Intensity-modulated Radiation Therapy Treatment Plans Research Articles

Optimizing patient positioning for intensity modulated radiation therapy in hippocampal-sparing whole brain radiation therapy

PurposeSparing the hippocampus during whole brain radiation therapy (WBRT) offers potential neurocognitive benefits. However, previously reported intensity modulated radiation therapy (IMRT) plans use multiple noncoplanar beams for treatment delivery. An optimized coplanar IMRT template for hippocampal-sparing WBRT would assist in clinical workflow and minimize resource utilization. In this study, we sought to determine the optimal patient position to facilitate coplanar treatment planning and delivery of hippocampal-sparing WBRT using IMRT. Methods and materialsA variable angle, inclined board was utilized for patient positioning. An anthropomorphic phantom underwent computed tomography simulation at various head angles. The IMRT goals were designed to achieve target coverage of the brain while maintaining hippocampal dose–volume constraints designed to conform to the Radiation Therapy Oncology Group 0933 protocol. Optimal head angle was then verified using data from 8 patients comparing coplanar and noncoplanar WBRT IMRT plans. ResultsHippocampal, hippocampal avoidance region, and whole brain mean volumes were 1.1 cm3, 12.5 cm3, and 1185.1 cm3, respectively. The hippocampal avoidance region occupied 1.1% of the whole brain planning volume. For the 30-degree head angle, a 7-field coplanar IMRT plan was generated, sparing the hippocampus to a maximum dose of 14.7 Gy; D100% of the hippocampus was 7.4 Gy and mean hippocampal dose was 9.3 Gy. In comparison, for flat head positioning the hippocampal Dmax was 22.9 Gy with a D100% of 9.2 Gy and mean dose of 11.7 Gy. Target coverage and dose homogeneity was comparable with previously published noncoplanar IMRT plans. ConclusionsCompared with conventional supine positioning, an inclined head board at 30 degrees optimizes coplanar whole brain IMRT treatment planning. Clinically acceptable hippocampal-sparing WBRT dosimetry can be obtained using a simplified coplanar plan at a 30-degree head angle, thus obviating the need for complex and time consuming noncoplanar IMRT plans.

Comparison of the Dose Distributions with Beam Arrangements in the Stereotactic Body Radiotherapy (SBRT) for Primary Lung Cancer

To compare 2 beam arrangements, circumferential equally angles (EA) beams or partially angles (PA) beams for stereotactic body radiation therapy (SBRT) of primary lung cancer for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) delivery techniques with respect to target, ipsilateral lung, contralateral lung, and organs-at-risk (OAR) dose-volume metrics, as well as treatment delivery efficiency. Data from 12 patients, four treatment plans were generated per data sets (IMRT EA, IMRTPA, VMATEA, VMATPA). The prescribed dose (PD) was 60 Gy in 4 fractions to 95% of the planning target volume (PTV) for a 6-MV photon beam. When compared with the IMRT and VMAT treatment plan for 2 beams, conformity index, homogeneity index, high dose spillage, D2 cm (Dmax at a distance ≥2 cm beyond the PTV), R50 (ratio of volume circumscribed by the 50% isodose line and the PTV), resulted in similar. But Dmax of the Organ at risk (OAR), spinal cord, trachea, resulted in differ between four treatment plans. Especially HDS location showed big difference in 21.63% vs. 26.46%.

A dose homogeneity and conformity evaluation between ViewRay and pinnacle-based linear accelerator IMRT treatment plans.

ViewRay, a novel technology providing soft-tissue imaging during radiotherapy is investigated for treatment planning capabilities assessing treatment plan dose homogeneity and conformity compared with linear accelerator plans. ViewRay offers both adaptive radiotherapy and image guidance. The combination of cobalt-60 (Co-60) with 0.35 Tesla magnetic resonance imaging (MRI) allows for magnetic resonance (MR)-guided intensity-modulated radiation therapy (IMRT) delivery with multiple beams. This study investigated head and neck, lung, and prostate treatment plans to understand what is possible on ViewRay to narrow focus toward sites with optimal dosimetry. The goal is not to provide a rigorous assessment of planning capabilities, but rather a first order demonstration of ViewRay planning abilities. Images, structure sets, points, and dose from treatment plans created in Pinnacle for patients in our clinic were imported into ViewRay. The same objectives were used to assess plan quality and all critical structures were treated as similarly as possible. Homogeneity index (HI), conformity index (CI), and volume receiving <20% of prescription dose (DRx) were calculated to assess the plans. The 95% confidence intervals were recorded for all measurements and presented with the associated bars in graphs. The homogeneity index (D5/D95) had a 1-5% inhomogeneity increase for head and neck, 3-8% for lung, and 4-16% for prostate. CI revealed a modest conformity increase for lung. The volume receiving 20% of the prescription dose increased 2-8% for head and neck and up to 4% for lung and prostate. Overall, for head and neck Co-60 ViewRay treatments planned with its Monte Carlo treatment planning software were comparable with 6 MV plans computed with convolution superposition algorithm on Pinnacle treatment planning system.

