Shoot Intensity Modulated Radiation Therapy Research Articles

Forward versus inverse planning in oropharyngeal cancer: A comparative study using physical and biological indices

Possible benefits of inverse planning. To analyze possible benefits of inverse planning intensity modulated radiation therapy (IMRT) over field-in-field 3D conformal radiation therapy (FIF-3DCRT) and to evaluate the differences if any, between low (6 Million Volts) and high energy (15 Million Volts) IMRT plans. Ten patients with squamous cell carcinoma of oropharynx, previously treated with 6 MV step and shoot IMRT were studied. V100, V33, V66 , mean dose and normal tissue complication probabilities (NTCP) were evaluated for parotid glands. Maximum dose and NTCP were the parameters for spinal cord. A two-tailed t-test was applied to analyze statistical significance between the different techniques. For combined parotid gland, a reduction of 4.374 Gy, 9.343 Gy and 7.883 Gy were achieved for D100, D66 and D33, respectively in 6 MV-IMRT when compared with FIF-3DCRT. Spinal cord sparing was better in 6 MV-IMRT (40.963 ± 2.650), with an average reduction of maximum spinal cord dose by 7.355 Gy from that using the FIF-3DCRT technique. The uncomplicated tumor control probabilities values were higher in IMRT plans thus leading to a possibility of dose escalation. Though low-energy IMRT is the preferred choice for treatment of oropharyngeal cancers, FIF-3DCRT must be given due consideration as a second choice for its well established advantages over traditional conventioan technique.

Volumetric Arc Therapy Seems More Promising to Spare Organ at Risk in Adjuvant Postoperative Radiation Therapy for Pancreas Adenocarcinoma in Comparison to Step and Shoot Intensity Modulated Radiation Therapy

We aimed to define whether Volumetric arc therapy (VMAT) provides a superior dose distribution in comparison to Intensity modulated radiation therapy (Step and shoot: ssIMRT) based on target volume coverage and organ at risk (OAR) doses in adjuvant postoperative radiation therapy for pancreas adenocarcinoma patients.

Comparison and evaluation of VMAT and IMRT for the treatment of initial treated nasopharyngeal carcinoma

Objective To compare planning quality,treatment efficiency and delivery accuracy for initial treated nasopharyngeal carcinoma (NPC) with step & shoot intensity modulated radiation therapy (ssIMRT) and volumetric modulated arc therapy (VMAT).These results will help make a clinic choice of the therapeutical technique.Methods Twenty-six NPC cases were planned with the same dose prescription and objective constrains by means of 9-field ss-IMRT and VMAT respectively.Compare:( 1 ) plan dosimetric distribution,conformity index and homogeneity index of the targets,the average dose,maximum dose and interested dose-volume histograms of organs at risk (OAR) et al;(2) delivery times of the therapy plans ;(3) the accuracy of treatment plans dose verification.Results Both therapeutical plans can achieve the clinic dosimetric demands.Compared to ss-IMRT,VMAT had less inferior target coverage.The CI and HI of the PGTV was 0.57 and 0.08 ( ss-IMRT),0.48 and 0.12 (VMAT) respectively ( t =-4.52,- 8.33,P =0.000,0.000).Except of brain stem,VMAT had higher mean dose and maximum dose of OARs than ss-IMRT (t=-9.57 - -3.71,P=0.000 -0.001).The spinal cord D1cc and parotids D50％ were increased by 11.9％ and 6.5％ averagely.The treatment times of ss-IMRT and VMAT were 803.7 s and 389.3 s respectively (t =24.12,P =0.000),while V MAT decreasing by 51.6％.The pass ratios of γ (3mm,3％ ) from the dose verification were 99.4％ (ss-IMRT) and 98.0％ (VMAT) respectively ( t =5.19,P =0.000).Conclusions The dose distribution of VMAT for initial treated nasopharyngeal carcinoma can achieve the clinic demands,but slightly worse than 9-field ss-IMRT.VMAT has the advantage of high efficiency and dosimetric accuracy. Key words: Nasopharyngeal neoplasms/radiotherapy; Radiotherapy,step and shoot intensity modulated; Radiotherapy,volumetric modulated arc; Plan evaluation

