G. Arun

Feasibility Study of Deep Inspiration Breath-Hold Based Volumetric Modulated Arc Therapy for Locally Advanced Left Sided Breast Cancer Patients

BACKGROUND The purpose of this study was to assess the feasibility of deep inspiration breath-hold (DIBH) based volumetric modulated arc therapy (VMAT) for locally advanced left sided breast cancer patients undergoing radical mastectomy. DIBH immobilizes the tumor bed providing dosimetric benefits over free breathing (FB). MATERIALS AND METHODS Ten left sided post mastectomy patients were immobilized in a supine position with both the arms lifted above the head on a hemi-body vaclock. Two thermoplastic masks were prepared for each patient, one for normal free breathing and a second made with breath-hold to maintain reproducibility. DIBH CT scans were performed in the prospective mode of the Varian real time position management (RPM) system. The planning target volume (PTV) included the left chest wall and supraclavicular nodes and PTV prescription dose was 5000cGy in 25 fractions. DIBH-3DCRT planning was performed with the single iso-centre technique using a 6MV photon beam and the field-in-field technique. VMAT plans for FB and DIBH contained two partial arcs (179o-300oCCW/CW). Dose volume histograms of PTV and OARs were analyzed for DIBH-VMAT, FB-VMAT and DIBH-3DCRT. In DIBH mode daily orthogonal (0o and 90o) KV images were taken to determine the setup variability and weekly twice CBCT to verify gating threshold level reproducibility. RESULTS DIBH-VMAT reduced the lung and heart dose compared to FB-VMAT, while maintaining similar PTV coverage. The mean heart V30Gy was 2.3% ±2.7, 5.1% ±3.2 and 3.3% ±7.2 and for left lung V20Gy was 18.57% ±2.9, 21.7% ±3.9 and 23.5% ±5.1 for DIBH-VMAT, FB-VMAT and DIBH-3DCRT respectively. CONCLUSIONS DIBH-VMAT significantly reduced the heart and lung dose for left side chest wall patients compared to FB-VMAT. PTV conformity index, homogeneity index, ipsilateral lung dose and heart dose were better for DIBH-VMAT compared to DIBH-3DCRT. However, contralateral lung and breast volumes exposed to low doses were increased with DIBH-VMAT.

Planning and Dosimetric Study of Volumetric Modulated Arc Based Hypofractionated Stereotactic Radiotherapy for Acoustic Schwannoma--6MV Flattening Filter Free Photon Beam.

BACKGROUND The purpose of this study was to assess the dosimetric and clinical feasibility of volumetric modulated arc based hypofractionated stereotactic radiotherapy (RapidArc) treatment for large acoustic schwannoma (AS>10 cc). MATERIALS AND METHODS Ten AS patients were immobilized using BrainLab mask. They were subject to multimodality imaging (magnetic resonance and computed tomography) to contour target and organs at risk (brainstem and cochlea). Volumetric modulated arc therapy (VMAT) based stereotactic plans were optimized in Eclipse (V11) treatment planning system (TPS) using progressive resolution optimizer-III and final dose calculations were performed using analytical anisotropic algorithm with 1.5 mm grid resolution. All AS presented in this study were treated with VMAT based HSRT to a total dose of 25 Gy in 5 fractions (5 fractions/ week). VMAT plan contains 2-4 non-coplanar arcs. Treatment planning was performed to achieve at least 99% of PTV volume (D99) receives 100% of prescription dose (25 Gy), while dose to OARs were kept below the tolerance limits. Dose-volume histograms (DVH) were analyzed to assess plan quality. Treatments were delivered using upgraded 6 MV un-flattened photon beam (FFF) from Clinac-iX machine. Extensive pretreatment quality assurance measurements were carried out to report on quality of delivery. Point dosimetry was performed using three different detectors, which includes CC13 ion-chamber, Exradin A14 ion-chamber and Exradin W1 plastic scintillator detector (PSD) which have measuring volume of 0.13 cm3, 0.009 cm3 and 0.002 cm3 respectively. RESULTS Average PTV volume of AS was 11.3 cc (±4.8), and located in eloquent areas. VMAT plans provided complete PTV coverage with average conformity index of 1.06 (±0.05). OARs dose were kept below tolerance limit recommend by American Association of Physicist in Medicine task group-101(brainstem V0.5 cc<23 Gy, cochlea maximum<25 Gy and Optic pathway<25 Gy). PSD resulted in superior dosimetric accuracy compared with other two detectors (p=0.021 for PSD.

