Rajesh Thiyagarajan

Surface dose measurements and comparison of unflattened and flattened photon beams

The purpose of this study was to evaluate the central axis dose in the build-up region and the surface dose of a 6 MV and 10 MV flattened photon beam (FB) and flattening filter free (FFF) therapeutic photon beam for different square field sizes (FSs) for a Varian Truebeam linear accelerator using parallel-plate ionization chamber and Gafchromic film. Knowledge of dosimetric characteristics in the build-up region and surface dose of the FFF is essential for clinical care. The dose measurements were also obtained empirically using two different commonly used dosimeters: a p-type photon semiconductor dosimeter and a cylindrical ionization chamber. Surface dose increased linearly with FS for both FB and FFF photon beams. The surface dose values of FFF were higher than the FB FSs. The measured surface dose clearly increases with increasing FS. The FFF beams have a modestly higher surface dose in the build-up region than the FB. The dependence of source to skin distance (SSD) is less significant in FFF beams when compared to the flattened beams at extended SSDs.

Comparison of Head Scatter Factor for 6MV and 10MV flattened (FB) and Unflattened (FFF) Photon Beam using indigenously Designed Columnar Mini Phantom.

To measure and compare the head scatter factor for flattened (FB) and unflattened (FFF) of 6MV and 10MV photon beam using indigenously designed mini phantom. A columnar mini phantom was designed as recommended by AAPM Task Group 74 with low and high atomic number materials at 10 cm (mini phantom) and at approximately twice the depth of maximum dose water equivalent thickness (brass build-up cap). Scatter in the accelerator (Sc) values of 6MV-FFF photon beams are lesser than that of the 6MV-FB photon beams (0.66-2.8%; Clinac iX, 2300CD) and (0.47-1.74%; True beam) for field sizes ranging from 10 × 10 cm2 to 40 × 40 cm2. Sc values of 10MV-FFF photon beams are lesser (0.61-2.19%; True beam) than that of the 10MV-FB photons beams for field sizes ranging from 10 × 10 cm2 to 40 × 40 cm2. The SSD had no influence on head scatter for both flattened and unflattened beams and irrespective of head design of the different linear accelerators. The presence of field shaping device influences the Sc values. The collimator exchange effect reveals that the opening of the upper jaw increases Sc irrespective of FB or FFF photon beams and different linear accelerators, and it is less significant in FFF beams. Sc values of 6MV-FB square field were in good agreement with that of AAPM, TG-74 published data for Varian (Clinac iX, 2300CD) accelerator. Our results confirm that the removal of flattening filter decreases in the head scatter factor compared to flattened beam. This could reduce the out-of-field dose in advanced treatment delivery techniques.

SU‐E‐T‐149: Scatter Factors Comparison of 6MV Flattened and 7MV Unflattened Beams

PURPOSE To compare the Collimator scatter factor (Sc), Phantom scatter factor (Sp) and Total scatter factors (Sc,p) of 6MV flattened Beam (6MV FB) and 7MV Unflattened beams (7MV UFB). METHODS The flattening filter and primary collimator are the major sources of producing the scattered radiation. In this study, the field sizes from 5×5 cm2 to 40 × 40 cm2 compared for 6MV FB and 7MV UFB. We measured Sc,p with CC 13 chamber at the depth of 10 g/cm2 using IBA blue phantom and Sc measured with CC 13 chamber at the depth of 10 g /cm2 using columnar phantom (TG 74) for 6MV FB and 7MV UFB x-ray beams from a Siemens - ARTISTE linear accelerator. The Sp values derived from the Sc,p and Sc Values. RESULTS All the values of Sc,p, Sc and Sp are normalized to 10 × 10 cm2 field size the measured values of Sc,p for 6MV FB and 7MV UFB varies from 0.9437 to 1.0651 and 0.9690 to 1.0283 respectively. The Sc values for 6MV FB and 7MV UFB varies from 0.9676 to 1.0212 and 0.9882 to 1.0075 respectively. The Sp values for 6MV FB and 7MV UFB varies from 0.9752 to 1.0429 and 0.9806 to 1.0206 respectively. Our study results shows that Sc, Sp & Sc,p for 7MV UFB for smaller fields up to 10 × 10 cm2 were higher than 6MV FB and for larger fields greater than 10 ×10 cm2 it is vice versa. CONCLUSIONS A scatter factors are measured and compared for 6MV FB and 7MV UFB. A significant variation observed for Sc, Sp and Sc,p values of 6MV FB and 7MV UFB .This is may be due to flattening filter and beam quality.