Dosimetric Study Of Helical Tomotherapy, Step And Shoot Intensity Modulated Radiation Therapy And Three-dimensional Conformal Radiation Therapy For Liver Cancers

Image-guided helical tomotherapy (HT) technology has been widely used in the clinical treatments. Treatment benefits have been observed in the management of head and neck, lung and pelvic cancers. HT, compared with step and shoot intensity-modulated radiation therapy (SaS-IMRT) and three-dimensional conformal radiation therapy (3D-CRT), had certain dosimetric advantages of its unique gantry-ring design. This study aims to investigate the dosimetric characteristics of HT, SaS-IMRT and 3D-CRT during the treatment of liver tumor as well as their dose distribution comparison of the associated critical normal tissues.

Improving cardiac dosimetry: Alternative beam arrangements for intensity modulated radiation therapy planning in patients with carcinoma of the distal esophagus

PurposeRadiation for carcinoma of the distal esophagus is associated with cardiac perfusion deficits and pericardial effusion. We performed a dosimetric analysis of alternative beam arrangements for use in intensity modulated radiation therapy (IMRT) planning, seeking to lower radiation dose to the heart. Methods and MaterialsTreatment plans using 4 separate beam arrangements were generated and optimized for 12 patients. Hemispheric and butterfly beam arrangements were compared with plans with posterior and lateral beam entries. Radiotherapy was planned to 50.4 Gy in 28 fractions, using step and shoot IMRT with 6 MV photons. Mean heart dose and volumes of heart and lung receiving up to specified doses (V5-V40) were recorded. Isodose distributions were evaluated for target coverage and normal tissue exposure. ResultsIMRT plans utilizing posterior-lateral beam arrangements significantly reduced mean cardiac doses (32.5 ± 3.9 Gy, 33.3 ± 3.2 Gy vs 24.3 ± 3.7 Gy, and 23.4 ± 4.2 Gy, P < .05, paired Student t test with post hoc Bonferroni correction) as well as the total heart volumes receiving at least 20 and 30 Gy. IMRT allowed the maximum cord dose to be limited to less than 40 Gy. While both posterior-lateral beam arrangements lead to improved cardiac dosimetry, mean lung doses as well as V5 and V20 were slightly higher, although within accepted limits. Target coverage, homogeneity, and conformality were similar or improved with the use of alternative beam configurations. ConclusionsThe use of IMRT with posterior-lateral beams can significantly reduce radiation dose to cardiac structures with minimal increased dose to the lung. Future studies will assess the physiologic and clinical impact of cardiac sparing.

SU‐E‐T‐861: Assessment of Collimator Jaw Optimization in Reducing Normal Tissue Irradiation with Intensity Modulated Radiation Therapy

Purpose: To evaluate normal tissue dose reduction in step‐and‐shoot intensity‐modulated radiation therapy (IMRT) on the Varian 2100 platform by tracking the multileaf collimator (MLC) apertures with the accelerator jaws. Methods: Clinical radiation treatment plans for 10 thoracic, 3 pediatric and 3 head and neck patients were converted to plans with the jaws tracking each segmentˈs MLC apertures and compared to the original plans in a commercial radiation treatment planning system (TPS). Each segment was then renormalized to account for the change in collimator scatter to obtain target coverage within 1 % of that in the original plan. Reduction in normal tissue dose was evaluated in the new plan by using the parameters V5, V10, and V20 in the cumulative dose‐volume histogram for the following structures: total lung minus GTV (gross target volume), heart, esophagus, spinal cord, liver, parotids, and brainstem. In order to validate the accuracy of our beam model, MLC transmission measurements were made and compared to those predicted by the TPS. Results: The greatest change between the original plan and new plan occurred at lower dose levels. The reduction in V20 was never more than 6.3% and was typically less than 1% for all patients. The reduction in V5 was 16.7% maximum and was typically less than 3% for all patients. The variation in normal tissue dose reduction was not predictable, and we found no clear parameters that indicated which patients would benefit most from jaw tracking. Our TPS model of MLC transmission agreed with measurements with absolute transmission differences of less than 0.1 % and thus uncertainties in the model did not contribute significantly to the uncertainty in the dose determination. Conclusion: The amount of dose reduction achieved by collimating the jaws around each MLC aperture in step and shoot IMRT is probably not clinically significant.

