Conformity Of Dose Distribution Research Articles

Short course irradiation in rectal cancer: Dosimetry study comparing 3D conformal radiotherapy versus intensity-modulated radiotherapy versus volumetric modulated arc therapy

The standard radiotherapy planning technique for locally advanced rectal cancer (LARC) was 3D Conformal Radiotherapy (3DCRT). Modern radiation delivery techniques, including Intensity-Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) techniques, have been developed in recent years in terms of dose delivery to the target while minimizing the dose to the organ at risk (OAR). The objective of this study was to identify the most suitable treatment plan technique from the available options of 3DCRT, IMRT and VMAT for LARC using short course radiotherapy (SCRT). Therefore, twenty-five adult patients with LARC in the last five-year period (2017–2021) were involved in this study. For each patient, three radiotherapy treatment plans were generated for dosimetric comparison of short course irradiation (25 Gy/5 fractions, which is the new standard of care) using Varian Eclipse software to generate 3DCRT in comparison with IMRT and VMAT plans. The conformity index (CI), the homogeneity index (HI), and the organs at risk (OAR), including bladder, small bowel, and femoral heads, were compared between the three plans. The results revealed that VMAT and IMRT plans had a higher level of conformity for the planning target volume (PTV) with 0.996 and 0.994, respectively, compared to the 3DCRT planning of 0.947 (both p < 0.005). The PTV dose distribution was less homogeneous with 3DCRT than with VMAT or IMRT plans. VMAT offered superior homogeneity to IMRT and 3DCRT plans with 1.055, 1.066, and 1.089, respectively (p < 0.005). The findings showed that VMAT is superior to IMRT and 3DCRT in almost all parameters, including dose distribution conformity and homogeneity of the target volume, as well as providing better OAR sparing in LARC patients.

Proton therapy in pediatric neuro-oncology. Potential advantages and the relationship between cost and results

The aim of study: to analyze the accumulated experience and try to identify those clinical situations in which the use of protons will be economically and clinically more effective than photon radiation therapy. Material and methods. The articles devoted to the study of the evaluation and comparison of the effectiveness of proton and photon radiation beams in the treatment of tumors of the central nervous system and published over the past 25 years were searched in the Medline, Embase and the Cochrane Library databases. Results. the analysis of available publications has shown that accelerated protons do not improve survival rates and disease-free rates in all forms of brain tumors compared with photon therapy. However, protons can significantly increase the level of dose distribution conformity and reduce the dose to critical structures (pituitary gland, cochlea, eye lenses, hypothalamus), thus reducing the risk of hearing and visual impairment as well as hormonal and cognitive disorders. All this is critically important for potentially curable malignant tumors, such as medulloblastoma and germinoma, for low malignant potential tumors (grade 1–2 glioma) or tumors with decreased metastatic potential, since proton therapy compared to photon therapy reduces the risk of late side effects that worsen the quality of life of cured children. Conclusion. central nervous system tumors are one of the most common solid malignant neoplasms in children. Radiation therapy (RT) is recognized as an important therapeutic component of treatment and is often used in strategies for multimodal therapy of tumors of the central nervous system in children. Proton radiation therapy is one of the attractive methods of radiotherapy with minimal dose distribution to normal tissues and a decrease in the absorbed dose. The precision of protons reduces the risk of long-term side effects associated with this type of treatment and the induction of secondary malignancies, which is of particular importance for the quality of life.

Toward an improved assessment of dose conformity in radiotherapy.

