W. De Gersem

Validation of MR-Based Polymer Gel Dosimetry as a Preclinical Three-Dimensional Verification Tool in Conformal Radiotherapy

The aim of this work was to investigate MR‐based polymer gel dosimetry as a three‐dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water‐filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin‐spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root‐mean‐square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized. Magn Reson Med 43:116–125, 2000.

Validation and application of polymer gel dosimetry for the dose verification of an intensity-modulated arc therapy (IMAT) treatment

Polymer gel dosimetry was used to assess an intensity-modulated arc therapy (IMAT) treatment for whole abdominopelvic radiotherapy. Prior to the actual dosimetry experiment, a uniformity study on an unirradiated anthropomorphic phantom was carried out. A correction was performed to minimize deviations in the R2 maps due to radiofrequency non-uniformities. In addition, compensation strategies were implemented to limit R2 deviations caused by temperature drift during scanning. Inter- and intra-slice R2 deviations in the phantom were thereby significantly reduced. This was verified in an investigative study where the same phantom was irradiated with two rectangular superimposed beams: structural deviations between gel measurements and computational results remained below 3% outside high dose gradient regions; the spatial shift in those regions was within 2.5 mm. When comparing gel measurements with computational results for the IMAT treatment, dose deviations were noted in the liver and right kidney, but the dose-volume constraints were met. Root-mean-square differences between both dose distributions were within 5% with spatial deviations not more than 2.5 mm. Dose fluctuations due to gantry angle discretization in the dose computation algorithm were particularly noticeable in the low-dose region.

Monte Carlo modeling of the ModuLeaf miniature MLC for small field dosimetry and quality assurance of the clinical treatment planning system

The purpose of this investigation was the verification of both the measured data and quality of the implementation of the add-on ModuLeaf miniature multileaf collimator (ML mMLC) into the clinical treatment planning system for conformal stereotactic radiosurgery treatment. To this end the treatment head with ML mMLC was modeled in the BEAMnrc Monte Carlo (MC) code. The 6 MV photon beams used in the setup were first benchmarked with a set of measurements. A total ML mMLC transmission of 1.13% of the 10 x 10 cm2 open field dose was measured and reproduced with the BEAMnrc/DOSXYZnrc code. Correspondence between calculated and measured output factors (OFs) was within 2%. Correspondence between MC and measured profiles was within 2% dose and 2 mm distance, only for the smallest 0.5 x 0.5 cm2 field the results were within 3% dose. In the next step, the MC model was compared with Gafchromic film measurements and Pinnacle(3) 7.4 f (convolution superposition algorithm) calculated dose distributions, using a gamma evaluation comparison, for a multi-beam patient setup delivered to a Lucytrade mark phantom. The gamma evaluation of the MC versus Gafchromic film resulted in 3.4% of points not fulfilling gamma <or= 1 for a 2%/2 mm criterion, the Pinnacle(3) 7.4 f versus Gafchromic results 3.8% and Pinnacle versus MC less than 1%. For specific patients with lesions of 8 cc and 0.2 cc, Monte Carlo and Pinnacle simulations of the plans were performed and compared using DVH evaluation. DVHs corresponded within 2% dose and 2% volume.

MCDE: a new Monte Carlo dose engine for IMRT

A new accurate Monte Carlo code for IMRT dose computations, MCDE (Monte Carlo dose engine), is introduced. MCDE is based on BEAMnrc/DOSXYZnrc and consequently the accurate EGSnrc electron transport. DOSXYZnrc is reprogrammed as a component module for BEAMnrc. In this way both codes are interconnected elegantly, while maintaining the BEAM structure and only minimal changes to BEAMnrc.mortran are necessary. The treatment head of the Elekta SLiplus linear accelerator is modelled in detail. CT grids consisting of up to 200 slices of 512 x 512 voxels can be introduced and up to 100 beams can be handled simultaneously. The beams and CT data are imported from the treatment planning system GRATIS via a DICOM interface. To enable the handling of up to 50 x 10(6) voxels the system was programmed in Fortran95 to enable dynamic memory management. All region-dependent arrays (dose, statistics, transport arrays) were redefined. A scoring grid was introduced and superimposed on the geometry grid, to be able to limit the number of scoring voxels. The whole system uses approximately 200 MB of RAM and runs on a PC cluster consisting of 38 1.0 GHz processors. A set of in-house made scripts handle the parallellization and the centralization of the Monte Carlo calculations on a server. As an illustration of MCDE, a clinical example is discussed and compared with collapsed cone convolution calculations. At present, the system is still rather slow and is intended to be a tool for reliable verification of IMRT treatment planning in the case of the presence of tissue inhomogeneities such as air cavities.