SU‐E‐T‐766: Verification of Volumetric Modulated Arc Therapy Plans with Independent Three Dimensional Dose Computation Algorithm

Purpose: To verify the volumetric modulated arc therapy (Rapidarc) plans using a independent three dimensional dose computation algorithm using COMPASS system. Methods: Rapidarc is a treatment technique which produces conformal dose distribution by delivering the dose in a rotational fashion while simultaneously changing MLC position, dose rate as well as gantry speed. COMPASS dosimetry system uses the Collapsed Cone Convolution(CCC) algorithm as a dose calculation engine which requires machine model information and beam modeling to display the 3D dose distribution on a patient CT data. Treatment plans (10 patients) generated for Rapidarc on Eclipse (version 8.9) Treatment Planning System (TPS) using Analytical Anisotropic Algorithm (AAA) were exported as DICOM file to COMPASS for recalculation using CCC algorithm. Pilot studies were performed for 2DRT, 3DCRT and IMRT plans prior to VMAT plan verification. The doses and dose‐volume histograms computed using CCC were compared with TPS calculated plans. Plans were analyzed in terms of Conformity Index (CI) for PTV, maximum and mean doses for OARs and difference in three dimensional gamma. Results: The average 3D mean gamma for 1mm(DTA) and 1%(DD) criteria for 2DRT and 3DCRT was 0.11 ±0.002 . Maximum and minimum deviation of PTV Volume receiving 95% of the prescribed dose(V95%) for IMRT and Rapidarc were found to be 1.01%, 0.79% and 1.24%, 0.86% respectively. Average deviation of maximum and mean doses of OARs for IMRT and Rapidarc were 0.91±0.002 %, 0.55±0.008 % and 0.25±0.005 %, 1.05±0.009 % respectively. Conclusions: This study illustrates that the compass three dimensional dosimetry system can be used as an accurate and effective tool to clinically validate the Rapidarc plans independently.

Multi-isocentric 4π volumetric-modulated arc therapy approach for head and neck cancer

Abstract Objectives To explore the feasibility of multi‐isocentric 4π volumetric‐modulated arc therapy (MI4π‐VMAT) for the complex targets of head and neck cancers. Methods Twenty‐five previously treated patients of HNC underwent re‐planning to improve the dose distributions with either coplanar VMAT technique (CP‐VMAT) or noncoplanar MI4π‐VMAT plans. The latter, involving 3–6 noncoplanar arcs and 2–3 isocenters were re‐optimized using the same priorities and objectives. Dosimetric comparison on standard metrics from dose‐volume histograms was performed to appraise relative merits of the two techniques. Pretreatment quality assurance was performed with IMRT phantoms to assess deliverability and accuracy of the MI4π‐VMAT plans. The gamma agreement index (GAI) analysis with criteria of 3 mm distance to agreement (DTA) and 3% dose difference (DD) was applied. Results CP‐VMAT and MI4π‐VMAT plans achieved the same degree of coverage for all target volumes related to near‐to‐minimum and near‐to‐maximum doses. MI4π‐VΜΑΤ plans resulted in an improved sparing of organs at risk. The average mean dose reduction to the parotids, larynx, oral cavity, and pharyngeal muscles were 3 Gy, 4 Gy, 5 Gy, and 4.3 Gy, respectively. The average maximum dose reduction to the brain stem, spinal cord, and oral cavity was 6.0 Gy, 3.8 Gy, and 2.4 Gy. Pretreatment QA results showed that plans can be reliably delivered with mean gamma agreement index of 97.0 ± 1.1%. Conclusions MI4π‐VMAT plans allowed to decrease the dose‐volume‐metrics for relevant OAR and results are reliable from a dosimetric standpoint. Early clinical experience has begun and future studies will report treatment outcome.

PO-0787: Clinical validation of Gated RapidArc using aS1000 Electronic Portal Imaging Device

observed in 6 and 8 measurements (total of 26.9%), respectively, but only 2 measurement were over 10%. Four of the six measurements on the anal verge showed difference of 5% or more between the calculated and estimated dose. Conclusions: With high dose gradients in VMAT treatments it is essential to know the correct position of TLDs in order to properly analyze the results of in-vivo dosimetry. This new procedure seems dealing with this issue, allowing validating and monitoring doses delivered to patients.

PD-0597: Is 5mm Millennium MLC adequate for VMAT based SBRT?- A comparative study with 2.5mm high definition MLC

