Dynamic IMRT Research Articles

SU‐FF‐T‐286: Dosimetric Differences in Dynamic MLC Performance as a Result of Alignment and Software Configuration

Purpose: During commissioning of a new linear accelerator (Varian Clinac 21iX), differences were found in dynamic MLC (sliding window) delivery of 2.5–5.5% when compared to an existing matched linear accelerator (Varian Trilogy). An investigation was done to determine the causes of these differences. Method and Materials: Ion chamber and film measurements were done on several accelerators while varying MLC alignment parameters. Test deliveries, including open fields, MLC fields, “sweeping gap”, and dynamic MLC fields were made using a central ion chamber and film. A difference in the software configuration of the MLC (“SureTouch”) resulted in a significant dosimetric effect and measurements were performed to ascertain its impact. Results: Except for dynamic MLC delivery, the accelerators were well matched, with all differences less than 0.5%. Differences in delivery of dynamic IMRT treatments were found in three areas: 1) Calibration of the MLC alignment can introduce up to 2% variation in dMLC performance; 2) A variation of 1.5% appears to be inherent in the mechanical performance of MLCs; 3) A 1% difference appears to be a result of the software configuration of the MLC by service engineers. This configuration can be optionally introduced in cases where poor performance of the MLC is noted during patient deliveries. Conclusion: With proper configuration and calibration of an MLC, machine‐to‐machine performance should be within 2%. As noted in several previous publications, careful calibration and testing of MLC leaf position and alignment are critical to proper delivery of dynamic MLC treatments. Software configuration of the MLC by service engineers can affect dosimetric performance for IMRT deliveries. Medical physicists should monitor MLC configuration changes installed by service personnel carefully, as these changes may require alterations in treatment planning system models.

SU-FF-T-213: Evaluation of RadCalc V5.2 as An Independent Monitor Unit Checking Program for Dynamic IMRT Plans

Purpose: To evaluate the accuracy of an independent monitor unit check program for checking Head and Neck IMRT plans generated by Eclipse v8.6 Treatment Planning System (Varian, Palo Alto, USA). Method and Materials: The independent monitor unit check program, RadCalc v5.2 (Lifeline Software, Texas, USA), was first commissioned to check conventional 3D conformal radiotherapy treatment plans. The IMRT module in RadClac was then optimized to test the controlled dynamic fluences used for IMRT delivery by Varian linear accelerators. The optimization involved adjustment of four RadCalc parameters: phantom scatter factor represented by Sp to the value of 1.5% from published values of C. McKerracher and D. I. Thwaites (1), Collimator scatter factor correction Sc to 0.650, Radiation Light offset to 0.95 and MLC transmission to 0.022. Preliminary data obtained from RadCalc has been compared to predicted doses by Eclipse TPS and to the ionization chamber measurements for a chair optimal fluence proposed by Van Esch et al (2). Preliminary analysis of three IMRT plans for Tonsil and Parotid treatments was also carried out using clinical verification data. Results: Initial investigation has shown that RadCalc is capable of predicting doses to within 2% for “chair” optimal fluence. For clinical IMRT cases RadCalc predicted dose agreed within 5% of ionization chamber measurements in a homogenous medium. Conclusion: RadCalc has the potential to be a useful independent monitor check QA tool for checking IMRT treatment plans. Further analysis is required to evaluate the accuracy and adequacy in a clinical IMRT program.

SU-FF-T-191: Considerations of Reference Level Definition for IMRT Plan Verification and Application of a Three Dimensional Gamma-Index Analysis