The importance of accurate linear accelerator head modelling for IMRT Monte Carlo calculations

Two Monte Carlo dose engines for radiotherapy treatment planning, namely a beta release of Peregrine and MCDE (Monte Carlo dose engine), were compared with Helax-TMS (collapsed cone superposition convolution) for a head and neck patient for the Elekta SLi plus linear accelerator. Deviations between the beta release of Peregrine and MCDE up to 10% were obtained in the dose volume histogram of the optical chiasm. It was illustrated that the differences are not caused by the particle transport in the patient, but by the modelling of the Elekta SLi plus accelerator head and more specifically the multileaf collimator (MLC). In MCDE two MLC modules (MLCQ and MLCE) were introduced to study the influence of the tongue-and-groove geometry, leaf bank tilt and leakage on the actual dose volume histograms. Differences in integral dose in the optical chiasm up to 3% between the two modules have been obtained. For single small offset beams though the FWHM of lateral profiles obtained with MLCE can differ by more than 1.5 mm from profiles obtained with MLCQ. Therefore, and because the recent version of MLCE is as fast as MLCQ, we advise to use MLCE for modelling the Elekta MLC. Nevertheless there still remains a large difference (up to 10%) between Peregrine and MCDE. By studying small offset beams we have shown that the profiles obtained with Peregrine are shifted, too wide and too flat compared with MCDE and phantom measurements. The overestimated integral doses for small beam segments explain the deviations observed in the dose volume histograms. The Helax-TMS results are in better agreement with MCDE, although deviations exceeding 5% have been observed in the optical chiasm. Monte Carlo dose deviations of more than 10% as found with Peregrine are unacceptable as an influence on the clinical outcome is possible and as the purpose of Monte Carlo treatment planning is to obtain an accuracy of 2%. We would like to emphasize that only the Elekta MLC has been tested in this work, so it is certainly possible that alpha releases of Peregrine provide more accurate results for other accelerators.

A comparison of the acute toxicity profile between two-dimensional and three-dimensional image-guided radiotherapy for postoperative prostate cancer

AIMS To compare acute gastrointestinal and genitourinary toxicity for patients positioned with an electronic portal imaging device (EPID) and patients positioned with kilovoltage cone beam computed tomography (CBCT) during postoperative prostate radiotherapy. MATERIALS AND METHODS Between 1999 and April 2010, 196 prostate cancer patients were referred for postoperative salvage radiotherapy. Patient position was corrected using EPID (1999 to December 2006, n=116) or CBCT (January 2007 to present, n=80). The treatment technique, number of beams, dose prescription, dose computation algorithm and planning target volume margins were not altered over time. Grade 1-3 acute gastrointestinal and genitourinary toxicity were compared between the EPID group and the CBCT group. RESULTS The incidence of grade 1 and 2 genitourinary toxicity was significantly reduced by 17 and 14%, respectively, in the CBCT group compared with the EPID group (P<0.05). This was mainly attributed to a decrease in the following grade 1 symptoms: frequency (P<0.05), nocturia (P=0.06) and urgency (P=0.07). Grade 2 incontinence (P=0.06) and frequency (P=0.06) were lower in the CBCT group. Grade 3 genitourinary toxicity was comparably low (EPID 3% versus CBCT 1%). There was no significant difference in gastrointestinal grade 1-2 toxicity between both groups. No grade 3 gastrointestinal toxicity was observed. CONCLUSIONS Patient positioning with CBCT significantly reduces acute genitourinary toxicity compared with positioning with EPID.