Postoperative intensity-modulated radiotherapy with simultaneous integrated boost in prostate cancer: A dose-escalation trial

ObjectivesTo determine the recommended phase II dose of postoperative accelerated intensity modulated radiotherapy (IMRT) for prostate cancer. Material and methodsStep and shoot IMRT with simultaneous integrated boost (SIB) was delivered in 25 fractions over 5 weeks to patients with high risk resected prostate adenocarcinoma (stage pT3-4 and/or positive surgical margins). Pelvic nodes received 45 Gy at 1.8 Gy/fraction; dose escalation was performed only to the prostate bed (planned dose escalation: 56.8 Gy at 2.27 Gy/fraction, 59.7 Gy at 2.39 Gy/fraction, 61.25 Gy at 2.45 Gy/fraction, 62.5 Gy at 2.5 Gy/fraction). Dose-limiting toxicity (DLT) was any grade ≥ 3 acute toxicity (RTOG score). ResultsTwenty-five patients were treated: 7 patients at the 56.75 Gy dose level, 6 patients at each subsequent dose level. Pathologic stages were: pT2c: 2; pT3a: 11; pT3b: 12; pN0: 22; pN1: 3; R0: 7; R1: 18. Median follow-up time was 19 months (range: 6–36 months). No patient experienced DLT. Grade 1–2 acute rectal and urologic toxicity was common (17 and 22 patients, respectively). ConclusionsThe recommended dose was 62.5 Gy in 2.5 Gy/fraction. Postoperative hypofractionated IMRT SIB for prostate cancer seemed to be well tolerated and could be tested in phase II studies.

Dosimetric impact of systematic MLC positional errors on step and shoot IMRT for prostate cancer: a planning study

The positional accuracy of multileaf collimators (MLC) is crucial in ensuring precise delivery of intensity-modulated radiotherapy (IMRT). The aim of this planning study was to investigate the dosimetric impact of systematic MLC positional errors on step and shoot IMRT of prostate cancer. A total of 12 perturbations of MLC leaf banks were introduced to six prostate IMRT treatment plans to simulate MLC systematic positional errors. Dose volume histograms (DVHs) were generated for the extraction of dose endpoint parameters. Plans were evaluated in terms of changes to the defined endpoint dose parameters, conformity index (CI) and healthy tissue avoidance (HTA) to planning target volume (PTV), rectum and bladder. Negative perturbations of MLC had been found to produce greater changes to endpoint dose parameters than positive perturbations of MLC (p<0.01). Negative and positive asynchronised MLC perturbations of -1mm resulted in median changes in D(95) of -1.2 and 0.9% respectively. Negative and positive synchronised MLC perturbations of 1mm in one direction resulted in median changes in D(95) of -2.3 and 1.8% respectively. Doses to rectum were generally more sensitive to systematic MLC errors compared to bladder (p<0.01). Negative and positive synchronised MLC perturbations of 1mm in one direction resulted in median changes in endpoint dose parameters of rectum and bladder from 1.0 to 2.5%. Maximum reduction of -4.4 and -7.3% were recorded for conformity index (CI) and healthy tissue avoidance (HTA) respectively due to synchronised MLC perturbation of 1mm. MLC errors resulted in dosimetric changes in IMRT plans for prostate.

Planning outcomes and acute toxicity of intensity-modulated radiotherapy (IMRT) for treatment of bladder and pelvic lymph nodes.