Dosimetric evaluation of integrated IMRT treatment of the chest wall and supraclavicular region for breast cancer after modified radical mastectomy

To investigate the dosimetric characteristics of irradiation of the chest wall and supraclavicular region as an integrated volume with intensity-modulated radiation therapy (IMRT) after modified radical mastectomy. This study included 246 patients who received modified radical mastectomy. The patients were scanned with computed tomography, and the chest wall (with or without the internal mammary lymph nodes) and supraclavicular region were delineated. For 143 patients, the chest wall and supraclavicular region were combined as an integrated planning volume and treated with IMRT. For 103 patients, conventional treatments were employed with 2 tangential fields for the chest wall, abutting a mixed field of 6-MV x-rays (16Gy) and 9-MeV electrons (34Gy) for the upper supraclavicular region. The common prescription dose was 50Gy/25Fx/5W to 90% of the target volume. The dosimetric characteristics of the chest wall, the supraclavicular region, and normal organs were compared. For the chest wall target, compared with conventional treatments, the integrated IMRT plans lowered the maximum dose, increased the minimum dose, and resulted in better conformity and uniformity of the target volume. There was an increase in minimum, average, and 95% prescription dose for the integrated IMRT plans in the supraclavicular region, and conformity and uniformity were improved. The V30 of the ipsilateral lung and V10, V30, and mean dose of the heart on the integrated IMRT plans were lower than those of the conventional plans. The V5 and V10 of the ipsilateral lung and V5 of the heart were higher on the integrated IMRT plans (p < 0.05) than on conventional plans. Without an increase in the radiation dose to organs at risk, the integrated IMRT treatment plans improved the dose distribution of the supraclavicular region and showed better dose conformity and uniformity of the integrated target volume of the chest wall and supraclavicular region.

Comparative analysis of several detectors for the measurement of radiation transmission and leakage from a multileaf collimator

The multileaf collimator (MLC) is the standard device used to shape radiation beams for 3-d conformal and intensity-modulated radiation therapy (IMRT). Due to the inherent properties of MLC, there is a small amount of radiation transmitted through the leaves, called radiation transmission (RT). Accurate measurements of this radiation are required to commission and validate IMRT-capable treatment planning systems because this radiation may impact the dosimetry of IMRT-calculated dose distributions. This work compares several detectors in the measurement of RT for a micro-multileaf collimation system. The results show that there are statistically significant differences in the measured RT values between detectors from 3.5 to 12.5% for the same MLC model and less than 0.2% relative to the isocentre dose for an open reference field. However, although small in magnitude, these differences may impact the dosimetry of IMRT treatment planning by up to 1.78 Gy to the healthy tissue surrounding the target for a treatment of 60 Gy in 30 fractions. By the later, these differences must be included as a source of uncertainty in IMRT dose delivery. Also, it must be established which detector offers the most reliable results in the measurement of the RT by using Monte Carlo simulation methods.

Predicting objective function weights from patient anatomy in prostate IMRT treatment planning

Intensity-modulated radiation therapy (IMRT) treatment planning typically combines multiple criteria into a single objective function by taking a weighted sum. The authors propose a statistical model that predicts objective function weights from patient anatomy for prostate IMRT treatment planning. This study provides a proof of concept for geometry-driven weight determination. A previously developed inverse optimization method (IOM) was used to generate optimal objective function weights for 24 patients using their historical treatment plans (i.e., dose distributions). These IOM weights were around 1% for each of the femoral heads, while bladder and rectum weights varied greatly between patients. A regression model was developed to predict a patient's rectum weight using the ratio of the overlap volume of the rectum and bladder with the planning target volume at a 1 cm expansion as the independent variable. The femoral head weights were fixed to 1% each and the bladder weight was calculated as one minus the rectum and femoral head weights. The model was validated using leave-one-out cross validation. Objective values and dose distributions generated through inverse planning using the predicted weights were compared to those generated using the original IOM weights, as well as an average of the IOM weights across all patients. The IOM weight vectors were on average six times closer to the predicted weight vectors than to the average weight vector, using l2 distance. Likewise, the bladder and rectum objective values achieved by the predicted weights were more similar to the objective values achieved by the IOM weights. The difference in objective value performance between the predicted and average weights was statistically significant according to a one-sided sign test. For all patients, the difference in rectum V54.3 Gy, rectum V70.0 Gy, bladder V54.3 Gy, and bladder V70.0 Gy values between the dose distributions generated by the predicted weights and IOM weights was less than 5 percentage points. Similarly, the difference in femoral head V54.3 Gy values between the two dose distributions was less than 5 percentage points for all but one patient. This study demonstrates a proof of concept that patient anatomy can be used to predict appropriate objective function weights for treatment planning. In the long term, such geometry-driven weights may serve as a starting point for iterative treatment plan design or may provide information about the most clinically relevant region of the Pareto surface to explore.