Purpose: A 3D Gamma Index analysis and different local reference levels for the dose axis are evaluated. Method and Materials: The Gamma‐Index method is usually performed with two spatial dimensions and the dose as variables. In the 3D‐Gamma Index method all three spatial dimensions and the dose are used, and different reference level methods (maximum, local dose, predefined dose level) can be chosen. Results: When the maximum dose is taken as the reference, the dose half axis of the acceptance ellipsoid becomes larger compared to using the local dose. This kind of reference tends to suppress Gamma‐Index failures at low doses. While this may be useful in situations in which the dose modulation in the distribution is comparably smooth, and when no larger dose gradients are to be expected (e.g. prostate treatments with dynamic IMRT techniques), critical areas may be overseen in more complex situations such as H&N plans. The reference to a user‐chosen dose level is a method focusing on deviations from a predefined dose‐level and can be used to analyze possible errors in the regions of an organs‐at‐risk. The 3D‐Gamma‐Index allows for smaller differences in the z‐position of the 2D‐ARRAY and the planning system. In combination with the reference to the local dose, the critical errors have as well been detected. Conclusion: It is shown that that the often used reference to the maximum dose tends to underestimate larger deviations in the low dose regions. Because these regions are often in the projection of organs‐at‐risk possible impacts may be overseen. We show that reference levels such as the local dose or a pre‐defined dose level do not have this drawback. Conflict of Interest: This work was performed in cooperation with PTW‐Freiburg.

Sparing the penile bulb in the radical irradiation of clinically localised prostate carcinoma: A comparison between MRI and CT prostatic apex definition in 3DCRT, Linac-IMRT and Helical Tomotherapy

Background and purpose To assess the impact of using MRI and Helical Tomotherapy (HT) compared to 3DCRT and dynamic IMRT on the dose to the penile bulb (PB). Materials and methods Eight patients diagnosed with prostate cancer entered a treatment protocol including CT and MRI simulation. The prostate apex was defined on both MRI and CT. Treatment plans (HT, Linac-IMRT, 3DCRT and conventional technique), were elaborated on both MRI and CT images. A dose of 71.4 Gy (2.55 Gy/fraction) was prescribed; it was requested that PTVs be covered by 95% isodose line. The mean dose and V50 of PB were evaluated. Results PTV-MRI plans reduced PB mean dose and V50 compared to PTV-CT plans. This improvement, deriving also from the treatment modality, was 89% for 3DCRT, 99% for Linac-IMRT and 97% for HT ( p < 0.01), considering V50. Conventional plans resulted in a significantly higher mean PB dose/V50 compared to 3DCRT-PTV-CT (+27%/+38%), Linac-IMRT-PTV-CT (+42%/+57%) and HT-PTV-CT (+32%/+48%) ( p < 0.01). The comparison between conventional and PTV-MRI techniques showed a still larger increase: +73%/+93% 3DCRT; +86%/+99% Linac-IMRT; +56%/+99% HT ( p < 0.01). The PB mean dose reduction with Linac-IMRT compared to 3DCRT was 24% ( p = 0.034) and 40% ( p = 0.027) for PTV-CT and PTV-MRI, respectively. This gain remained significant even when comparing Linac-IMRT to HT: 21% ( p = 0.07) PTV-CT and 68% ( p = 0.00002) PTV-MRI. HT was superior to 3DCRT with respect to PTV-CT (average gain 4%, p = 0.044), whereas it resulted to be detrimental considering PTV-MRI (26 Gy vs 16.5 Gy), possibly due to the helical delivery of HT; however, in a patient where the distance bulb-PTV <1 cm, HT provided better PB sparing than 3DCRT (29.5 Gy vs 45.2 Gy). Conclusions MRI allowed efficient sparing of PB irrespective of the treatment modality. Linac-IMRT was shown to further reduce the dose to the bulb compared to 3DCRT and HT.

Study of 2D ion chamber array for angular response and QA of dynamic MLC and pretreatment IMRT plans

Aim To study of 2 Dimensional ion chamber array for angular response and its utility for quality assurance of dynamic multileaf collimator and pretreatment intensity modulated radiotherapy plans. Materials and Methods The MLC QA test patterns and IMRT plans were executed on 2D ion chamber array having 1020 vented pixel ionization chambers. The dynamic MLC QA test patterns were chair test, x–wedge, pyramid, open swipe field, garden fence and picket fence. Performance of Dynamic wedges was compared with physical wedges. For IMRT verification, five patients with localized prostate carcinoma were planned using dynamic IMRT technique. Angular response of MatriXX was measured by exposing the system from different gantry angles. Results Dynamic MLC QA tests such as chair, x-wedge, pyramid, and open swipe field were successfully verified. MatriXX was not able to recognize the bar pattern of picket test and garden fence test. The response of MatriXX gradually decreases from 0° to 180° angles and it was 7.7% less at 180° angle. The dynamic wedge profiles were matching with corresponding physical wedge profiles. For pretreatment IMRT QA, the average dose difference between planned and measured dose was 1.26% with standard deviation of 1.06. Conclusion I'mRT MatriXX can be used for routine dynamic MLC and IMRT pretreatment QA but care should be taken while taking measurements in penumbra region because of its limited spatial resolution.