Evaluation of dose conformity is important to ensure minimum dose to normal tissue and sufficient dose coverage of the planning target volume (PTV). The existing conformity indices depend on the PTV volume and do not differentiate between two different scenarios: overdosing normal tissue and underdosing PTV. In this study, we introduce a novel index to assess conformity of dose distributions in radiotherapy. The suggested conformity index is defined by the ratio of the volume representing actual "non-conformity" of the planned dose and the volume representing acceptable "non-conformity." The latter volume is produced by expanding the PTV. If both the average distance ( ) between the reference isodose surface and planning target volume and arbitrarily selected PTV expansion margin ( ) are much smaller than the size of the PTV, approximately equals the ratio . In this work, was utilized to analyze 90 cases of brain metastases treated with stereotactic radiation therapy (SRS) and 102 cases of lung cancer treated with stereotactic body radiation therapy (SBRT). For =0.1cm, all considered SRS treatment plans were characterized by while 2 out of 102 SBRT plans had . The average values of for SRS and SBRT plans were 0.31 and 0.43, respectively. For =0.2cm, all studied treatment plans had , and the average values of for SRS and SBRT plans were 0.15 and 0.25, respectively. The suggested conformity index varies less with PTV volume than the RTOG and Riet-Paddick indices frequently used for evaluation of dose conformity. In addition, can be expressed as a sum of two terms which describe "over-coverage" and "under-coverage" of the treatment target. The results confirm that can be used for evaluation of dose conformity in SRS and SBRT.

Comparison of helical and TomoDirect techniques with simultaneous integrated boost in early breast cancer patients.

The aim of the study was to perform dosimetric comparisons of helical (H) and TomoDirect (TD) plans for whole-breast irradiation (WBI) with simultaneous integrated boost (SIB) in early-stage breast cancer patients undergoing breast conserving surgery. Fifty patients, 25 with left-side and 25 with right-side tumors, were determined for a treatment planning system for a total dose of 50.4Gy in 1.8Gy per fraction to WBI, with a SIB of 2.3Gy per fraction delivered to the tumor bed. The planning target volume (PTV) doses and the conformity (CI) and homogeneity indices (HI) for PTVbreast and PTVboost, as well as organ-at-risk (OAR) doses and treatment times, were compared between the H and TD plans. All plans met the PTV coverage criteria for the H plan, except for mean V107 of PTVbreast for TD plan. The H plan yielded better homogeneity and conformity of dose distribution compared to the TD plan. The ipsilateral mean lung doses were not significantly different between the two plans. The TD plans is advantageous for mean doses to the heart, contralateral breast and lung, spinal cord, and esophagus than the H plans. In both the H and TD plans, the right-sided breast patients had lower heart dose parameters than the left-sided breast patients. The TD plan is superior to the H plan in sparing the contralateral breast and lung by decreasing low-dose volumes. While the OAR dose advantages of TD are appealing, shorter treatment times or improved dose homogeneity and conformity for target volume may be advantageous for H plan.

Advances in Skin Brachytherapy: Cosmesis and Function Preservation

Skin brachytherapy represents an excellent alternative treatment for patients with non-melanoma skin cancers. It offers superior conformity of dose distribution with rapid dose fall off, reducing the risk of radiotherapy-related treatment toxicity. A smaller treatment volume in brachytherapy, when compared with external beam radiotherapy, is conducive for hypofractionation, which is an attractive option for decreasing outpatient visits to the cancer centre, especially for elderly and frail patients. Skin brachytherapy is an excellent option to preserve function and cosmesis, especially in skin cancers located in the head and neck region. Electronic brachytherapy, image-guided superficial brachytherapy and 3D printed moulds are all emerging advances in skin brachytherapy.

Angle of concavity in planning target volume can be adopted as selection criteria for intensity-modulated radiation therapy or three-dimensional conformal radiotherapy technique in brain tumors.

With innovation of medical imaging, radiotherapy attempts to conform the high dose region to the planning target volume (PTV). The present work aimed to assess the angle of concavity in PTV can be adopted as selection criteria for intensity-modulated radiation therapy (IMRT) or three-dimensional conformal radiotherapy (3DCRT) technique in Brain tumors. Thirty previously irradiated patients with brain tumors were replanned with both 3DCRT and IMRT technique. Angle of concavity (dip) in the PTV near the organs at risk was measured in the contoured structure set images of each patient. These cases were divided into three groups where angles were 0°, >120° and <120°. Dose of 60 Gy/30# was fixed. In Group 1, the IMRT plan had better TV95% as compared to 3DCRT respectively with significant P value (P = 0.002). Mean of conformity index (CI) and Homogeneity Index (HI) were comparable. For Group 2 (angle >120°), the IMRT plan had better TV95% as compared to 3DCRT respectively with a significant P value (P = 0.021). HI and CI were not significant. For Group 3 (<120°), IMRT plan had better TV95% as compared to 3DCRT respectively with a significant P value (P = 0.001). HI and CI were better in IMRT arm with significant P value. The results from this study showed that the angle of concavity can be considered as an additional objective tool for selection criteria whether tumor can be treated with IMRT or 3DCRT. Tumors where angle of concavity was <120°, HI and CI provided more uniformity and conformity of dose distribution inside PTV with significant P values.