influencethe resulting dose distribution. In the present work, we study the effect of different reconstruction parameters on dose distribution for both dose painting by contours (DPC) and dose painting by numbers (DPBN) techniques. Materials and Methods: FDG-PET/CT was performed for 8 advanced stage non-small cell lung cancer patients on a Siemens Biograph 40 PET/CT-scanner.For each patient, 6 different PET reconstructions were applied; ordered subsets expectation maximization (OSEM) with 21 subsets, using 2 (OSEM2i), 3 (OSEM3i)and 4 (OSEM4i) iterations. In addition, the data was reconstructed using theSiemens HD PET algorithm that corrects for the varying point spread function(PSF) inside the scanner, with 21 subsets, 2 (PSF2i), 3(PSF3i) and 4 (PSF4i)iterations. Gaussian post filtering was 5 mm for all cases. A GTV was generated for each patient, encompassing the primary lung tumor including all voxels with an SUV > 2.0 in the standard OSEM2i reconstruction. For DPC, a boost volume was defined as the 50% of SUVmax for each reconstruction[1]. The boost volumes were assigned doses corresponding to an increase in mean GTV dose from 60 to 70 Gy. For each boost volume the quality factor (QF) [2] of the dose distribution was calculated with reference to the standard reconstruction as well as differences in boost volumes. For DPBN, a boost dose was distributed ranging from 60 to 130 Gy within the GTV assuming a linear relationship between FDG voxel intensity and prescribed dose distribution [2],using SUVmax as threshold for voxels receiving maximum dose. Dose volume histograms (DVH) were extracted for all reconstructions, as well as quality factors (QF) compared to the standard reconstruction OSEM2i. Results: For DPC, the mean boost volume was 32 ± 9 % and 31 ± 9 % of the GTV for OSEM2i and PSF2i, respectively. Correspondingly, the mean dose to the boost volume was 91 ± 10 Gyand 95 ± 13 Gy for OSEM2i and PSF2i. Considering the different reconstructions, the largest observed mean difference in boost volume was -11 ± 6 %, between OSEM4i and PSF2i. The mean difference in boost volume for OSEM2i and PSF2i was -9 ± 6%. The largest mean QF was 2.1 %, for PSF2i. For DPBN, for all patients except one, DVHs of the different reconstructions were approximately similar. The mean dose to the GTV was 81 ± 4 Gy and 80 ± 4 Gy for OSEM2i andPSF2i, respectively. The mean QF ofPSF2i relative to OSEM2i was 1.8 ± 0.8 %. Conclusions: PET reconstruction settings have an effect on PET-boost dose distributions, although the effect is small or moderate for most patients. [1] van Elmpt, W., De Ruysscher,D., van der Salm, A., et al. Radiother Oncol 2012; 104: 67-71. [2]Vanderstraten B,De Gersem W, Derie C, et al. Radiother Oncol 2006; 79; 249-258

PD-0229: Evaluation of gated volumetric modulated arc therapy using COMPASS 3D dosimetry system

further improved to 0.3±0.6%, 0.2±0.4%, and 0.1±0.3% by combining 3, 4, and 5 fractions, respectively. Conclusions: A system was developed for high accuracy assessment of actual delivered IMRT fluence profiles in treatment fractions for doseguided radiotherapy. For prostate cancer patients, intra-fraction anatomy changes did sometimes result in reduced accuracy. Combination of measurements performed in 2 or more fractions could largely reduce uncertainties.

POSTER: CLINICAL TRACK: GASTROINTESTINAL TUMOURS (UPPER AND LOWER GI)PO-0693: Image guided volumetric modulated arc therapy with concurrent Cisplatin for inoperable esophageal cancer

adjuvant intent. We compared plans with Foward Planning -IMRT (FP– IMRT) adjusted to the delineated breast volume with two other plans without the breast delineated (one with standard tangential beams with wedges and another with FP-IMRT). The ACOSOG Z0011 trial showed the non inferiority of the irradiation of axillary levels I and II (included in the irradiated volume when using standard tangential beam radiotherapy to the breast) when compared with axillary dissection of the same levels, in selected patients with breast cancer. Materials and Methods: We analyzed data from 40 patients undergoing radiotherapy after breast conserving surgery, with a negative sentinel node biopsy. The patients were submitted to a CT for virtual simulation with 3mm slice. The data from the CT were doubled. In one CT, 2 dosimetry plans were done: one with conventional RT with tangential beams with wedges (2-3 segments were added if necessary to achieve dose homogeneity) and another with FP-IMRT. On the other CT data, target volumes were delineated: breast CTV and PTV (expansions of 10 mm were made for all directions except for the posterior one, which was 7 mm), and a plan was calculated using the FP-IMRT technique, adjusted to the PTV. We then outlined the axillary levels I and II, for analysis, on one CT data and copied it to the other. Mean dose (Dmean) and V95 were evaluated for axillary levels I and II. The Conformity Index (CI) of the PTV was also analysed. Results: Both V95 and Dmean for axillary level I were higher in the standard tangential beams with wedges technique and in the FP-IMRT technique without the breast volume delineated. When compared with these plans, the plan adjusted to the breast PTV achieved an inferior V95 value to axillary level I. When evaluating the axillary level II, these differences were more pronounced. We found a higher CI value in the plan adjusted to the breast target volume. Conclusions: We found a higher conformity index to the breast target volume using the FP-IMRT technique with the breast target volumes delineated, and a lower V95 for the axillary levels evaluated. This shows that when conforming the dose to the breast alone the unintended irradiation of the axilla is lower. Although a higher V95 was achieved with the standard tangential breast irradiation with wedges, it happens at the expenses of a lower conformity index to the breast volume, and this technique has shown a higher rate of skin toxicity, which lead to being less used. We conclude that the delineation of axillary levels I and II is mandatory when there is the need to irradiate them, and the information obtained by the sentinel node biopsy is therefore important to the radiotherapy treatment.