297 Background: We implemented an IMRT protocol for simultaneous treatment of bladder and pelvic lymph nodes (LN) and report the planning outcomes and acute toxicity of patients treated using this technique. Methods: 13 patients were treated with a 5-field step and shoot IMRT technique. Inclusion criteria were: radiological evidence of pelvic LN metastases; stage T3/T4; high risk pathology or post-cystectomy with persistent/recurrent disease. Anisotropic margins were applied to the whole bladder CTV (0.5 cm laterally/inferiorly, 1 cm posteriorly, 1.5 cm superiorly/anteriorly) to create PTV1. 0.5 cm margins were applied to the pelvic LN CTV to create PTV2. PTVs 1/2 were prescribed 52 Gy in 32 fractions. 1 cm margins were applied to the involved bladder to create PTV3 (prescribed 64Gy). 0.5 cm margins were applied to the involved LN volumes to create PTV4 (prescribed 60 Gy). Post cystectomy patients were planned using PTVs 2/4 only. Dose volume histograms for organs at risk (rectum, femoral heads and other bowel) were calculated and compared with local dose constraints. Acute toxicity was assessed according to common toxicity criteria v3.0 and recorded weekly during treatment. Treatment verification was performed by cone beam CT. Results: All treatment plans achieved target coverage of &gt; 95% volume to &gt; 95% prescription dose for each PTV. All patients achieved rectal and femoral head dose constraints. 2 patients did not meet V45 other bowel constraints but proceeded with treatment as all other dose limits were achieved. At the time of abstract submission 13 patients had completed treatment. 2 were treated for relapse post-cystectomy and did not have GU toxicities recorded. Maximum experienced acute toxicities were recorded. 9/11 patients experienced acute GU toxicity (G1=3, G2=1, G3=5). 6/13 patients experienced acute GI toxicity (G1=3, G2=3). 9/13 patients developed other acute toxicities (G1=7, G2=2). No patients developed ≥G3 non-GU toxicity. Conclusions: Bladder and pelvic LN IMRT allows patients with high risk locoregional bladder cancer to meet preset dose constraints, appears feasible and has a comparable rate of acute toxicity to conventional bladder-only radiotherapy treatment. No significant financial relationships to disclose.

Fast intensity-modulated arc therapy based on 2-step beam segmentation

Single or few are intensity-modulated arc therapy (IMAT) is intended to be a time saving irradiation method, potentially replacing classical intensity-modulated radiotherapy (IMRT). The aim of this work was to evaluate the quality of different IMAT methods with the potential of fast delivery, which also has the possibility of adapting to the daily shape of the target volume. A planning study was performed. Novel double and triple IMAT techniques based on the geometrical analysis of the target organ at risk geometry (2-step IMAT) were evaluated. They were compared to step and shoot IMRT reference plans generated using direct machine parameter optimization (DMPO). Volumetric arc (VMAT) plans from commercial preclinical software (SMARTARC) were used as an additional benchmark to classify the quality of the novel techniques. Four cases with concave planning target volumes (PTV) with one dominating organ at risk (OAR), viz., the PTV/OAR combination of the ESTRO Quasimodo phantom, breast/lung, spine metastasis/ spinal cord, and prostate/rectum, were used for the study. The composite objective value (COV) and other parameters representing the plan quality were studied. The novel 2-step IMAT techniques with geometry based segment definition were as good as or better than DMPO and were superior to the SMARTARC VMAT techniques. For the spine metastasis, the quality measured by the COV differed only by 3%, whereas the COV of the 2-step IMAT for the other three cases decreased by a factor of 1.4-2.4 with respect to the reference plans. Rotational techniques based on geometrical analysis of the optimization problem (2-step IMAT) provide similar or better plan quality than DMPO or the research version of SMARTARC VMAT variants. The results justify pursuing the goal of fast IMAT adaptation based on 2-step IMAT techniques.