SU-E-T-651: Quantification of Dosimetric Accuracy of Respiratory Gated Stereotactic Body Radiation Therapy

Purpose: To quantify the dosimetric accuracy of respiratory gated stereotactic body radiation therapy delivery using dynamic thorax phantom. Methods: Three patients with mobile target (2 lung, 1liver) were chosen. Retrospective 4DCT image sets were acquired for using Varian RPM system. An in-house MATLAB program was designed for MIP, MinIP and AvgIP generation. ITV was contoured on MIP image set for lung patients and on MinIP for liver patient. Dynamic IMRT plans were generated on selected phase bin image set in Eclipse (v10.0) planning system. CIRS dynamic thorax phantom was used to perform the dosimetric quality assurance. Patient breathing pattern file from RPM system was converted to phantom compatible file by an in-house MATLAB program. This respiratory pattern fed to the CIRS dynamic thorax phantom. 4DCT image set was acquired for this phantom using patient breathing pattern. Verification plans were generated using patient gating window and delivered on the phantom. Measurements were carried out using with ion chamber and EBT2 film. Exposed films were analyzed and evaluated in FilmQA software. Results: The stability of gated output in comparison with un-gated output was within 0.5%. The Ion chamber measured and TPS calculated dose compared for all the patients. The difference observed was 0.45%, −0.52% and −0.54 for Patient 1, Patient2 and Patient 3 respectively.Gamma value evaluated from EBT film shows pass rates from 92.41% to 99.93% for 3% dose difference and 3mm distance to agreement criteria. Conclusion: Dosimetric accuracy of respiratory gated SBRT delivery for lung and liver was dosimetrically acceptable. The Ion chamber measured dose was within 0.203±0.5659% of the expected dose. Gamma pass rates were within 96.63±3.84% of the expected dose.

SU-E-T-456: Impact of Dose Calculation Algorithms On Biologically Optimized VMAT Plans for Esophageal Cancer

Purpose: To evaluate the impact of dose calculation algorithm on the dose distribution of biologically optimized Volumatric Modulated Arc Therapy (VMAT) plans for Esophgeal cancer. Methods: Eighteen retrospectively treated patients with carcinoma esophagus were studied. VMAT plans were optimized using biological objectives in Monaco (5.0) TPS for 6MV photon beam (Elekta Infinity). These plans were calculated for final dose using Monte Carlo (MC), Collapsed Cone Convolution (CCC) & Pencil Beam Convolution (PBC) algorithms from Monaco and Oncentra Masterplan TPS. A dose grid of 2mm was used for all algorithms and 1% per plan uncertainty maintained for MC calculation. MC based calculations were considered as the reference for CCC & PBC. Dose volume histogram (DVH) indices (D95, D98, D50 etc) of Target (PTV) and critical structures were compared to study the impact of all three algorithms. Results: Beam models were consistent with measured data. The mean difference observed in reference with MC calculation for D98, D95, D50 & D2 of PTV were 0.37%, −0.21%, 1.51% & 1.18% respectively for CCC and 3.28%, 2.75%, 3.61% & 3.08% for PBC. Heart D25 mean difference was 4.94% & 11.21% for CCC and PBC respectively. Lung Dmean mean difference was 1.5% (CCC) and 4.1% (PBC). Spinal cord D2 mean difference was 2.35% (CCC) and 3.98% (PBC). Similar differences were observed for liver and kidneys. The overall mean difference found for target and critical structures was 0.71±1.52%, 2.71±3.10% for CCC and 3.18±1.55%, 6.61±5.1% for PBC respectively. Conclusion: We observed a significant overestimate of dose distribution by CCC and PBC as compared to MC. The dose prediction of CCC is closer (<3%) to MC than that of PBC. This can be attributed to poor performance of CCC and PBC in inhomogeneous regions around esophagus. CCC can be considered as an alternate in the absence of MC algorithm.