Introducing new plan evaluation indices for prostate dose painting IMRT plans based on apparent diffusion coefficient images

BackgroundDose painting planning would be more complicated due to different levels of prescribed doses and more complex evaluation with conventional plan quality indices considering uniform dose prescription. Therefore, we tried to introduce new indices for evaluating the dose distribution conformity and homogeneity of treatment volumes based on the tumoral cell density and relative volumes of each lesion in prostate IMRT.MethodsCT and MRI scans of 20 male patients having local prostate cancer were used for IMRT DP planning. Apparent diffusion coefficient (ADC) images were imported to a MATLAB program to identify lesion regions based on ADC values automatically. Regions with ADC values lower than 750 mm2/s and regions with ADC values higher than 750 and less than 1500 mm2/s were considered CTV70Gy (clinical tumor volume with 70 Gy prescribed dose), and CTV60Gy, respectively. Other regions of the prostate were considered as CTV53Gy. New plan evaluation indices based on evaluating the homogeneity (IOE(H)), and conformity (IOE(C)) were introduced, considering the relative volume of each lesion and cellular density obtained from ADC images. These indices were compared with conventional homogeneity and conformity indices and IOEs without considering cellular density. Furthermore, tumor control probability (TCP) was calculated for each patient, and the relationship of the assessed indices were evaluated with TCP values.ResultsIOE (H) and IOE (C) with considering cellular density had significantly lower values compared to conventional indices and IOEs without considering cellular density. (P < 0.05). TCP values had a stronger relationship with IOE(H) considering cell density (R2 = -0.415), and IOE(C) without considering cell density (R2 = 0.624).ConclusionIOE plan evaluation indices proposed in this study can be used for evaluating prostate IMRT dose painting plans. We suggested to consider cell densities in the IOE(H) calculation formula and it’s appropriate to calculate IOE(C) without considering cell density values.

Novel approach for the evaluation of dose conformity in radiotherapy.

We describe a new approach to evaluate conformity of dose distributions in radiotherapy. The suggested conformity factor λ is defined by using existing conformity indices and expansion of the planning target volume (PTV). If the average distance ( ) between the PTV and reference isodose surface and an arbitrarily selected PTV expansion margin ( ) are both much smaller than the size of the PTV, then λ approximately equals the ratio . We use λ to analyze several cases of stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). In the case of SRS with a single target or multiple targets, treatment plans produced with the help of volumetric modulated arc therapy (VMAT) have smaller λ than plans produced by using dynamic conformal arcs (DCA). Likewise, it is demonstrated that in the case of SBRT, λ is reduced by employing VMAT instead of DCA. It is also shown that if the distance between the reference isodose surface and surface of the PTV is fixed, λ varies less with variations in PTV volume compared to frequently used conformity indices. The described conformity factor λ can be applied clinically to compare and rank treatment plans for lesions of different sizes. It is suggested that conditions and λ>1 can be employed as "pass" and "fail" criteria, respectively, for dose conformity assessment with appropriate choice of .