Integration of Real-Time Internal Electromagnetic Position Monitoring Coupled With Dynamic Multileaf Collimator Tracking: An Intensity-Modulated Radiation Therapy Feasibility Study

Continuous tumor position measurement coupled with a tumor tracking system would result in a highly accurate radiation therapy system. Previous internal position monitoring systems have been limited by fluoroscopic radiation dose and low delivery efficiency. We aimed to incorporate a continuous, electromagnetic, three-dimensional position tracking system (Calypso 4D Localization System) with a dynamic multileaf collimator (DMLC)-based dose delivery system. A research version of the Calypso System provided real-time position of three Beacon transponders. These real-time three-dimensional positions were sent to research MLC controller with a motion-tracking algorithm that changed the planned leaf sequence. Electromagnetic transponders were embedded in a solid water film phantom that moved with patient lung trajectories while being irradiated with two different plans: a step-and-shoot intensity-modulated radiation therapy (S-IMRT) field and a dynamic IMRT (D-IMRT) field. Dosimetric results were recorded under three conditions: no intervention, DMLC tracking, and a spatial gating system. Dosimetric accuracy was comparable for gating and DMLC tracking. Failure rates for gating/DMLC tracking are as follows: +/-3 cGy 10.9/ 7.5% for S-IMRT, 3.3/7.2% for D-IMRT; gamma (3mm/3%) 0.2/1.2% for S-IMRT, 0.2/0.2% for D-IMRT. DMLC tracking proved to be as efficient as standard delivery, with a two- to fivefold efficiency increase over gating. Real-time target position information was successfully integrated into a DMLC effector system to modify dose delivery. Experimental results show both comparable dosimetric accuracy as well as improved efficiency compared with spatial gating.

Development and Experimental Evaluation of Guidelines for Planning and Treating Moving Tumors using Dynamic IMRT

Development and Experimental Evaluation of Guidelines for Planning and Treating Moving Tumors using Dynamic IMRT

Changes in 3T-MRI Prostate Volumes in High Risk Prostate Cancer Treated with Hypofractionated Radiation using Dynamic IMRT and Varying Hormonal Therapy Duration

Changes in 3T-MRI Prostate Volumes in High Risk Prostate Cancer Treated with Hypofractionated Radiation using Dynamic IMRT and Varying Hormonal Therapy Duration

Dosimetric Properties of an Amorphous Silicon Electric Portal Imaging Device for Verification of Dynamic IMRT

Dosimetric Properties of an Amorphous Silicon Electric Portal Imaging Device for Verification of Dynamic IMRT

Sci-Thurs PM: Delivery-06: 2-D lag and response nonlinearity corrections for dynamic IMRT verification using an EPID.

In recent years, EPIDs have been used for pre-treatment IMRT verification. Although EPID lag and signal nonlinearities have been investigated, they have not been implemented in the verification process. In dynamic sliding-window IMRT delivery, the dose delivered, and the time between the end of dose delivery and the end of image acquisition differ between pixels. The resulting differences in lag and signal-response across the image can cause artificial asymmetries and amplitude changes in measured EPID dose images. These artifacts alter the agreement between measured and predicted images, potentially complicating the assessment of clinical IMRT verifications. A method of 2-D (pixel-by-pixel) correction was developed based on data from sets of experiments performed to independently quantify the lag and nonlinearity characteristics of Varian's aS500 EPID. To test the correction, it was applied to two sweeping window 10×10 cm2 fields that differ only in sweeping direction. The correction resolved discrepancies in the symmetry between these two cases, and the differences between measured and predicted amplitudes evident when small numbers of MUs were delivered. To illustrate its potential use, the correction technique was applied to a measured image of a clinical IMRT field that produced a relatively poor verification result. The correction partially accounted for discrepancies between measured and Eclipse-predicted images of this field, reducing the percentage of pixels failing a Gamma analysis (3 %, 3 mm) from 8.5 to 5.6 %. This correction technique can be used to help resolve the source of discrepancies in troublesome clinical IMRT verifications.