Dose distribution features of periodic motion target based on MatriXX system

Objective To investigate the dose distribution features of periodic motion target based on MatriXX system. Methods The static and periodic motion targets were separately irradiated by step and shoot and sliding window intensity modulated radiation therapy (IMRT) beams. The dose distribution was measured by MatfiXX system combined with domestic made periodic motion platform. Then the dose distribu-tions were compared by using the OmniPro I'mRT software system. Results When the target was irradiated by one type of IMRT beams (step and shoot or sliding window), the differences of dose distributions between periodic motion target and static were mainly located at the edge of the target, about 15%. When the target (static or motion) was irradiated separately by two types of IMRT beams (step and shoot and sliding window), the differences of dose distributions between step and shoot and sliding window irradiation were very small. Con-clusions The differences of dose distributions between periodic motion and static target were mainly located at the edge of the fields, and we should pay more attention. In the central area of the target, the dose distribution differences were very small. Key words: Radiotherapy; Step and shoot IMRT; Two-dimensional ionization chamber array MatriXX system; Sliding window IMRT

An investigation of the accuracy of Monte Carlo portal dosimetry for verification of IMRT with extended fields

This work investigated the accuracy of Monte Carlo (MC) simulations of amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) for the dosimetric verification of intensity-modulated radiotherapy (IMRT). In particular, the suitability of the method for verification of head and neck IMRT with extended field segments (≈20 cm superior–inferior), covering almost the entire detector area, was studied. A solution involving schematic modelling of backscatter materials has been established to account for non-uniform backscatter to the imager from supporting structures. 96% of points within the IMRT fields evaluated passed a ‘gamma’ evaluation criterion of 2%, 2 mm at isocentre at a dose rate of 100 MU min−1 with this solution included. Only 79% of points passed this gamma criterion without the correction for backscatter included. This work has also demonstrated the ability of the technique to detect systematic delivery errors in step and shoot IMRT. The technique identified a systematic overshoot on the first segment and an undershoot on the final segment. Results were verified by ion chamber measurements and agreed well with those reported in the literature, averaging approximately 0.1 and 0.3 MU for 100 and 300 MU min−1 deliveries, respectively. MC portal verification has the potential to become a key tool in the verification of IMRT and can also facilitate selection of optimal delivery parameters, thus improving treatment accuracy. This approach can be applied to the verification of other new treatment techniques and should also enable development of methodologies to detect and correct for delivery errors, both before and during treatment.

SU‐GG‐J‐100: A Comparison of the Effect of Intra‐Fraction Motion on Tomotherapy and IMRT Delivery Systems

Purpose: To illustrate the variation in dose distribution caused by a moving target volume that can be expected from a Tomotherapy treatment plan and a conventional step and shoot intensity modulated radiation therapy (IMRT) plan. Method and Materials: A two‐dimensional Motionsim platform was used with Mapcheck2 to obtain the measurements. Tumor motion with a superior‐inferior amplitude of 1cm and lateral amplitude of 0.2cm was used. Similar IMRT treatment plans were created for a volume vertically centered on the diode array using Tomotherapy and CMS Xio, respectively. A planned dose of 2Gy per fraction was delivered onto both a stationary Mapcheck2 diode array, as well as on the array when the tumor motion was simulated. The process was then repeated using a 9 beam step and shoot IMRT plan on a conventional Linac. Results: When compared to the plan with a stationary target, the motion file introduced single fraction dose variation at the superior and inferior borders of the field of up to 20%. This was the case for both the helical Tomotherapy and co‐planar IMRT scenarios. Detectors inferior of the target volume received more dose than planned, while superior detectors received up to 20% less dose than planned. The penumbra for these resulting hot/cold spots for the Tomotherapy plan was larger than for the IMRT plan. Conclusion: The results reiterate the need for tumor motion management when IMRT is used during treatment. Motion effects may even be more pronounced for helical tomotherapy treatments than for conventional IMRT.

