that the use of an 18MV IMRT may achieve better target coverage and normal tissue sparing such that this benefit outweighs the risks. This paper investigates whether 18MV IMRT offer better target coverage and normal tissue sparing. Computed Tomography (CT) image sets of ten prostate cancer patients were acquired and two separate IMRT plans were created for each patient. One plan used 6 MV beams, and the other used 18 MV, both in a coplanar, non-opposed beam geometry. Beam arrangements and optimization constraints were the same for all plans. This work includes a comparison and discussion of the total integral dose, neutron dose conformity index, and total number of MU for plans generated with both energies.

Reduction of IMRT beam complexity through the use of beam modulation penalties in the objective function

Inverse planned intensity modulated radiation therapy (IMRT) has become commonplace in treatment centers across the world. Due to the implications of beam complexity on treatment planning, delivery, and quality assurance, several methods have been proposed to reduce the complexity. These methods include beamlet intensity restrictions, smoothing procedures, and direct aperture optimization. Many of these methods typically sacrifice target coverage and/or normal tissue sparing in return for increased beam smoothness and delivery efficiency. In the present work, we penalize beam modulation in the inverse planning cost function to reduce beam complexity and increase delivery efficiency, while maintaining dosimetric quality. Three modulation penalties were tested: two that penalized deviation from Savitzky-Golay filtered versions of the optimized beams, and one that penalized the plan intensity map variation (a measure of overall beam modulation). The modulation penalties were applied at varying weights in a weighted sum objective (or cost) function to investigate their ability to reduce beam complexity while preserving IMRT plan quality. The behavior of the penalties was characterized on a CT phantom, and then clinical optimization comparisons were performed in the brain, prostate, and head/neck. Comparisons were made between (i) plans with a baseline cost function (ii) plans with a baseline cost function employing maximum beamlet intensity limits, and (iii) plans with each of the modulation penalties added to the baseline cost function. Plan analysis was based upon dose-volume histograms, relevant dose metrics, beam modulation, and monitor units required for step and shoot delivery. Each of the techniques yielded improvements over a baseline cost function in terms of MU reduction. In most cases, this was achieved with minimal change to the plan DVHs and metrics. In all cases, an acceptable plan was reached with each of the methods while reducing MU substantially. Each individual method has merit as a tool for reducing IMRT beam complexity and could be easily applied in the clinic to improve overall inverse plan quality. However, the penalty based upon the plan intensity map variation consistently produced the most delivery-efficient plans with the fewest computations.

Effect of beamlet step‐size on IMRT plan quality

We have studied the degree to which beamlet step-size impacts the quality of intensity modulated radiation therapy (IMRT) treatment plans. Treatment planning for IMRT begins with the application of a grid that divides each beam's-eye-view of the target into a number of smaller beamlets (pencil beams) of radiation. The total dose is computed as a weighted sum of the dose delivered by the individual beamlets. The width of each beamlet is set to match the width of the corresponding leaf of the multileaf collimator (MLC). The length of each beamlet (beamlet step-size) is parallel to the direction of leaf travel. The beamlet step-size represents the minimum stepping distance of the leaves of the MLC and is typically predetermined by the treatment planning system. This selection imposes an artificial constraint because the leaves of the MLC and the jaws can both move continuously. Removing the constraint can potentially improve the IMRT plan quality. In this study, the optimized results were achieved using an aperture-based inverse planning technique called direct aperture optimization (DAO). We have tested the relationship between pencil beam step-size and plan quality using the American College of Radiology's IMRT test case. For this case, a series of IMRT treatment plans were produced using beamlet step-sizes of 1, 2, 5, and 10 mm. Continuous improvements were seen with each reduction in beamlet step size. The maximum dose to the planning target volume (PTV) was reduced from 134.7% to 121.5% and the mean dose to the organ at risk (OAR) was reduced from 38.5% to 28.2% as the beamlet step-size was reduced from 10 to 1 mm. The smaller pencil beam sizes also led to steeper dose gradients at the junction between the target and the critical structure with gradients of 6.0, 7.6, 8.7, and 9.1 dose%/mm achieved for beamlet step sizes of 10, 5, 2, and 1 mm, respectively.