A comparison of anatomical and dosimetric variations in the first 15 fractions, and between fractions 16 and 25, of intensity‐modulated radiotherapy for nasopharyngeal carcinoma*

The purpose of this study was to compare anatomical and dosimetric variations in first 15 fractions, and between fractions 16 and 25, during intensity‐modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). Twenty‐three NPC patients who received IMRT in 33 fractions were enrolled. Each patient had two repeat computed tomography (CT) scans before the 16th and 25th fraction. Hybrid IMRT plans were generated to evaluate the dosimetric changes. There was a significant decrease of the transverse diameter of nasopharyngeal and neck as well as gross tumor volume (GTV) in the primary nasopharyngeal carcinoma (GTVnx) and involved lymph nodes (GTVnd) during the first 15 fractions, and between fraction 16 and 25 (p<0.05). Consequently, there was a significant reduction of the percentage of the volume receiving the prescribed dose (V100) of CTV1 and GTVnd, which was more prominent after the first 15 fractions treatment compared to that between fraction 16 and 25 (p<0.05). Additionally, there was a significant increase in the mean dose (Dmean) and percentage of volume receiving ≥30Gy(V30) to the bilateral parotid in the first 15 fractions (p<0.05), but not between fraction 16 and 25. While the maximum dose to the spinal cord was significantly increased both in the first 15 fractions, and between fraction 16 and 25 (p<0.05), the increase of the percent of spinal cord volume receiving ≥40Gy(V40) was significantly higher in the first 15 fractions compared to that between fraction 16and25(p<0.05). Based on the dose constraint criterion in the RTOG0225 protocol, a total 39.1%(9/23) of phantom plan 1 (generated by applying the beam configurations of the original IMRT treatment plan to the anatomy of the second CT scan) and 17.4%(4/23) of phantom 2 (generated by applying the beam configurations of the replan 1 to the anatomy of the third CT scan) were out of limit for the dose to the normal critical structures. In conclusion, our data indicated that anatomic changes resulted in more predominant dosimetric effects in the first 15 fractions, and between fractions 16 and 25, of IMRT.PACS number: 87.53.Bn, 87.55.de, 87.55.Qr

Dosimetric comparison of different multileaf collimator leaves in treatment planning of intensity-modulated radiotherapy for cervical cancer

To study the effect of multileaf collimator (MLC) leaf widths (standard MLC [sMLC] width of 10mm and micro-MLC [mMLC] width of 4mm) on intensity-modulated radiotherapy (IMRT) for cervical cancer. Between January 2010 and August 2010, a retrospective analysis was conducted on 12 patients with cervical cancer. The treatment plans for all patients were generated with the same machine setup parameters and optimization methods in a treatment planning system (TPS) based on 2 commercial Elekta MLC devices. The dose distribution for the planning tumor volume (PTV), the dose sparing for organs at risk (OARs), the monitor units (MUs), and the number of IMRT segments were evaluated. For the delivery efficiency, the MUs were significantly higher in the sMLC-IMRT plan than in the mMLC-IMRT plan (802 ± 56.9 vs 702 ± 56.7; p < 0.05). The number of segments in the plans were 58.75 ± 1.8 and 59 ± 1.04 (p > 0.05). For the planning quality, the conformity index (CI) between the 2 paired IMRT plans with the mMLC and the sMLC did not differ significantly (average: 0.817 ± 0.024 vs 0.810 ± 0.028; p > 0.05). The differences of the homogeneity index (HI) between the 2 paired plans were statistically significant (average: 1.122 ± 0.010 vs 1.132 ± 0.014; p < 0.01). For OARs, the rectum, bladder, small intestine, and bony pelvis were evaluated in terms of V10, V20, V30, and V40, percentage of contoured OAR volumes receiving 10, 20, 30, and 40Gy, respectively, and the mean dose (Dmean) received. The IMRT plans with the mMLC protected the OARs better than the plans with the sMLC. There were significant differences (p < 0.05) in evaluated parameters between the 2 paired IMRT plans, except for V30 and V40 of the rectum and V10, V20, V40, and Dmean of the bladder. IMRT plans with the mMLC showed advantages over the plans with the sMLC in dose homogeneity for targets, dose sparing of OARs, and fewer MUs in cervical cancer.