MR-Guided Adaptive Radiotherapy for OAR Sparing in Head and Neck Cancers

Simple SummaryNormal tissue toxicities in head and neck cancer persist as a cause of decreased quality of life and are associated with poorer treatment outcomes. The aim of this article is to review organ at risk (OAR) sparing approaches available in MR-guided adaptive radiotherapy and present future developments which hope to improve treatment outcomes. Increasing the spatial conformity of dose distributions in radiotherapy is an important first step in reducing normal tissue toxicities, and MR-guided treatment devices presents a new opportunity to use biological information to drive treatment decisions on a personalized basis.MR-linac devices offer the potential for advancements in radiotherapy (RT) treatment of head and neck cancer (HNC) by using daily MR imaging performed at the time and setup of treatment delivery. This article aims to present a review of current adaptive RT (ART) methods on MR-Linac devices directed towards the sparing of organs at risk (OAR) and a view of future adaptive techniques seeking to improve the therapeutic ratio. This ratio expresses the relationship between the probability of tumor control and the probability of normal tissue damage and is thus an important conceptual metric of success in the sparing of OARs. Increasing spatial conformity of dose distributions to target volume and OARs is an initial step in achieving therapeutic improvements, followed by the use of imaging and clinical biomarkers to inform the clinical decision-making process in an ART paradigm. Pre-clinical and clinical findings support the incorporation of biomarkers into ART protocols and investment into further research to explore imaging biomarkers by taking advantage of the daily MR imaging workflow. A coherent understanding of this road map for RT in HNC is critical for directing future research efforts related to sparing OARs using image-guided radiotherapy (IGRT).

Feasibility of interstitial stepping-source electronic brachytherapy to locally inoperable tumors.

PurposeRadiotherapy is the mainstay in the treatment of locally inoperable tumors. Interstitial electronic needle-based kilovoltage brachytherapy (EBT) could be an economic alternative to high-dose-rate (HDR) brachytherapy or permanent seed implantation (PSI). In this work, we evaluated if locally inoperable tumors treated with PSI at our institution may be suitable for EBT.Material and methodsA total of 10 post-interventional computed tomography (CT) scans of patients, who received PSI and simulated stepping-source EBT applied with Intrabeam system and needle applicator were used. EBT treatment planning software with 3-dimensional image and projection of applicator were applied for designing trajectories and establishing dwell positions. Dwell position doses were summarized, and doses covering 90% of the target volume (D90) achieved with stepping-source EBT were compared to those of PSI. Additionally, conformality of dose distributions and total irradiation time were assessed using conformation number (CN) or conformal index (COIN).ResultsIn all patients, D90 of EBT exceeded the prescribed dose or D90 of PSI on average by 4.7% or 21.3% relative to the prescribed dose, respectively. Mean number of trajectories was 5.0 for EBT and 6.9 for PSI. Average CN/COIN for EBT was 0.69, with a mean irradiation time of 27.8 minutes for standardized dose of 13 Gy.ConclusionsStepping-source EBT allowed for a conformal treatment of inoperable interstitial tumors with similar D90. Fewer trajectories were required for EBT in majority of cases.

Analysis of different evaluation indexes for prostate stereotactic body radiation therapy plans: conformity index, homogeneity index and gradient index

AbstractObjectiveThis study analyzed different definitions of the conformity index (CI), homogeneity index (HI), and gradient index (GI) used in evaluation of prostate cancer stereotactic body radiation therapy.MethodsA total of 10 patients with localized prostate cancer staged T1–T2a were selected randomly, from which two stereotactic body radiation therapy plans were designed with CyberKnife and EDGE systems for each patient based on the same images and contours. CI, HI, and GI with different definitions were calculated based on a dose‐volume histogram of treatment plans.ResultsFor four definitions of CI, the results showed that the values calculated by the Radiation Therapy Oncology Group, Lomax and Scheib, and Van't Riet were closer to 1 for EDGE plans, except for that by the Saint‐Anne, Lariboisiere, and Tenon group. Meanwhile, CI values of Van't Riet et al were lower than other definitions, which showed high accuracy to describe the conformity between the target volume and prescribed isodose line. For five definitions of HI, CyberKnife plans showed lower values than EDGE plans. HI calculated based on the D2 (D5) and D98 (D95) were lower than that calculated by the Dmax to Dmin, whereas the standard deviation was higher than others. For the dose GI, the results of effective radius and modified GI showed that EDGE plans had a steeper dose fall‐off, which is inverse to the conventional GI.ConclusionsThese indexes can all be objective tools for the evaluation of plan quality. Our results showed better conformity of dose distribution and dose gradient for EDGE plans, but better uniformity for CyberKnife plans. The conformation number and S‐index were recommended to accurately describe the conformity and uniformity of dose distribution. The ∆Riso and modified GI were also recommended to calculate the dose gradient.