OC-0452: Prospective randomized adaptive dose-de-escalation in the elective neck: late toxicity and control

German Cancer Research Center DKFZ, Heidelberg and German Cancer Consortium DKTK partner site Dresden, Dresden, Germany , F. Lohaus, S. Löck, V. Gudziol, A. Nowak, C. Von Neubeck, I. Tinhofer, V. Budach, A. Sak, M. Stuschke, P. Balermpas, C. Rödel, M. Avlar, A.L. Grosu, A. Abdollahi, J. Debus, C. Belka, S. Pigorsch, S.E. Combs, D. Mönnich, D. Zips, G.B. Baretton, F. Buchholz, M. Baumann, M. Krause

SU‐E‐J‐49: Evaluation of Deformable Image Co‐Registration in Adaptive Dose Painting by Numbers for Head and Neck Cancer

Purpose: To evaluate the accuracy of automated contour deformation for head‐and‐neck cancer in adaptive treatment. Methods: Data from 13 head‐and‐neck patients in a phase I trial for adaptive treatment were used. Adaptation was based on [18F]FDG‐PET‐guided dose painting by numbers (DPBN) plans. Each patient had two DPBN plans based on: (i) a pretreatment PET/CT scan and (ii) a during‐treatment PET/CT scan acquired after 8 fractions. Contours manually drawn on the pretreatment CT scan were deformed using commercial deformable image registrationsoftware onto the during‐treatment CT scan. Deformed contours of regions of interest (ROIdef) were visually inspected by an experienced radiation oncologist and, if necessary, adjusted (ROIdef_ad) and both sets of contours were compared to manually redrawn ROIs (ROIm) using Jaccard (JI) and overlap indices (OI). ROI indices and volumes were compared for all contour sets used a paired t‐test and one‐way ANOVA pairwise comparison, respectively. Results: Almost all deformed ROIs in all patients required adjustment after visual inspection. The largest adjustments were made in GTVs when substantial tumor regression occured, e.g., ROIdef=9.2 cm3 vs. ROIdef_ad=2.2 cm3 vs. ROIm=2.1 cm3. The swallowing structures were the most frequently adjusted ROIs. The mandible was the most acurately propagated ROI requiring little or no adaptation: JI=0.7 and OI=0.8. The upper esophageal sphincter was the worst propagated ROI: JI=0.3 and OI=0.3 for the ROIdef, JI=0.5 and OI=0.6 for the ROIdef_ad. Despite the variation in indices, there was no statistically significant difference between ROIdef, ROIdef_ad and ROIm volumes. Generating ROIm took 4–6 hours, generating ROIdef took a few minutes and generating ROIdef_ad took less than 2 hours. Conclusions: Deformable image co‐registration followed by visual inspection does require adjustment of most deformed ROIs. Nevertheless, fast automatic ROI propagation followed by user‐driven adjustments appears to be more efficient than labor intensive de‐novo re‐contouring.

Clinical Implementation of IMRT by MLC Technology

The case in favour of IMRT is made as IMRT allows to shape the dose distribution in such a way that at least three challenges in clinical radiotherapy can be addressed. These are i) conforming dose to concave PTV shapes (i.e. head and neck (HN ii) reducing the volume of surrounding tissues irradiated to intermediate dose levels by using beam combinations with restricted hinge angles (lung, mediastinal tumours) and iii) generating controlled 3D-inhomogeneous dose distributions (i.e. prostate, paranasal sinus tumours). The two largest groups of patients that we treat with IMRT are H&N and prostate cancer patients.

PO-0638: Adaptive dose painting by numbers for head and neck cancer: interim analysis of a randomised trial

ESTRO 35 2016 ______________________________________________________________________________________________________ studies, topical clonidine shown activity in reducing NF-κB activation and incidence of severe OM (SOM). In a randomized double blind, placebo-controlled study, a novel mucoadhesive buccal tablet (MBT) containing clonidine reduced the incidence of SOM in HNC patients being treated with CRT. We now report overall survival (OS), tolerability and systemic exposure of clonidine of study subjects.