Dosimetric effects of endorectal balloons on intensity-modulated radiation therapy plans for prostate cancer

We used an endorectal balloon (ERB) for prostate immobilization during intensity-modulated radiotherapy (IMRT) for prostate cancer treatment. To investigate the dosimetric effects of ERB-filling materials, we changed the ERB Hounsfield unit (HU) from 0 to 1000 HU in 200-HU intervals to simulate the various ERB fillings; 0 HU simulated a water-filled ERB, and 1000 HU simulated the densest material-filled ERB. Dosimetric data (coverage, homogeneity, conformity, maximal dose, and typical volume dose) for the tumor and the organs at risk (OARs) were evaluated in prostate IMRT treatment plans with 6-MV and 15-MV beams. The tumor coverage appeared to differ by approximately 1%, except for the clinical target volume (CTV) V100% and the planning target volume (PTV) V100%. The largest difference for the various ERB fillings was observed in the PTV V100%. In spite of increasing HU, the prostate IMRT plans at both energies had relatively low dosimetric effects on the PTV and the CTV. However, the maximal and the typical volume doses (D25%, D30%, and D50%) to the rectal wall and the bladder increased with increasing HU. For an air-filled ERB, the maximal doses to the rectal wall and the monitor units were lower than the corresponding values for the water-filled and the densest material-filled ERBs. An air-filled ERB spared the rectal wall because of its dosimetric effect. Thus, we conclude that the use of an air-filled ERB provides a dosimetric benefit to the rectal wall without a loss of target coverage and is an effective option for prostate IMRT treatment.

Case study of functional performance characteristics and test methods for intensity modulated radiation treatment planning system

Objective To study a series of tests to the IMRT planning RayStation,and verify the feasibility of this Standard.Referring to the standard Performance and test methods for intensity modulated radiation therapy (IMRT) treatment planning system (Standard).Methods Referring to the Standard,the tests include:dose goals used for planning,point dose calculation accuracy and dose distribution calculation accuracy.And test phantoms include:AAPM TG119 report's test cases and solid water phantom for verification,and IMRT test cases include:multitarget,mock prostate,head/neck and CTshape (easier version and harder version) and so on.Referring to the Standard,we optimize and calculate doses of interest in RayStation,and use the common measurement equipments and tools,such as an ionization chamber,films,a detector array,and etc,to measure the doses on the accelerator,then calculate and analyze the errors between them.Results The results of dose goals for five different mock plans meet the requirements of the Standard.And the point dose ionization chamber measurements are in line with the requirements of the Standard,and the total error of the mock plans was (0.83 ± 1.65) ％.The γpass rates of per-field measurements using detector array are all greater than 99.0％,and the γpass rates of composite film measurements are all greater than 92.5％.Conclusions The results of three trials to IMRT performance of RayStation are in full compliance with the requirements of the Standard.And also it is an evidence for the feasibility of this Standard. Key words: Intensity modulated radiation therapy; Performance testing; Dosimetric verification; Point dose; Dose distribution

The choice of IMRT for early-stage right breast cancer.

81 Background: The benefit of Intensity Modulated Radiation Therapy (IMRT) has been established for left breast cancer. However, IMRT is not routinely employed for right breast cancer. The purpose of this study is to (1) compare dosimetric parameters in women with early stage right breast cancer undergoing whole breast radiation therapy using 3-dimensional conformal radiation (3DCRT) versus intensity modulated radiation therapy (IMRT) (2) determine treatment planning and/or anatomical correlates for patients who may benefit from IMRT. Methods: IMRT versus 3DCRT treatment plans were compared for 20 consecutive female patients with early stage (T1-T2 N0M0 ) right breast cancer treated at our center from 1/2012 to 12/2012. The planned target volume (PTV), contoured according to the RTOG atlas guidelines, was prescribed to 46-8-50.4 Gy in 26-28 fractions with PTV coverage goal of ≥ 95 % receiving the prescription dose. Dosimetric and anatomic parameters: % right lung receiving 20 Gy (lung V20), maximum breast dose (Dmax), maximum skin dose (Dmax skin), chest wall separation (CWS) and medial gantry angle (MGA) were determined for each paired treatment plan. Wilcoxon-signed rank test was utilized to compare IMRT and 3DCRT dosimetric parameters. Univariate analysis was used to identify planning or anatomic correlates associated with favorable dosimetric outcomes. Results: The lung V20Gy for IMRT ( 10.86±2.81%) was significantly lower than for 3DCRT (12.92±3.45 %, p &lt; 0.043). IMRT significantly reduced Dmax to 53.79 ± 2.17 Gy from 56.7 ± 2.24 Gy with 3DCRT (p 0.001). 3DCRT yielded significantly higher Dmax skin (33.47±3.21 Gy) than IMRT (24.16 ±2.65 Gy, p&lt; 0.0001). Mean CWS and MGA were 22.4 ± 2.6 cm and 126 ± 5.1° respectively. There was a significant association between CWS and Dmax for 3DCRT (p &lt; 0.026). Conclusions: The most significant impact of IMRT for right breast cancer was the reduction of Dmax and Dmax skin, with a more tempered advantage in the reduction of lung V20. Reduction of maximum breast and skin dose harbors clinical relevance as it relates to acute skin toxicity, long- term fibrosis and adverse cosmetic outcomes. Chest wall separation may be a potential planning surrogate marker to determine which right-sided breast cancer patients may benefit from IMRT.