Dose Distribution Analysis of Rapid Arc and Intensity Modulated Radiotherapy Plan in Head and Neck Cancer

Introduction: Globally, intensity-modulated radiation therapy (IMRT) is considered as highly precise and accurate method of radiotherapy planning. This technique amplifies spatial dose distribution conformity by modulating the intensity of radiation beams in each sub-volume. Additionally, it can reduces the dose to surrounding critical organs and deliver the planned dose to targets with the nominal risk of side effects. Material and Methods: In this study, 13 patients with head and neck cancer were randomly taken for analysis. The IMRT and Rapid Arc plans were generated for each case in the Eclipse treatment planning system, version 11.0. There were seven to nine beams deployed in IMRT plan, while Rapid Arc plans were performed using two arcs with opposite direction of rotation. Portal dosimetry plans were created and analyzed before executing the plan on the patient. Results: The mean of V95%(Target’s volume covered by 95% isodose line)was 97.89% and 97.47% for Rapid Arc and IMRT plans, respectively. Moreover, mean standard deviations were found 1.93 and 1.70 in Rapid Arc and IMRT plans, respectively. The mean gamma index was97.55% and 98.43% in Rapid Arc and IMRT, respectively. Conclusion: IMRT technique was slightly better in the treatment of head and neck cancer compared to the Rapid Arc method. The only advantage of Rapid Arc was saving the treatment time by two to three times on an average compared with IMRT. It is prudent to use IMRT technique in head and neck cancer treatment.

Image guided high-dose-rate brachytherapy versus volumetric modulated arc therapy for head and neck cancer: A comparative analysis of dosimetry for target volume and organs at risk.

BackgroundThe aim of the study was to present dosimetric comparison of image guided high-dose-rate brachytherapy (IGBT) with volumetric modulated arc therapy (VMAT) for head and neck cancer regarding conformity of dose distribution to planning target volume (PTV) and doses to organs at risk (OARs).Patients and methodsThirty-eight consecutive patients with T1-4 mobile tongue, floor of mouth and base of tongue cancer treated with IGBT were selected. For these patients additional VMAT treatment plans were also prepared using identical computed tomography data. OARs and PTV related parameters (e.g. V98, D0.1cm3, Dmean, etc.) were compared.ResultsMean V98 of the PTV was 90.2% vs. 90.4% (p > 0.05) for IGBT and VMAT, respectively. Mean D0.1cm3 to the mandible was 77.0% vs. 85.4% (p < 0.05). Dmean to ipsilateral and contralateral parotid glands was 4.6% vs. 4.6% and 3.0% vs. 3.9% (p > 0.05). Dmean to ipsilateral and contralateral submandibular glands was 16.4% vs. 21.9% (p > 0.05) and 8.2% vs. 16.9% (p < 0.05), respectively.ConclusionsBoth techniques showed excellent target coverage. With IGBT dose to normal tissues was lower than with VMAT. The results prove the superiority of IGBT in the protection of OARs and the important role of this invasive method in the era of new external beam techniques.