Effectiveness of noncoplanar IMRT planning using a parallelized multiresolution beam angle optimization method for paranasal sinus carcinoma

To determine the effectiveness of noncoplanar beam configurations and the benefit of plans using fewer but optimally placed beams designed by a parallelized multiple-resolution beam angle optimization (PMBAO) approach. The PMBAO approach uses a combination of coplanar and noncoplanar beam configurations for intensity-modulated radiation therapy (IMRT) treatment planning of paranasal sinus cancers. A smaller number of beams (e.g. 3) are first used to explore the solution space to determine the best and worst beam directions. The results of this exploration are then used as a starting point for determining an optimum beam orientation configuration with more beams (e.g. 5). This process is parallelized using a message passing interface, which greatly reduces the overall computation time for routine clinical practice. To test this approach, treatment for 10 patients with paranasal sinus cancer was planned using a total of 5 beams from a pool of 46 possible beam angles. The PMBAO treatment plans were also compared with IMRT plans designed using 9 equally spaced coplanar beams, which is the standard approach in our clinic. Plans with these two different beam configurations were compared with respect to dose conformity, dose heterogeneity, dose-volume histograms, and doses to organs at risk (i.e., eyes, optic nerve, optic chiasm, and brain). The noncoplanar beam configuration was superior in most paranasal sinus carcinoma cases. The target dose homogeneity was better using a PMBAO 5-beam configuration. However, the dose conformity using PMBAO was not improved and was case dependent. Compared with the 9-beam configuration, the PMBAO configuration significantly reduced the mean dose to the eyes and optic nerves and the maximum dose to the contralateral optical path (e.g. the contralateral eye and optic nerve). The maximum dose to the ipsilateral eye and optic nerve was also lower using the PMBAO configuration than using the 9-beam configuration, although this difference was not significant. The mean doses to the optic chiasm and brain are marginally lower using the PMBAO configuration than using 9-beam configuration. The maximum doses to the optic chiasm and brain are the same with the PMBAO configuration and the 9-beam configuration. Parallelized multiple-resolution beam angle optimization with an optimized noncoplanar beam configuration is an effective and practical approach for IMRT treatment planning. Five-beam treatment plans optimized using the PMBAO are at least equivalent to, and overall better than, the plans using 9 equally spaced coplanar beams.

Effects of field parameters on IMRT plan quality for gynecological cancer: A case study

Traditional external beam radiotherapy of gynecological cancer consists of a 3D, four‐field‐box technique. The radiation treatment area is a large region of normal tissue, with greater inhomogeneity over the treatment volume, which could benefit more with intensity‐modulated radiation therapy (IMRT). This is a case report of IMRT planning for a patient with endometrial cancer. The planning target volume (PTV) spanned the intrapelvic and periaortic lymph nodes to a 33‐cm length. Planning and treatment were accomplished using double isocenters. The IMRT plan was compared with a 3D plan, and the effects of field parameters were studied. Delineated anatomical contours included the intrapelvic nodes (PTV), bone marrow, small bowel, bladder, rectum, sigmoid colon, periaortic nodes (PTV), spinal cord, left kidney, right kidney, large bowel, liver, and tissue (excluding the PTVs). Comparisons were made between IMRT and 3D plans, 23‐MV and 6‐MV energies, zero and rotated collimator angles, different numbers of segments, and opposite gantry angle configurations. The plans were evaluated based on dose‐volume histograms (DVHs). Compared with the 3D plan, the IMRT plan had superior dose conformity and spared the bladder and sigmoid colon embedded in the intrapelvic nodes. The higher energy (23 MV) reduced the dose to most critical organs and delivered less integral dose. Zero collimator angles resulted in a better plan than “optimized” collimator angles, with lower dose to most of the normal structures. The number of segments did not have much effect on isodose distribution, but a reasonable number of segments was necessary to keep treatment time from being prohibitively long. Gantry angles, when evenly spaced, had no noticeable effect on the plan. The patient tolerated the treatment well, and the initial complete blood count was favorable. Our results indicated that large‐volume tumor sites may also benefit from precise conformal delivery of IMRT.PACS numbers: 87.53.Kn, 87.53.Tf

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