Radiation-induced temporal lobe injury after intensity modulated radiotherapy in nasopharyngeal carcinoma patients: a dose-volume-outcome analysis

BackgroundTo identify the radiation volume effect and significant dosimetric parameters for temporal lobe injury (TLI) and determine the radiation dose tolerance of the temporal lobe (TL) in nasopharyngeal carcinoma (NPC) patients treated with intensity modulated radiation therapy (IMRT).MethodsTwenty NPC patients with magnetic resonance imaging (MRI)-diagnosed unilateral TLI were reviewed. Dose-volume data was retrospectively analyzed.ResultsPaired samples t-tests showed all dosimetric parameters significantly correlated with TLI, except the TL volume (TLV) and V75 (the TLV that received ≥75 Gy, P = 0.73 and 0.22, respectively). Receiver operating characteristic (ROC) curves showed V10 and V20 (P = 0.552 and 0.11, respectively) were the only non-significant predictors from V10 to V70 for TLI. D0.5cc (dose to 0.5 ml of the TLV) was an independent predictor for TLI (P < 0.001) in multivariate analysis; the area under the ROC curve for D0.5cc was 0.843 (P < 0.001), and the cutoff point 69 Gy was deemed as the radiation dose limit. The distribution of high dose ‘hot spot’ regions and the location of TLI were consistent.ConclusionsA D0.5cc of 69 Gy may be the dose tolerance of the TL. The risk of TLI was highly dependent on high dose ‘hot spots’ in the TL; physicians should be cautious of such ‘hot spots’ in the TL during IMRT treatment plan optimization, review and approval.

Dosimetric and Radiobiological Consequences of Computed Tomography–Guided Adaptive Strategies for Intensity Modulated Radiation Therapy of the Prostate

To examine a range of scenarios for image-guided adaptive radiation therapy of prostate cancer, including different schedules for megavoltage CT imaging, patient repositioning, and dose replanning. We simulated multifraction dose distributions with deformable registration using 35 sets of megavoltage CT scans of 13 patients. We computed cumulative dose-volume histograms, from which tumor control probabilities and normal tissue complication probabilities (NTCPs) for rectum were calculated. Five-field intensity modulated radiation therapy (IMRT) with 18-MV x-rays was planned to achieve an isocentric dose of 76 Gy to the clinical target volume (CTV). The differences between D95, tumor control probability, V70Gy, and NTCP for rectum, for accumulated versus planned dose distributions, were compared for different target volume sizes, margins, and adaptive strategies. The CTV D95 for IMRT treatment plans, averaged over 13 patients, was 75.2 Gy. Using the largest CTV margins (10/7 mm), the D95 values accumulated over 35 fractions were within 2% of the planned value, regardless of the adaptive strategy used. For tighter margins (5 mm), the average D95 values dropped to approximately 73.0 Gy even with frequent repositioning, and daily replanning was necessary to correct this deficit. When personalized margins were applied to an adaptive CTV derived from the first 6 treatment fractions using the STAPLE (Simultaneous Truth and Performance Level Estimation) algorithm, target coverage could be maintained using a single replan 1 week into therapy. For all approaches, normal tissue parameters (rectum V(70Gy) and NTCP) remained within acceptable limits. The frequency of adaptive interventions depends on the size of the CTV combined with target margins used during IMRT optimization. The application of adaptive target margins (<5 mm) to an adaptive CTV determined 1 week into therapy minimizes the need for subsequent dose replanning.