SU‐E‐T‐423: Effect of X‐Ray Source Spot Size Modeling in AcurosXB Dose Calculation Algorithm On VMAT Based SBRT Treatments

PURPOSE Aim of this study is to evaluate AcurosXB(AXB) dose calculation algorithm dosimetrically with different X-ray source spot size modeling and its clinical effect on Volumetric Modulated Arc Therapy(VMAT) based Stereotactic Body Radiotherapy(SBRT). METHODS 10 SBRT cases were purposely selected to analyze the effect of source spot size for smaller field size. All the cases were planned in Varian Eclipse(V10.0) treatment planning system(TPS) which uses AXB algorithm for VMAT dose computation. For this study source spot sizes varied from 0.5mm, 1.0mm, 1.5mm and 2.0mm in X as well as Y-direction in the beam data configuration. SBRT plans were optimized using Progressive Resolution Optimizer III(PRO-III) and dose was calculated by AXB with different source spot sizes. Plan quality index such as conformity index(CI) and gradient measure for PTV and for normal tissues volume of 90% and 25% prescribed dose were used for analysis. VMAT plans were delivered with Varian Clinac-iX and 2D-planner dose was measured using iba iMatrixx 2D array in Multicube phantom. Gamma (3mm/3%) analyses were performed between measured and predicted doses for different spot sizes. RESULTS Conformity index value for PTV were found to be 1.16±0.21, 1.14±0.20, 1.13±0.2 & 1.08±0.17 and gradient measure were 1.45±0.26, 1.46±0.26, 1.47±0.25 & 1.52±0.27 for spot size of 0.5mm, 1.0mm, 1.5mm & 2.0mm respectively. Volume of normal tissue receiving 90% of prescribed dose for 2mm spot size was 2.7% more in compare to 0.5mm spot size. Gamma analysis(3mm/3%) were found to be 98.77±1.03%, 98.96±0.91%, 99.18±0.76% & 99.43±0.60% for spot size 0.5mm, 1.0mm, 1.5mm & 2.0mm respectively. CONCLUSION The Result showed that spot size configuration of 1.5-2.0mm were matching very closely with our measurements. Based on the variation in the PTV coverage and dose to normal tissues it is highly recommended to fine tune spot size parameters for VMAT based SBRT.

SU‐E‐T‐188: Patient Specific Quality Assurance for Volumetric Modulated Arc Therapy (RapidArc) Using COMPASS 3D Dosimetry System

Purpose: To implement the COMPASS (IBA, Inc) quality assurance (QA) system as a patient specific QA tool for Volumetric Modulated Arc Therapy. Methods: RapidArc is a treatment technique which produces conformal dose distribution by delivering the dose in a rotational fashion while simultaneously changing MLC position, dose rate as well as gantry speed. The COMPASS has the potential to calculate and display the delivered 3D dose distribution on a patient CT data by using beam modeling, dose map from detector measurements (I‐MatriXX Evolution) and dose map reconstruction using Collapsed Cone Convolution Algorithm. Dose maps for 10 Ten RapidArc plans were measured using I‐MatriXX Evolution with a gantry mount (SSD=76.2cm) along with gantry angle sensor. This device captures the RapidArc plan delivery in real‐time in a pre‐treatment QA context. The measurement data were read directly by the control software, which provides the ability to import patient plan data from the treatment planning system via DICOM export. The COMPASS software also provides the user a dose calculation engine, including a physics based head fluence model. The doses and dose‐volume histograms reconstructed from the fluence measurements were compared to the TPS calculated plans.Results: Maximum and minimum deviation of PTV Volume receiving 95% of the prescribed dose(V95%) was found to be −2.89% and 0.16% respectively and mean deviation −1.12%. Average deviation of maximum and mean doses of OARs (organ at risk) were −0.5880±0.012% and − 0.6952±0.011% respectively. Average 3D mean gamma for 3mm and 3% criteria was found to be 0.067±0.0013 and Maximum absolute dose deviation at isocentre was 1.3573% Conclusions: Compass system can be used as an accurate and visually enhanced patient specific QA tool for VMAT plans to provide a complete clinical relevance of dose discrepancies for better patient treatment.