Remarks on reporting and recording consistent with the ICRU Reference Dose

BackgroundICRU 50/62 provides a framework to facilitate the reporting of external beam radiotherapy treatments from different institutions. A predominant role is played by points that represent "the PTV dose". However, for new techniques like Intensity Modulated Radiotherapy (IMRT) - especially step and shoot IMRT - it is difficult to define a point whose dose can be called "characteristic" of the PTV dose distribution. Therefore different volume based methods of reporting of the prescribed dose are in use worldwide. Several of them were compared regarding their usability for IMRT and compatibility with the ICRU Reference Point dose for conformal radiotherapy (CRT) in this study.MethodsThe dose distributions of 45 arbitrarily chosen volumes treated by CRT plans and 57 volumes treated by IMRT plans were used for comparison. Some of the IMRT methods distinguish the planning target volume (PTV) and its central part PTVx (PTV minus a margin region of × mm). The reporting of dose prescriptions based on mean and median doses together with the dose to 95% of the considered volume (D95) were compared with each other and in respect of a prescription report with the aid of one or several possible ICRU Reference Points. The correlation between all methods was determined using the standard deviation of the ratio of all possible pairs of prescription reports. In addition the effects of boluses and the characteristics of simultaneous integrated boosts (SIB) were examined.ResultsTwo types of methods result in a high degree of consistency with the hitherto valid ICRU dose reporting concept: the median dose of the PTV and the mean dose to the central part of the PTV (PTVx). The latter is similar to the CTV, if no nested PTVs are used and no patient model surfaces are involved. A reporting of dose prescription using the CTV mean dose tends to overestimate the plateau doses of the lower dose plateaus of SIB plans. PTVx provides the possibility to approach biological effects using the standard deviation of the dose within this volume.ConclusionThe authors advocate reporting the PTV median dose or preferably the mean dose of the central dose plateau PTVx as a potential replacement or successor of the ICRU Reference Dose - both usable for CRT and IMRT.

Step and shoot IMRT to mobile targets and techniques to mitigate the interplay effect

The purpose of this work is to evaluate a method to mitigate temporal dose variation due to the interplay effect as well as investigate the effect of randomly varying motion patterns. The multi-leaf collimator (MLC) settings from 5, 9 and 11 field step and shoot intensity modulated radiation therapy (IMRT) of non-small cell lung cancer (NSCLC) treatment plans with tumor motion of 1.53, 1.03 and 1.95 cm, respectively, were used. Static planar dose distributions were determined for each treatment field using the Planar Dose Module in the Pinnacle3 treatment planning system. The MotionSIM XY/4D robotic diode array was used to recreate the tumor motion orthogonal to each treatment beam. Dose rate modulation was investigated as a method to mitigate temporal dose variation due to the interplay effect. Computer simulation was able to identify individual fields where interplay effects are greatest. Computer simulation and physical measurement have shown that temporal dose variation can be mitigated by the selection of the dose rate or by selective dose rate modulation within a given IMRT treatment field. Selective dose rate modulation within a given IMRT treatment field reduced temporal dose variation to levels comparable to whole field dose rate reduction, while also producing shorter radiation delivery times in six of the seven cases investigated. For the cases considered, the interplay effect did not appear to have a greater effect on hypofractionation compared to traditional fractionation even though fewer fractions were delivered. Randomized motion kernel variation was also considered. For this portion of the study, a nine field step and shoot IMRT configuration was considered with a 1.03 cm tumor motion rather than the five field case. In general, if the extent of the variant motion pattern was mostly contained within the target volume, limited impact on the temporal dose variation was observed. In cases where the variant motion kernels increasingly exceeded the target volume limits, increases in temporal dose variation were observed.

A Monte Carlo-based procedure for independent monitor unit calculation in IMRT treatment plans

Intensity-modulated radiotherapy (IMRT) treatment plan verification by comparison with measured data requires having access to the linear accelerator and is time consuming. In this paper, we propose a method for monitor unit (MU) calculation and plan comparison for step and shoot IMRT based on the Monte Carlo code EGSnrc/BEAMnrc. The beamlets of an IMRT treatment plan are individually simulated using Monte Carlo and converted into absorbed dose to water per MU. The dose of the whole treatment can be expressed through a linear matrix equation of the MU and dose per MU of every beamlet. Due to the positivity of the absorbed dose and MU values, this equation is solved for the MU values using a non-negative least-squares fit optimization algorithm (NNLS). The Monte Carlo plan is formed by multiplying the Monte Carlo absorbed dose to water per MU with the Monte Carlo/NNLS MU. Several treatment plan localizations calculated with a commercial treatment planning system (TPS) are compared with the proposed method for validation. The Monte Carlo/NNLS MUs are close to the ones calculated by the TPS and lead to a treatment dose distribution which is clinically equivalent to the one calculated by the TPS. This procedure can be used as an IMRT QA and further development could allow this technique to be used for other radiotherapy techniques like tomotherapy or volumetric modulated arc therapy.