A fluence map optimization model for restoring traditional fractionation in IMRT treatment planning

One of the core problems in intensity modulated radiation therapy (IMRT) treatment planning is the fluence map optimization (FMO) problem, which determines a fluence map (or profile) for each beam used in the delivery of treatment. Radiation therapy is administered in multiple so-called daily fractions to allow for healthy tissue to recover from damage caused by the treatment. Before the advent of IMRT, the treatment was designed to ensure a constant dose to cells in the target (the areas in the patient where cancerous cells are present or suspected). In the presence of multiple targets with different prescribed doses, this design meant that treatment had to be delivered in a sequence of unequal fractions, one per prescription dose level. For example, in case of two targets treatment would consist of an initial plan aimed at treating both targets to a lower total dose, followed by a so-called boost plan aimed at delivering the additional dose at the target with higher prescribed dose. In contrast, IMRT treatment plans are often delivered with equal treatment plan for each fraction, which means that the dose per fraction cannot be the same for all targets. The important problem of restoring traditional fractionation to IMRT treatments has not yet received much attention in the literature. In this paper we propose a new optimization model that explicitly restores fractionation into the FMO problem, yielding an optimal set of fluence maps for each fraction. We illustrate the capabilities of our approach on clinical head-and-neck cancer cases.

Protons offer reduced bone marrow, small bowel, and urinary bladder exposure for patients receiving neoadjuvant radiotherapy for resectable rectal cancer.

To assess the potential benefit of proton therapy (PT) over photon therapy, we compared 3-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiotherapy (IMRT), and PT plans in patients undergoing neoadjuvant chemoradiation for resectable rectal cancer at our institution. Eight consecutive patients with resectable (T2-T3) rectal cancers underwent 3DCRT, IMRT, and 3-dimensional conformal PT treatment planning. Initial target volumes (PTV1) were contoured using the Radiation Therapy Oncology Group anorectal atlas guidelines. Boost target volumes (PTV2) consisted of the gross rectal tumor plus a uniform 2-cm expansion. Plans delivered 45 Gray (Gy) or Cobalt Gray Equivalent (CGE) to the PTV1 and a 5.4-Gy (CGE) boost to the PTV2. Ninety-five percent of the PTVs received 100% of the target dose and 100% of the PTVs received 95% of the target dose. Standard normal-tissue constraints were utilized. Wilcoxon paired t-tests were performed to compare various dosimetric points between the 3 plans for each patient. All plans met all normal-tissue constraints and were isoeffective in terms of PTV coverage. The proton plans offered significantly reduced median normal-tissue exposure over the 3DCRT and IMRT plans with respect to pelvic bone marrow at the V5Gy, V10Gy, V15Gy, and V20Gy levels and the small bowel space at the V10Gy and V20Gy levels. The proton plans also offered significantly reduced median normal-tissue exposure over the 3DCRT plans with respect to the small bowel at the V30Gy and V40Gy levels and the urinary bladder at the V40Gy level. By reducing bone marrow exposure, PT may reduce the acute hematologic toxicity of neoadjuvant chemoradiation and increase the likelihood of uninterrupted chemotherapy delivery. Bone marrow sparing may also facilitate the delivery of salvage chemotherapy for patients who subsequently develop hematogenous metastasis. Reduced small bowel exposure using PT may also reduce toxicity and possibly facilitate the use of more-aggressive chemotherapy with radiotherapy.

Investigation of the dose accuracy and benefits of gated IMRT and VMAT for clinical implementation