Spatial frequency performance limitations of radiation dose optimization and beam positioning

The flexibility and sophistication of modern radiotherapy treatment planning and delivery methods have advanced techniques to improve the therapeutic ratio. Contemporary dose optimization and calculation algorithms facilitate radiotherapy plans which closely conform the three-dimensional dose distribution to the target, with beam shaping devices and image guided field targeting ensuring the fidelity and accuracy of treatment delivery. Ultimately, dose distribution conformity is limited by the maximum deliverable dose gradient; shallow dose gradients challenge techniques to deliver a tumoricidal radiation dose while minimizing dose to surrounding tissue. In this work, this ‘dose delivery resolution’ observation is rigorously formalized for a general dose delivery model based on the superposition of dose kernel primitives. It is proven that the spatial resolution of a delivered dose is bounded by the spatial frequency content of the underlying dose kernel, which in turn defines a lower bound in the minimization of a dose optimization objective function. In addition, it is shown that this optimization is penalized by a dose deposition strategy which enforces a constant relative phase (or constant spacing) between individual radiation beams. These results are further refined to provide a direct, analytic method to estimate the dose distribution arising from the minimization of such an optimization function. The efficacy of the overall framework is demonstrated on an image guided small animal microirradiator for a set of two-dimensional hypoxia guided dose prescriptions.

The Role of SABR for Eradicative Treatment of Thymomas Recurrences

Surgery usually represents the standard treatment of Thymoma with neoadjuvant or adjuvant chemoradiotherapy in locally advanced stages.Frequently the recurrences of disease appears on the pleura. The relative radiosensitivity of thymomas and the small tendency to systemic spread offer curative possibility to local eradicative therapies.We have evaluated in terms to local control, OS and toxicity the SABR directed to all sites of relapses. Between April 2014 and Dicember 2016, 14 pts with thymoma underwent SABR for pleural relapses Median age was 43 ys, with KPS >90. 9 pts presented myasthenia gravis at diagnosis, at the onset all patients were treated by surgery .Thymic epithelial tumours were staged according to the Masaoka-Koga staging system, 8/14 pts were stage B3, 4/14 pts were stage B2, 1/14 was B1, 1/14 was stage AB. Relapses in pleural regions were detected by CT in 100% pts, and PET-CT in 86% pts. 50% pts were treated with somatostatin analogue and all pts received corticosteroid. 51 lesions were treated with SABR. SBRT was delivered by 6MV Linac using beam modulator equipped with 4 mm MLC, through two coplanar arcs with VMAT optimization. Median GTV volume was 36.6 cc (2-173 cc). The prescription dose was 30Gy/3fx , 26 Gy /1fx , 24 Gy/3fx to the 80% isodose, in 39/51, 6/51, 7/51 lesions respectively. Set-up and isocenter position was controlled before each fx using CBCT .The response was evaluated 60 days after SBRT by CT and PET scan and every 4 months successively.Toxicity was assessed by CTCAE score. Only 12/14 pts and 39/51 lesions have been evaluated for response. Median FU was 19.28 (range 7-37) months the median survival was 17.7 months. All Pts are still alive and all lesions treated are in complete response. 6/12 pts had relapses outside of treatment sites, these recurrences were treated by SABR. Only grade 2 pneumonitis occurred in 1 pts, who underwent to SBRT for three ipsilateral pleural lesions simulataneously. In our experience SABR appears safe and effective for curative treatment of thymoma relapses. The LC of 100% of primary and following relapses and the disease chest limited lead to curative effect of SABR. All pts are still alive without any evidence of disease. The high conformity of dose distribution using VMAT limits the treatment relate toxicity.

Dosimetric comparison between helical tomotherapy and intensity‐modulated radiotherapy for esophageal carcinoma

AbstractObjectiveTo compare the effectiveness of different radiotherapy techniques – tomotherapy (TOMO) and intensity‐modulated radiation therapy (IMRT) – for patients with esophageal carcinoma on the basis of dosimetric analysis.MethodsThe target areas and organs at risk in 10 patients with esophageal carcinoma were underlined and transmitted to the Raystation and TOMO planning systems. The homogeneity index (HI) and conformal index (CI) values, and dose distributions to organs at risk were compared on the basis of dose–volume histograms parameters.ResultsThe median of HI of gross planning target volume by TOMO (HI = 0.0575) showed superior homogeneity dose distribution compared with the IMRT technique (HI = 0.0735, P = 0.047). The median of CI of planning target volume by TOMO (CI = 0.785) showed improved conformity of dose distribution compared with IMRT (CI = 0.6825, P = 0.009). The median of the maximum dose to planning gross tumor volume (PGTV) and planning target volume by TOMO was significantly decreased compared with IMRT. The median of total lung doses, total heart doses, and maximum doses to the spinal cord were all significantly reduced by TOMO compared with IMRT.ConclusionsCompared with IMRT, TOMO could provide better conformal target coverage, more homogeneity dose distribution, and significantly decreased the radiation dose to the lungs, heart, and spinal cord.