Radiographic film dosimetry for IMRT fields in the near‐surface buildup region

Radiographic film dosimetry provides fast, convenient 2‐D dose distributions, but is challenged by the dependence of film response on scatter conditions (i.e., energy dependence). Verification of delivered dose in the surface buildup region is important for intensity modulated radiation therapy (IMRT) when volumes of interest encroach on these regions (e.g., head/neck, breast). The current work demonstrates that film dosimetry can accurately predict the dose in the buildup region for IMRT, since 1) film dosimetry can be performed with sufficient accuracy for small fields and 2) IMRT is delivered by a series of “small” segments (step and shoot IMRT). This work evaluates the accuracy of X‐OMAT V (XV) and Extended Dose Range (EDR) film for measurements from 2 mm to 15 mm depths for small fields and clinical IMRT beams. Film measurements have been compared to single point measurements made with a stereotactic diode and parallel plate ionization chamber (P11) and thermoluminescent dosimeters (TLD) at various depths for square (diode, P11) and IMRT (diode, TLD) fields. Film calibration was performed using an 8‐field step exposure on a single film at 5 cm depth, which has been corrected to represent either small field or large field depth dependent film calibration techniques. Up to 10% correction for film response variation as a function of depth was required for measurements in the buildup region. A depth‐dependent calibration can sufficiently improve the accuracy for IMRT calculation verification (i.e., ≤5% uncertainty). A small field film calibration technique was most appropriate for IMRT field measurements. Improved buildup region dose measurements for clinical IMRT fields promotes improved dose estimation performance for (inverse) treatment planning and allows more quantitative treatment delivery validation.PACS numbers: 87.53.‐j, 87.53.Dq

SU‐GG‐T‐551: VMAT Compared to 3D‐RT and Step and Shoot IMRT for Anal Cancer Pelvis Treatment

Purpose: The shielding of organs at risk (OAR) like testis, small bowel and bladder increases the complexity of a treatment for anal cancer. The new rotation therapy volumetric intensity modulated arc therapy (VMAT) may have the potential to reduce the treatment time while producing a comparable dose distribution as the intensity modulated radiation therapy (IMRT). Method and Materials: CT datasets of 8 patients with anal cancer from our department formed the basis of this study. The routinely applied refined 3D‐Treatment Plan (Masterplan, Theranostic) was compared to a 9 beam step and shoot IMRT (Corvus 6.3, Nomos) and a VMAT plan with 2 rotations generated with ERGO++ 1.6 by Elekta. All three treatment planning systems used the identical CT datasets and the same OAR's. To compare these three techniques, the dose volume histograms (DVH) of PTV and OAR's as well as the total treatment time (TTT) were used. The homogeneity index (HI) and the conformality index (CI) calculated as suggested by the RTOG guidelines were analysed. Results: VMAT provided the best PTV coverage as indicated by the following metrics (isodose as percentage of prescription dose (PD) encompassing 95% of the PTV / percentage of tissue outside the PTV encompassed by 95% of PD). For 3D‐RT the mean values were (94.7±0.6% / 5.4±1.5%), for IMRT (89.8±1% / 0.7±2.6%) and for VMAT (93.7±1% / 2.9±1.3%). The mean CI and HI over all 8 patients was (2.00±0.17, 1.06±0.01) for 3D‐RT, for IMRT (1.56±0.16, 1.15±0.02) and for VMAT (1.71±0.13, 1.09±0.03). The TTT for 3D‐RT (220 seconds) was much shorter than for IMRT (557 seconds). The estimated TTT of the VMAT technique is 180 seconds. Conclusion: The new VMAT technique provides excellent treatment planes with highest conformality and homogeneity. The short treatment delivery time together with low primary monitor units is the most important advantage.