Introduction To evaluate the dose accuracy and benefits of gated intensity modulated radiation therapy (IMRT) and gated volumetric modulated arc therapy (VMAT) for clinical implementation. Methods and materials The Dynamic Thorax Phantom from CIRS (DTP) was used for this study. The phantom mimics a human thorax and contains a moving and programmable rod enclosing a spherical tumor in the left lung. Thirteen different settings were tested combining three spheres of different diameters (1, 2 or 3 cm) and seven breathing patterns. One 4D-CT per setting was acquired using the Real-time Positioning Management system (Varian). Gated and non-gated IMRT and VMAT treatment plans were calculated with Eclipse 10.0. Absolute dose measurements were performed with thermoluminescent dosimeters (TLD, diameter 2 mm, thickness 0.1 mm) and 2D relative dose distributions were measured with radiochromic films (Gafchromic EBT3, ISP). Dose-volume histograms were compared. Furthermore, an irregular patient-based breathing pattern was applied to the DTP. Gated and non-gated 4DCT-image based VMAT plans were created. PTV coverage was compared using a moving gafchromic film inserted in the DTP. Results The absolute calculated and measured doses were within 3%. The passing rate of 95% (gamma criterion 3%/3 mm) was achieved in the majority of the 2D dose distribution comparison plans. The reduction in dose to OAR was important for high amplitude pulmonary movements (>5 cm). For amplitudes of 5 cm, 10.2% of healthy lung tissue receiving 20 Gy was spared with IMRT and 9.7% with VMAT compared to non-gated treatments. The size of the tumor appeared to be an important selection criterion: the volume of healthy tissue spared increased with tumors volume. Conclusion The commissioning of this new technique was satisfying. IMRT and VMAT gating should only be used clinically for large and mobile tumors to obtain the greatest dosimetric benefit and to counterbalance the decrease in patient comfort due to the longer treatment delivery time. To evaluate the dose accuracy and benefits of gated intensity modulated radiation therapy (IMRT) and gated volumetric modulated arc therapy (VMAT) for clinical implementation. The Dynamic Thorax Phantom from CIRS (DTP) was used for this study. The phantom mimics a human thorax and contains a moving and programmable rod enclosing a spherical tumor in the left lung. Thirteen different settings were tested combining three spheres of different diameters (1, 2 or 3 cm) and seven breathing patterns. One 4D-CT per setting was acquired using the Real-time Positioning Management system (Varian). Gated and non-gated IMRT and VMAT treatment plans were calculated with Eclipse 10.0. Absolute dose measurements were performed with thermoluminescent dosimeters (TLD, diameter 2 mm, thickness 0.1 mm) and 2D relative dose distributions were measured with radiochromic films (Gafchromic EBT3, ISP). Dose-volume histograms were compared. Furthermore, an irregular patient-based breathing pattern was applied to the DTP. Gated and non-gated 4DCT-image based VMAT plans were created. PTV coverage was compared using a moving gafchromic film inserted in the DTP. The absolute calculated and measured doses were within 3%. The passing rate of 95% (gamma criterion 3%/3 mm) was achieved in the majority of the 2D dose distribution comparison plans. The reduction in dose to OAR was important for high amplitude pulmonary movements (>5 cm). For amplitudes of 5 cm, 10.2% of healthy lung tissue receiving 20 Gy was spared with IMRT and 9.7% with VMAT compared to non-gated treatments. The size of the tumor appeared to be an important selection criterion: the volume of healthy tissue spared increased with tumors volume. The commissioning of this new technique was satisfying. IMRT and VMAT gating should only be used clinically for large and mobile tumors to obtain the greatest dosimetric benefit and to counterbalance the decrease in patient comfort due to the longer treatment delivery time.

SU‐E‐T‐275: Voxelized Second Cancer Risk Calculation — Comparison of Proton and Photon Radiotherapies for Hodgkin Lymphoma

Purpose: The purpose of this study was to compare the risk of developing a second lung or breast cancer for a female Hodgkin Lymphoma patient treated with passively scattered proton therapy (PSPT) and photon intensity modulated radiotherapy (IMRT). Particular objectives of this work were to fully account for stray dose, perform voxel by voxel risk calculations, and to visually display risk in a manner analogous to dose visualization used in commercial treatment planning systems (CTPS). Methods: Clinically approved PSPT and IMRT treatment plans were designed using a CTPS and prescribed 36 Gy to the target volume. Neutron and stray photon dose were calculated using an in house Monte Carlo code and a TLD measurement fitted analytical model for PSPT and IMRT, respectively. For lung and breast, therapeutic and stray doses were summed and input into an in‐house program that calculates Excess Absolute Risk (EAR) per voxel in the patient based on the RadRAT risk model. The radiation weighting factor (Wr) varied for neutron doses (Wr =10 and 30) to account for biological uncertainties. Results: Neutron equivalent doses added an additional 3% (Wr =10)/9% (Wr =30) of the prescribed dose to the CTV and near field organs. Scatter dose added an additional 5% (Wr =10 )/15% (Wr =30) of the prescribed dose to organ portions far out of field. PSPT registered a lower EAR compared to IMRT except for lung when Wr =30. Larger differences were seen in low dose regions of organs. Conclusion: EAR was lower for PSPT versus IMRT unless neutron Wr values were very high, e.g.&gt; ∼20. More work incorporating uncertainties must be done before definitive answers are determined. In the future, similar types of risk visualization tools could be incorporated into treatment plan analysis to prospectively select plans with the lowest predicted risk of second cancers. This research was supported in part, by the National Cancer Institute award 1R01CA131463‐01A1 (W.D. Newhauser, P.I.) and a subcontract of that award (R.M. Howell, P.I). No conflicts of interest to disclose.