SU‐E‐T‐92: Comparison of the Depth Dose in the Build‐Up Region and Surface Dose for 6MV Flattened and 7MV Unflattened Photon Beams with Different Detectors

PURPOSE Comparison of the depth dose in the build-up region and Surface dose for 6MV flattened and 7MV unflattened photon beams with different detectorsMethods: The percentage depth dose in the build-up region and the surface dose for the 6MV-FB and 7MV-UFB photon beams from a Siemens Artiste medical linear accelerator were measured for square field sizes of 5×5, 10×10 and 15×15 cm 2 using Scanditronix NACP ion chamber, CC 13, and Stereotactic field Detectors (SFD) along the central axis of the beam at 100 cm source to surface distance with IBA blue phantom 2 and solid phantom. RESULTS The consistency between the measured percentage depth dose data from all four detector types were observed for depths beyond the depth of the maximum dose, but were all clearly different from each other data in the build-up region this is observed for both 6MV-FB and 7MV-UFB beams. The measured surface dose (16.2 Percent) applied with improved Velkley correction factors used for 10X10 cm2 of 6MV-FB using NACP parallel plate chamber values were all in good agreement with previously published data. The measured percentage surface doses obtained using the NACP parallel plate chamber, CC 13 and SFD without any correction factor values are 0.3885,0.4330 and 0.4751, 0.4485,0.4922 and 0.5323,0.3652,0.4076 and 0.4580 respectively for field sizes of 5X5,10X10 and 15×15 cm2 for the 6MV-FB photon beams and 0.3800,0.4117 and 0.4398, 0.4364, 0.4655 and 0.4965, 0.3611,0.4100 and 0.4349 for the 7MV-UFB photon beams. CONCLUSION The measured surface dose clearly increases with increasing field size, regardless of the detector used in the measurement for both 6MV-FB and 7MV-UFB photon beams. Compare to NACP parallel plate chamber CC 13 chamber is showing over response and SFD is showing lesser surface dose.

SU‐E‐T‐93: A Comparison of Out‐Of‐Field Dose and Its Constituent Components for 6MV Flattened and 7MV Unflattened Photon Beam

PURPOSE A Comparison of out-of-field dose and its constituent components for 6MV Flattened and 7MV unflattened Photon BeamsMethods: The flattening filter and primary collimator are the major sources of producing the scattered radiation. The out-of-field photon dose, Sc,p, Sc and Sp were measured with CC 13 chamber for square open fields from 5 × 5 cm2 to 30 × 30 cm2 using IBA blue phantom and solid phantom as a function of distance from central axis and field size at depth of maximum for respective photon energies. Scatter radiation, which predominantly contributes to peripheral dose at larger distance from the field edge, was measured using a CC 13 ionization chamber. RESULTS All the values of Sc,p and Sc are normalized to 10 × 10 cm2 field size. The measured values of Sc,p for 6MV-FB and 7MV-UFB varies from 0.9437 to 1.0651 and 0.9690 to 1.0283 respectively. The Sc values for 6MV-FB maximum (max.) 2.1 percent lesser than 7MV-UFB for up to 10 ×10 cm2 field size and 6MV-FB max.1.7 percent higher than the 7MV-UFB for above 10 × 10cm2 field size. Out of field dose is max.42.0 percent higher for 6MV-FB compare to 7MV-UFB at 25 cm from the field edge for up to 10 × 10 cm2 field size and max.50.39 percent reduced dose for 7MV-UFB for above 10 × 10 cm2 . Out of field dose is max.70.0 percent reduced for 7MV-UFB compare to 6MV-FB above 50 cm away from the field edge for up to 10 × 10 cm2 field size and max.68 percent reduced dose for 7MV-UFB for above 10 × 10 cm2 . CONCLUSION 7MV-UFB may be advantageous by reduced out-of-field dose compare to 6MV-FB, but differences are small in absolute terms.

SU-E-T-42: Penumbral Width Calculation for Flattening Filter Free Beam 7MV and Flattened Beam 6MV Using Inflection Point