WE-E-AUD A-01: Controlling the Impact of Intensity Modulation When Treating Moving Targets with Dynamic IMRT

Purpose: The treatment of moving targets using dynamic IMRT can result in dosimetric effects that have a predictable component (dose blurring) and an unpredictable component (interplay between organ motion and leaf motion). We investigate the impact of the intensity modulation on the unpredictable component. Method and Materials: Software was written to simulate MLCs moving across a moving target using dose distributions exported from Eclipse. The dose distribution delivered to a static target was blurred with the target motion to give the expected dose distribution. Dose errors were calculated by comparing the dose delivered to a moving target with the expected dose distribution. The impact of the intensity modulation (1:1 to 6:1) on the maximum dose error was investigated for a wide range of MLC sequences and target motion. MLC sequences were identified which kept dose variations within 10% of the expected dose. Results were confirmed experimentally by measuring the dose delivered to an ion chamber array in a moving phantom. Results: The maximum dose error increases with increasing intensity modulation. Dose errors also increase with a decrease in the following parameters: target period, target amplitude, MLC separation and MLC speed. Simple rules were developed to determine whether a planned fluence / MLC sequence will give a dose error larger than 10%. If the MLC speed is restricted to 0.1cm/sec and target period ⩽6sec, then the maximum daily dose error is always less than 10% for all levels of intensity modulation. For some combinations of intensity modulation and MLC sequences, faster MLC speeds are acceptable depending on details of the MLC parameters and target motion. Conclusion: Intensity modulation as high as 6:1 can be used when treating moving targets with dynamic IMRT, but the MLC speed must be carefully restricted if unpredictable and large dose errors are to be avoided.

SU‐GG‐T‐154: Use of I'mRT MatriXX for Routine Dynamic MLC QA and IMRT Dose Verification

Purpose: To investigate the utility of 2D ion chamber array, I'mRT MatriXX™ (Scanditronix Wellhofer, Sweden) for dynamic MLC QA and IMRT dose verification. Method and Materials: The MLC QA test patterns designed for millennium 80 multileaf collimator were chair, x—wedge, pyramid, peak, open swipe field, garden fence and picket fence. For IMRT verification, five prostate cancer patients were planned using dynamic IMRT technique on Eclipse treatment planning system using 6 MV photons. The MLC QA test patterns and IMRT plans were executed on 2D ion chamber array, I'mRT MartiXX (24×24 cm2 active area, 1020 ventilated ion chambers) with equivalent 5 cm solid water buildup, keeping SSD 95. Planned and delivered dose delivery was compared using Gamma analysis (Δ dose 3% and Δ distance 3mm). Results: Dose response of MatriXX was linear in 0.2–10 Gy. Dose rate dependence was with in 0.15% in range of 1–6 Gy/min. Dynamic MLC QA tests such as chair, x‐wedge, pyramid, and open swipe field were successfully verified and passed the gamma analysis criterion with more than 95% pixels in defined field size. But because of less spatial resolution (7.62 mm), matrix was not able to recognize garden fence, picket fence, and 2.5 mm pattern in peak test. Mean and SD of difference between TPS calculated and measured CAX doses were 1.27% and 1.06 respectively for IMRT plans. Mean and SD of gamma analysis were 94.85% and 2.16 respectively. Dose differences were more than 3% along y direction in penumbra region because dose distribution can not be faithfully reflected by linear interpolation in high dose gradient region by MatriXX. Other than penumbra region, all pixels successfully passed the gamma analysis criterion. Conclusion: I'mRT MatriXX can be used for routine MLC and pretreatment IMRT QA. But care should be taken while taking measurements in penumbra region.