SU‐E‐T‐653: Predicting Objective Function Weights for IMRT Prostate Treatment Planning Using Patient Anatomy

Purpose: To develop a prediction model for objective function weights for intensity‐modulated radiation therapy (IMRT) prostate treatment planning with multiple objectives using geometry information from patient anatomy. Methods: A previously developed inverse optimization method (IOM) was used to reverse‐engineer optimal objective function weights (inverse weights) from an observed treatment plan. We developed a regression model to predict the weights for IMRT prostate treatment planning using patient anatomy from 25 patients. The ratio of the overlap volumes of the rectum and bladder with the planning target volume expanded by 1cm was used to predict the bladder and rectum weights. The femoral head weights were included in the model as a small fixed weight (1%). The model was validated using leave‐one‐out cross‐validation. We evaluated the model by comparing the treatment plans generated through inverse planning using the inverse weights from IOM and the predicted weights from the regression model. Results: On average, V54Gy for the bladder was 36.1% using the inverse weights and 36.6% using the predicted weights. V70Gy for the bladder was 23.2% (inverse) and 23.5% (predicted). For the rectum, V54Gy was 34.6% (inverse) and 33.9% (predicted), and V70Gy was 22.6% (inverse) and 22.3% (predicted). For each criterion, the difference between the inverse and predicted metrics was not statistically significant. All treatment plans from the predicted weights satisfied the clinical criteria. Conclusion: Our results show that objective function weights are well‐predicted by the regression model. This approach may support the genesis of personalized weights in IMRT treatment planning. This research was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Ontario Graduate Scholarship (OGS).

SU‐E‐T‐661: Nonlinear Continuous Method for Non‐Negatively Constrained Inverse Problem of IMRT Planning

Purpose: The intensity‐modulated radiation therapy (IMRT) is an effective therapeutic technique for the treatment of tumors in the human body without inflicting serious damage to normal tissues. IMRT plans include an optimization strategy for minimizing an objective function of radiation beam weights. We present a novel approach for solving non‐negatively constrained inverse problems in IMRT treatment planning, based on the idea of continuous‐time dynamical methods using nonlinear differential equations. Our method can compute a feasible solution without the second derivative of an objective function, which is required for gradient‐based optimization algorithms such as conjugate gradient and steepest descent methods. Methods: We first consider the stability of an equilibrium, which corresponds to the desired planning in the IMRT system, by the use of Lyapunov's stability theorem. Then we perform some phantom experiments using phantom data simulating a clinical setup. Results: We prove theoretically that a Kullback‐Leibler (KL) divergence can be a Lyapunov function for the IMRT planning system, in consistent case. It means that the KL‐divergence measure decreases along the solution to the differential equation. Additionally, we show that the intensity of any radiation beam is not negative. Because the planning system can be created as an analog electronic circuit, its implementation in actual hardware yields dramatically faster planning as a physical phenomenon with real parallel computing, than software‐based iterative methods. Conclusion: The proposed method provides not only the reduction of a computational cost but also no production of a solution with an unphysical negative radiation beam coefficient in solving IMRT planning inverse problems. Moreover the theoretical results on convergence properties of solutions are confirmed by numerical experiments. Based on the results, we found that we can obtain a feasible solution violating the physical dose constraints minimally, even if the system is inconsistent.

SU‐E‐T‐642: A Patient Population Based Approach for Disease Site Specific Intensity‐Modulated Radiation Therapy (IMRT) Treatment Planning

Purpose: To develop and evaluate a population based planning scheme for disease site specific IMRT treatment planning. Methods: A database of previously treated locally advanced head and neck patients from our institution was retrospectively used for this study. The treatment targets involved primary tumor, high risk and low risk nodal volumes. Organs‐at‐risk (OARs) included both parotids, oral cavity, brain stem and the spinal cord. Nine‐beam IMRT plans were generated using the Pinnacle3 planning system for 80% of patients. These plans served as the population basis. Thousands of apertures from these cases were extracted. From these apertures, prospective case‐specific apertures were either selected from the population basis or derived using machine learning for each of the remaining 20% of the patients. Aperture‐based 3D dose distributions were calculated in parallel using Monte Carlo‐based Kernel Superposition (MCKS) via a high throughput computing environment. Aperture weights of each case were optimized via an in‐house developed progressive dose optimization (PDO) approach. The PDO approach involved solving the DO problem by dividing the problem into sub‐problems, each of which was solved while maintaining a global view of the full problem. Our approach was compared with conventional IMRT and two‐arc volumetric modulated arc therapy (VMAT). Results: On average, the improvements achieved using population approach in parotid gland mean dose were 16% and 15%, in the maximum dose to the spinal cord were 21% and 28%, in mean oral cavity dose were 10% and 24% and in maximum brain stem dose were 6% and 15% compared with conventional IMRT and two‐arc VMAT. t‐test showed that these improvements were significant (p &lt; 0.001). Dose homogeneity for all PTVs (tumor and nodes) was higher for population based approach as well. Conclusion: Population based planning approach resulted in superior plan quality compared with conventional IMRT and VMAT.