Development of a new ridge filter with honeycomb geometry for a pencil beam scanning system in particle radiotherapy

A ridge filter (RGF), a beam energy modulation device, is usually used for particle radiotherapy with a pencil beam scanning system. The conventional RGF has a one-dimensional (1D) periodic laterally stepped structure in orthogonal plane with a central beam direction. The energy of a beam passing through the different thicknesses of the stepped RGF is modulated. Although the lateral pencil beam size is required to cover the several stepped RGF units to modulate its energy as designed, the current trend is to decrease lateral beam size to improve the scanning system. As a result, the beam size becomes smaller than the size of the individual RGF unit. The aim of this study was to develop a new RGF with two-dimensional (2D) honeycomb geometry to simultaneously achieve both a decrease in lateral beam size and the desired energy modulation.The conventional 1D-RGF and the 2D-RGF with honeycomb geometry were both designed so that the Bragg peak size of a 79MeV/u carbon ion pencil beam in water was 1mm RMS in the beam direction. To validate the design of the 2D-RGF, we calculated depth dose distributions in water using a simplified Monte Carlo method. In the calculations, we decreased the lateral pencil beam size at the entrance of the RGF and investigated the threshold of lateral beam size with which the pencil beam can reproduce the desired Bragg peak size for each type of RGF. In addition, we calculated lateral dose distributions in air downstream from the RGF and evaluated the inhomogeneity of the lateral dose distributions.Using the 2D-RGF, the threshold of lateral beam size with which the pencil beam can reproduce the desired Bragg peak size was smaller than that using the 1D-RGF. Moreover, the distance from the RGF at which the lateral dose distribution becomes uniform was shorter using the 2D-RGF than that using the 1D-RGF. These results indicate that when the periodic length of both RGFs is the same, the 2D-RGF allows use of a pencil beam with smaller lateral beam size and a shorter distance from the RGF to the target, resulting in improvement in the conformity of dose distribution in a tumor.

Improving Head and Neck Cancer Treatments Using Dynamic Collimation in Spot Scanning Proton Therapy.

Interest in using collimation for spot scanning proton therapy has recently increased in an attempt to improve the lateral penumbra. To investigate the advantages of such an approach for complex targets, a plan comparison between uncollimated and collimated beam spots was performed for patients with head and neck cancer. For 10 patients with head and neck cancer, previously treated with spot scanning proton therapy, uncollimated and collimated treatment plans were created using an in-house treatment-planning system capable of modeling asymmetric-beamlet dose distributions resulting from the use of a dynamic collimation system. Both uncollimated and collimated plans reproduced clinically delivered plans in terms of target coverage. A relative plan comparison was performed using both physical and radiobiological metrics on the organs at risk. The dynamic collimation system improved dose-distribution conformity while preserving target coverage. The median reduction of the mean dose to the esophagus, uninvolved larynx, and uninvolved parotids were -11.9% (minimum to maximum, -6.4% to -24.1%), -7.2% (-0.8% to -60.1%), and -5.2% (-0.2% to -21.5%), respectively, and depended on the organ location relative to the target and radiation beam angle. The collimation did not improve dose to some organs at risk surrounded by the target or located upstream of Bragg peaks because of the priority on the target coverage. In spot scanning proton therapy, the dynamic collimation system generally affords better target conformity, which results in improvement in organ-at-risk sparing in the head and neck region while preserving target coverage. However, the benefits of collimation and the increased complexity should be considered for each patient. Patients with large bilateral targets or organs at risk surrounded by the target showed the least benefit.