SU‐GG‐T‐542: Arc‐Modulated Radiation Therapy (AMRT): A Novel Method for Rotational Radiation Therapy

Purpose: To develop a prototype treatment planning system for Arc‐Modulated Radiation Therapy (AMRT), which delivers the optimal radiation distribution with only one beam rotation. Method and Materials: The AMRT planning starts with inverse planning, typically using 36 equally‐spaced (fixed) beams and producing 36 continuous intensity maps. After inverse planning, an AMRT leaf sequencing algorithm converts the optimized intensity patterns to a sequence of MLC apertures along a single arc. Finally, a homegrown Monte‐Carlo superposition‐based dose calculation engine is used to quickly and accurately evaluate the plan qualities. A big challenge to AMRT is leaf sequencing. Our algorithm is based on a key observation that if each optimized intensity pattern is delivered through its neighboring beam angles within a ±5° range, there is only negligible error on the resulting dose distribution. Thus, the sequencing problem becomes how to guarantee the interconnectedness of the MLC apertures along the single arc, while reproducing or closely approximating the optimized intensity patterns. We model this problem as computing shortest paths in directed acyclic graphs, and solve it efficiently using various algorithmic techniques. Results: We applied our AMRT approach to 18 clinical cases with a wide range of treatment sites. In terms of dose conformity, we observed that AMRT plans outperform gantry‐fixed IMRT plans and rival intensity‐modulated arc therapy (IMAT) plans. In terms of delivery efficiency, AMRT plans are much more efficient and can be delivered in less than 5 minutes for complex head‐and‐neck cases. The AMRT planning time is currently about 1–2 hours. Conclusion: We developed a prototype treatment planning system for AMRT, which delivers the optimal radiation distribution with only one beam rotation. Experimental studies demonstrated the potentials of our new approach to dynamic IMRT which warrants further investigations.

SU-GG-T-93: Dosimetry Evaluation and Correction of DMLC Control Points for Sliding Window IMRT

Purpose: The aim of this study is to investigate the sliding window‐intensity modulated radiation therapy (SW‐IMRT) dose profiles using the extreme dose rate and leaf velocity to reduce the irradiation time per beam and beam leakage for the field. Moreover, the accuracy of the dose profiles with multiple gradients generated by the SW‐IMRT was evaluated. Method and Materials:Radiographic films were used to measure the difference between the planned and delivered dynamic IMRTdose profiles, which were used to determine the point‐by‐point leaf correction function. Dose profiles of the flat, sinusoidal, exponential and steep linear functions were used with different prescribed doses up to 100 cGy per beam at the isocenter. Photon beams with different field sizes up to 12 × 12 cm2 were used with dose rates varying between 100 and 600 MU min−1, and leaf velocity was set from 1 to 5 cm s−1 in this study. Results: The correction methodology was examined for simple non‐clinical beam profiles that mimic the extremes of more complex fluence maps, and dynamic IMRT beams planned for the prostate and head‐and‐neck radiation treatments. Using the point‐by‐point correction functions, close agreement between the planned and delivered dose profiles can be achieved. Reduced field leakage, beam‐on time and deviation of dose profile in the range < 0.5% – 0.8% were found for all profiles with a high dose rate of 600 MU min−1 and leaf velocity of 5 cm s−1. Conclusion: Although this study was based on our in‐house developed software, the results are encouraging and the proposed model can be applied to a more complex optimization of clinical SW‐IMRT plans.

Comparison of fixed‐beam IMRT, helical tomotherapy, and IMPT for selected cases

A growing number of advanced intensity modulated treatment techniques is becoming available. In this study, the specific strengths and weaknesses of four techniques, static and dynamic multileaf collimator (MLC), conventional linac-based IMRT, helical tomotherapy (HT), and spot-scanning proton therapy (IMPT) are investigated in the framework of biological, EUD-based dose optimization. All techniques were implemented in the same in-house dose optimization tool. Monte Carlo dose computation was used in all cases. All dose-limiting, normal tissue objectives were treated as hard constraints so as to facilitate comparability. Five patient cases were selected to offer each technique a chance to show its strengths: a deep-seated prostate case (for 15 MV linac-based IMRT), a pediatric case (for IMPT), an extensive head-and-neck case (for HT), a lung tumor (for HT), and an optical neurinoma (for noncoplanar linac-based IMRT with a miniMLC). The plans were compared by dose statistics and equivalent uniform dose metrics. All techniques delivered results that were comparable with respect to target coverage and the most dose-limiting normal tissues. Static MLC IMRT struggled to achieve sufficient target coverage at the same level of dose homogeneity in the lung case. IMPT gained the greatest advantage when lung sparing was important, but did not significantly reduce the risk of nearby organs. Tomotherapy and dynamic MLC IMRT showed mostly the same performance. Despite the apparent conceptual differences, all four techniques fare equally well for standard patient cases. The absence of relevant differences is in part due to biological optimization, which offers more freedom to shape the dose than do, e.g., dose volume histogram constraints. Each technique excels for certain classes of highly complex cases, and hence the various modalities should be viewed as complementary, rather than competing.