SU‐GG‐T‐531: Comparison of Volumetric Modulated Arc Therapy (VMAT) with Serial Tomotherapy and Segmental (step and Shoot) IMRT for Boost Treatment of Prostate Cancer

Purpose: VMAT is about to be established clinically on a wide basis and may increase the efficiency of Intensity Modulated Radiotherapy (IMRT) treatment. The goal of this investigation was to compare VMAT to established IMRT delivery techniques of serial Tomotherapy and segmental IMRT for prostate cancer. Method and Materials: Using CT data sets of 9 patients treated for prostate cancer, treatment plans for all three techniques were created. Segmental IMRT was planned for a conventional MLC (Elekta MLCi, 2×40leaves, 1cm) and serial Tomotherapy for the MIMiC Multivane‐Collimator (NOMOS, Corp.), using the Corvus 6.3 (Nomos; Pencil beam). VMAT was planned with ERGO++ R1.6 (3D Line Medical Systems/Elekta; Pencil beam). Segmental IMRT plans with MLC used 7 isotropic beams. Serial Tomotherapy plans were calculated in 1cm mode and for a 300° rotation. VMAT plans were created with a 360° and two 100° rotations. Evaluation of plan quality used metrics of conformity index (CI=Vtissue Dmin/Vtarget), homogeneity index (HI=Dmax/Dprescribed), dose to normal tissue and the isodose which encompasses 95% of the target volume. Results: Median indices for conformity and homogeneity of the Corvus plans were CI 2,34/HI 1,21; MLC based segmental IMRT CI 2,89/HI 1,22; and VMAT plans CI 2,02/HI 1,11. 76Gy were prescribed to the median dose of the PTV. The resulting median doses to organ at risk (OAR) for (MIMiC/MLC/VMAT) were as follows: for anterior rectum (52,2Gy/52,4Gy/60,5Gy), posterior rectum (32,3Gy/33,5Gy/34,3Gy) and bladder (44,9Gy/44,1Gy/53,7Gy). Volumes for 50% of prescribed dose (38Gy) in normal tissue were 1091ml/1583ml/1191ml. Dose encompassing 95% of PTV was (MIMiC/MLC/VMAT) 68,8Gy/66,6Gy/70,7Gy. Median treatment time (MIMiC/MLC/VMAT) was 15min/7,5min/3min. Conclusion: Rotational therapy techniques showed higher conformity, coverage and lower normal tissue dose. Homogeneity and dose to posterior rectum were similar for all approaches. VMAT plans are deliverable within a 3minute time slot which is the most efficient treatment option compared to the other approaches.

A genetic algorithm used for the intensity level discretization in MLC leaf sequencing for step and shoot IMRT

The inverse planning for a step-and-shoot plan in intensity-modulated radiotherapy (IMRT) is usually a multiple step process. Before being converted into the MLC segments, the optimum intensity profiles of beams, which are generated by an optimization algorithm, shall be discretized into a few intensity levels. The discretization process of the optimum intensity profiles can induce deviations in the final dose distribution from the original optimum dose distribution. This paper describes a genetic algorithm for the discretization of given optimum intensity profiles. The algorithm minimizes an objective function written in terms of the intensity levels. Both the dose-based objective function, which is defined by the deviation between the dose distributions before and after the discretization, and the intensity-based objective function, which is defined by the deviation between the optimum intensity profiles and the discretization intensity profiles, have been adopted. To evaluate this algorithm, a series of simulation calculations had been carried out using the present algorithm, the even-spaced discretization and the k-means clustering algorithm respectively. By comparing the resultant discretization-induced deviations (DIDs) in intensity profiles and in dose distributions, we have found that the genetic algorithm induced less DIDs in comparison with that induced in the even-spaced discretization or the k-means clustering algorithm. Additionally, it has been found that the DIDs created in the genetic algorithm correlate with the complexity of the intensity profiles that is measured by the “fluence map complexity”.