Theoretical Benefits of Dynamic Collimation in Pencil Beam Scanning Proton Therapy for Brain Tumors: Dosimetric and Radiobiological Metrics

To quantify the dosimetric benefit of using a dynamic collimation system (DCS) for penumbra reduction during the treatment of brain tumors by pencil beam scanning proton therapy (PBS PT). Collimated and uncollimated brain treatment plans were created for 5 patients previously treated with PBS PT and retrospectively enrolled in an institutional review board-approved study. The in-house treatment planning system, RDX, was used to generate the plans because it is capable of modeling both collimated and uncollimated beamlets. The clinically delivered plans were reproduced with uncollimated plans in terms of target coverage and organ at risk (OAR) sparing to ensure a clinically relevant starting point, and collimated plans were generated to improve the OAR sparing while maintaining target coverage. Physical and biological comparison metrics, such as dose distribution conformity, mean and maximum doses, normal tissue complication probability, and risk of secondary brain cancer, were used to evaluate the plans. The DCS systematically improved the dose distribution conformity while preserving the target coverage. The average reduction of the mean dose to the 10-mm ring surrounding the target and the healthy brain were 13.7% (95% confidence interval [CI] 11.6%-15.7%; P<.0001) and 25.1% (95% CI 16.8%-33.4%; P<.001), respectively. This yielded an average reduction of 24.8% (95% CI 0.8%-48.8%; P<.05) for the brain necrosis normal tissue complication probability using the Flickinger model, and 25.1% (95% CI 16.8%-33.4%; P<.001) for the risk of secondary brain cancer. A general improvement of the OAR sparing was also observed. The lateral penumbra reduction afforded by the DCS increases the normal tissue sparing capabilities of PBS PT for brain cancer treatment while preserving target coverage.

TU-EF-304-11: Therapeutic Benefits of Collimation in Spot Scanning Proton Therapy in the Treatment of Brain Cancer

Purpose: A dynamic collimation system (DCS) based on two orthogonal pairs of mobile trimmer blades has recently been proposed to reduce the lateral penumbra in spot scanning proton therapy (SSPT). The purpose of this work is to quantify the therapeutic benefit of using the DCS for SSPT of brain cancer by comparing un-collimated and collimated treatment plans. Methods: Un-collimated and collimated brain treatment plans were created for five patients, previously treated with SSPT, using an in-house treatment planning system capable of modeling collimated and un-collimated beamlets. Un-collimated plans reproduced the clinically delivered plans in terms of target coverage and organ-at-risk (OAR) sparing, whereas collimated plans were re-optimized to improve the organ-at-risk sparing while maintaining target coverage. Physical and biological comparison metrics such as dose distribution conformity, mean and maximum doses, normal tissue complication probability (NTCP) and risk of secondary brain cancer were used to evaluate the plans. Results: The DCS systematically improved the dose distribution conformity while preserving the target coverage. The average reduction of the mean dose to the 10-mm ring surrounding the target and the healthy brain were 7.1% (95% CI: 4.2%–9.9%; p<0.01) and 14.3% (95% CI: 7.8%–20.8%; p<0.01), respectively. This yielded an average reduction of 12.0% (95% CI: 8.2%–15.7%; p<0.01) for the brain necrosis NTCP using the Flickinger model, and 14.2% (95% CI: 7.7%–20.8%; p<0.01) for the risk of secondary brain cancer. The average maximum dose reductions for the brainstem, chiasm, optic nerves, cochleae and pituitary gland when comparing un-collimated and collimated plans were 14.3%, 10.4%, 11.2%, 13.0%, 12.9% and 3.4%, respectively. Evaluating individual plans using the Lyman-Kutcher-Burman NTCP model also yielded improvements. Conclusion: The lateral penumbra reduction performed by the DCS increases the normal tissue sparing capabilities of SSPT for brain tumor treatment while preserving the target coverage. This research was financially supported by Ion Beam Applications S.A. (IBA, Louvain-La-Neuve, Belgium).