Modelling ionization chamber response to nonstandard beam configurations

Novel technologies aiming at improving target dose coverage while minimising dose to organs at risk use delivery of radiation fields that significantly deviate from reference conditions defined in protocols such as TG-51 and TRS-398. The use of ionization chambers for patient-specific quality assurance of these new delivery procedures calibrated in reference conditions increases the uncertainties on dose delivery. The conversion of the dose to the chamber cavity to the dose to water becomes uncertain; and the geometrical details of the chamber, as well as the details of the delivery, are expected to be significant. In this study, a realistic model of the Exradin® A12 Farmer chamber is simulated. A framework is applied for the calculation of ionization chamber response to arbitrarily modulated fields as a summation of responses to pencil beams. This approach is used with the chamber model and tested against measurements in static open fields and dynamic MLC IMRT fields. As a benchmark test of the model, quality conversion factors values calculated by Monte-Carlo simulation with the chamber model are in agreement within 0.1 % and 0.4 % with those in the AAPM TG-51, for 6 MV and 18 MV photon beams, respectively. Pencil-beam kernels show a strong dependence on the geometrical details of the chamber. Kernel summations with open fields show a relative agreement within 4.0 % with experimental data; the agreement is within 2.0 % for dynamic MLC IMRT beams. Simulations show a strong sensitivity of chamber response on positioning uncertainties, sometimes leading to dose uncertainties of 15 %.

An independent dose calculation algorithm for MLC-based radiotherapy including the spatial dependence of MLC transmission

An analytical dose calculation algorithm was developed and commissioned to calculate dose delivered with both static and dynamic multileaf collimator (MLC) in a homogenous phantom. The algorithm is general; however, it was designed specifically to accurately model dose for large and complex IMRT fields. For such fields the delivered dose may have a considerable contribution from MLC transmission, which is dependent upon spatial considerations. Specifically, the algorithm models different MLC effects, such as interleaf transmission, the tongue-and-groove effect, rounded leaf ends, MLC scatter, beam hardening and divergence of the beam, which results in a gradual MLC transmission fall-off with increasing off-axis distance. The calculated dose distributions were compared to measured dose using different methods (film, ionization chamber array, single ionization chamber), and the differences among the treatment planning system, the measurements and the developed algorithm were analysed for static MLC and dynamic IMRT fields. It was found that the calculated dose from the developed algorithm agrees very well with the measurements (mostly within 1.5%) and that a constant value for MLC transmission is insufficient to accurately predict dose for large targets and complex IMRT plans with many monitor units.

Clinical evaluation of monitor unit software and the application of action levels