Purpose: The purpose of the study was to investigate penumbra for Flattening filter free beam 7MV(7MV UFB) and 6MV(6MV FB) flattened beam by inflection point method and verify with conventional penumbra width for 6MV FB. Methods: The conventional definitions of the penumbra based 80 ‐ 20 % values are not applicable for unflattened beam. In house built in program in Matlab were developed to find inflection point in the penumbral region for penumbra. Further to find the penumbra, a factor of 0.4 and 1.4 was applied to the inflection point dose value to obtain the penumbral width. We measured profiles for various field size ranging from 5 × 5 cm2 to 30 × 30 cm2 with CC 13 chamber at the depth of 5.0 and 10 g/cm2 using IBA blue water phantom for 6MV FB and 7MV UFB x‐ray beams from a Siemens ‐ ARTISTE linear accelerator.Results: The calculated penumbra width for 6MV FB using inflection method are compared with conventional calculation and the values differ from 0.1 to 1.61 mm for jaw shaped fields and 0.1 to 1.75 mm for MLC shaped fields at 5.0 cm and 10.0 cm depth for various field size. The calculated penumbra widths in inflection point method values for 7MV UFB and 6MV FB varies from 5.13 to 10.7 mm and 5.55 to 11.85 mm respectively at 10.0 cm depth for 5 × 5 cm2 to 30 × 30 cm2field sizes. We observed the calculated penumbra widths with inflection point method values for 7MV UFB are lesser than the 6MV FB for various field sizes and depth. Conclusions: The penumbra values of 7MV UFB are lesser than the 6MV FB. This is due to softer photon beam spectrum and missing scatter from flattening filter and higher beam quality.

SU-E-T-143: Effect of Physical and Virtual Wedges on the Surface Dose at Various SSD for 6 and 15 MV Photon Beam

PURPOSE To study the effect of the virtual wedge and physical wedge filters on the surface and build-up region doses for 6 and 15MV high-energy photon beams for different field sizes and various source to surface distance(SSD). METHODS The measurements were made in water equivalent (PMMA) solid phantom in the build-up region at various SSD for various field sizes using virtual and physical wedge filters having different angles. A parallel-plate ion chamber (Markus) was used to measure the percent depth doses at surface and buildup region. Plane parallel ion chamber with fixed plate separation on the surface and buildup region would perturbate the dose measured, to get the proper dose over response correction factor was used. RESULTS The percentage depth dose at surface (PDD0) increased as the field size increased for open, virtual, and physical wedged beams. For open, 30 degree physical, and virtual wedged beams, the surface doses were found to be 15.4%, 11.2%, and 15.2% with 6-MV photons and 11.2%, 9.4%, 11.2% with 15-MV photons, respectively, at 10 × 10 cm2 field size at 100cm SSD.As SSD increases percentage depth dose at surface (PDD0) decreases for open,physical and virtual wedge field. CONCLUSIONS Percentage depth dose at surface (PDD0) of virtual wedged beams were similar to those of open beams. PDD0 of physical wedged beams were lower than those of open and virtual wedged beams. Surface doses for both PW and VW increases with field size and small increase in surface dose for both PW and VW fields as wedge angle increases especially for large fields.

SU‐E‐T‐130: IMAT Patient Specific Quality Assurance Using ArcCHECK Diode Array Detector

PURPOSE To evaluate the IMAT patient specific quality assurance (QA) performed using ArcCHECK detector array in reference with standard ion chamber for routine clinical use. METHODS Twelve patient plans having different tumor sites chosen for this study. On Eclipse planning system,IMAT patient plans were calculated on ArcCHECK phantom inserted with Ion chamber using superposition algorithm. ArcCHECK is a cylindrical phantom with a three-dimensional array of 1386 diode detectors, arranged in a spiral pattern, with 10 mm diode spacing. These plans delivered from Clinac-iX linac equipped with 120 MLC. Point dose and Dose/fluence map were measured simultaneously with ion chamber (IC-15) and ArcCHECK diode array detector respectively. Point doses, dose/fluences map and dose at central axis (CAX) on ArcCHECK phantom were compared with their respective TPS calculated values. RESULTS The ion chamber measurements are in good agreement with TPS calculated doses. Mean difference between them is 0.50% with standard deviation is 0.51%. Concordance correlation coefficient (CCC) obtained for ion chamber base absolute dose measurements is 0.9996. These results demonstrate a strong correlation between the absolute dose predicted by our TPS and the measured dose. The precision of the TPS software was 0.9999, and its accuracy was 0.9997.The agreement between ArcCHECK doses and TPS predictions on the CAX, shown CCC of 0.9978 (the mean difference in the central axis dose is 2.11%). The 95% Confidence Interval is from 0.9932 to 0.9995. In gamma analysis of dose/fluence map the mean passing rate was 98.53% for 3% dose difference and 3mm distance to agreement. CONCLUSIONS The IMAT patient specific QA with Ion chamber and ArcCHECK phantom are consistent with the TPS calculated dose. Statistically good agreement observed between ArcCHECK measured and TPS calculated. Hence it can be used for routine IMAT QA.