Purpose The aim of this study was the clinical evaluation of an independent dose and monitor unit verification (MUV) software which is based on sophisticated semi-analytical modelling. The software was developed within the framework of an ESTRO project. Finally, consistent handling of dose calculation deviations applying individual action levels is discussed. Materials and methods A Matlab-based software (“MUV”) was distributed to five well-established treatment centres in Europe (Vienna, Graz, Basel, Copenhagen, and Umeå) and evaluated as a quality assurance (QA) tool in clinical routine. Results were acquired for 226 individual treatment plans including a total of 815 radiation fields. About 150 beam verification measurements were performed for a portion of the individual treatment plans, mainly with time variable fluence patterns. The deviations between dose calculations performed with a treatment planning system (TPS) and the MUV software were scored with respect to treatment area, treatment technique, geometrical depth, radiological depth, etc. Results In general good agreement was found between calculations performed with the different TPSs and MUV, with a mean deviation per field of 0.2 ± 3.5% (1 SD) and mean deviations of 0.2 ± 2.2% for composite treatment plans. For pelvic treatments less than 10% of all fields showed deviations larger than 3%. In general, when using the radiological depth for verification calculations the results and the spread in the results improved significantly, especially for head-and-neck and for thorax treatments. For IMRT head-and-neck beams, mean deviations between MUV and the local TPS were −1.0 ± 7.3% for dynamic, and −1.3 ± 3.2% for step-and-shoot IMRT delivery. For dynamic IMRT beams in the pelvis good agreement was obtained between MUV and the local TPS (mean: −1.6 ± 1.5%). Treatment site and treatment technique dependent action levels between ±3% and ±5% seem to be clinically realistic if a radiological depth correction is performed, even for dynamic wedges and IMRT. Conclusion The software MUV is well suited for patient specific treatment plan QA applications and can handle all currently available treatment techniques that can be applied with standard linear accelerators. The highly sophisticated dose calculation model implemented in MUV allows investigation of systematic TPS deviations by performing calculations in homogeneous conditions.

Local Control (LC) and Overall Survival (OS) With Concomitant Temozolomide (TM) and High Dose “Shrinking Fields” Radiotherapy (SFHDRT) as Post Operative (PO) Treatment in Patients (pts) With Incomplete Resection (IR) of Glioblastoma (GBM)

LC and OS remain unsolved aspects of clinical history of GBM when treated with PO conventional radiotherapy doses. The study was aimed to evaluate local control and overall survival in pts incompletely resected for GBM, using concomitant TM and SFHDRT as adjuvant PO treatment. A series of 76 consecutive pts (50 male 26 female) with histological diagnosis of GBM, who underwent IR, entered a pilot study and were treated with concomitant TM and HDRT at our institution. Pts accrual started July 2003 and lasted up to July 2006 The pts median age was 52 years; all pts had a Karnofsky performance score no lower than 60. IR and the extent of the residual tumour burden after surgical removal were evaluated by MRI and 18FDG PET. The schedule of treatment was as follows: TM was administered orally at 75 mg/m2/die for all the length of the radiotherapy. SFHDRT, highly 3D conformal, was given in three phases: 46 Gy were given to a volume encompassing the MRI enhancement plus 4 cm margin, followed by 10 Gy to a volume as above but with only 3 cm margin; a boost of 10 Gy, for a total dose of 66 Gy, was then given to the previous volume with a margin of only 2 cm. Steroids and osmotic diuretics were used to prevent intracranial hypertension. Treatment response was evaluated according to MRI and PET scan, taken every three months for the first year and every 6 months, or if clinical symptoms occurred, thereafter. The response was considered complete if MRI and/or 18FDG-PET were negative, partial if MRI parameters and 18FDG-PET SUV were cut more than 50%, stable or no response when they remain unchanged. MRI aspects of post treatment necrosis were considered no tumour related if PET scan was negative. A complete response was documented in 11 patients; 30 pts experienced partial response while stable disease was obtained in 35 pts. MRI aspects of radionecrosis were observed in 37 pts. Nine pts developed local relapse within one year from the end of RT, and underwent as salvage treatment with a BED10 24 Gy dose given through a hypofractionated dynamic arc IMRT. Transient thrombocytopenia was observed in all patient as TM related acute toxicity. Neurological SFHDRT related toxicity was evaluated according to mini mental test. No significant alterations were seen, except in relapsed pts undergoing the salvage treatment. At median of follow-up of 26 months (range 6–36 months) the median survival was 22 months. Data obtained in the study support the conclusion that concomitant continuous daily TM and concomitant, SFHDRT, can be considered safe, well tolerated. It seems to improve LC and OS when compared with historical series of standard 60 Gy RT with concomitant TM.

Potential for Reduced Toxicity in the Treatment of Anal Canal Cancer: A Dosimetric Comparison of Dynamic IMRT, 7-Field IMRT, and 3-D Conformal Radiation Therapy Techniques

Potential for Reduced Toxicity in the Treatment of Anal Canal Cancer: A Dosimetric Comparison of Dynamic IMRT, 7-Field IMRT, and 3-D Conformal Radiation Therapy Techniques