Kothanda Raman

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.

The dosimetric impact of different photon beam energy on RapidArc radiotherapy planning for cervix carcinoma.

The main purpose of this study is to know the effect of three different photon energies viz., 6, 10, and 15 mega voltage (MV) on RapidArc (RA) planning for deep-seated cervix tumor and to develop clinically acceptable RA plans with suitable photon energy. RA plans were generated for 6, 10, and 15 MV photon energies for twenty patients reported with cervix carcinoma. RA plans were evaluated in terms of planning target volume (PTV) coverage, dose to organs at risk (OARs), conformity index (CI), homogeneity index (HI), gradient measure, external volume index of dose distribution produced, total number of monitor units (MUs), nontumor integral dose (ID), and low dose volume of normal tissue. A two-sample paired t-test was performed to compare the dosimetric parameters of RA plans. Irrespective of photon energy used for RA planning, plans were dosimetrically similar in terms of PTV coverage, OARs sparing, CI and HI. The numbers of MUs were 13.4 ± 1.4% and 18.2 ± 1.5% higher and IDs were 2.7 ± 0.8% and 3.7 ± 0.9% higher in 6 MV plans in comparison to that in the 10 and 15 MV plans, respectively. V1Gy, V2Gy, V3Gy, and V4Gywere higher in 6 MV plans in comparison to that in 10 and 15 MV plans. Based on this study, 6 MV photon beam is a good choice for RA planning in case of cervix carcinoma, as it does not deliver additional exposure to patients caused by photoneutrons produced in high energy beams.

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-687: Scatter Factors Comparison of 6MV FFF and Energy Matched 6MV EqFFF.

Purpose: To compare the Collimator scatter factor (Sc), Phantom scatter factor (Sp) and Total scatter factors (Sc,p) of 6MV FFF and energy matched 6MV FFF (6MV eqFFF) 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 FFF and 6MV eqFFF. We measured Sc,p with CC 13 chamber at the depth of 10 g/cm2 using IBA blue water phantom and Sc measured with CC 13 chamber at the depth of 10 g /cm2 using columnar phantom (TG 74) for 6MV FFF and 6MV eqFFF x-ray beams from a Siemens — ARTISTE(6MV eqFFF ) and Varian True beam (6MV FFF) 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 FFF and 6MV eqFFF varies from 0.9404 to 1.0760 and 0.9690 to 1.0283 respectively. The Sc values for 6MV FFF and 6MV eqFFF varies from 0.9880 to 1.133 and 0.9882 to 1.0075 respectively. The Sp values for 6MV FFF and 6MV eqFFF varies from 0.9518 to 1.0619 and 0.9806 to 1.0206 respectively. From the measured values, we found that Sc, Sc,p and Sp factors of 6MV FFF less than 10 × 10cm2 field is similar to the 6MV eqFFF. For greater than 10 × 10 cm2 field size Sc, Sc,p and Sp factors higher for 6MV FFF than 6MV eqFFF. Conclusion: The energy matched 6MV FFF (6MV eqFFF) photon beam producing lesser Scatter radiation compare to the conventional 6MV FFF photon beams for field size more than 10 × 10 cm2.

SU-E-T-493: Influence of Filtered and Flatting Filter Free Photon Beam of 10 Megavolts Energy On Rapid Arc Radiotherapy Planning for Cervix Carcinoma

Purpose: Aim of this study is to determine the dosimetric influence of Filtered and Flatting Filter Free Photon Beam of 10 MV energy on RA planning for Ca. Cervix. Methods: CT data sets of eleven patients reported with carcinoma cervix were used for RA planning for 10MV _FFB and 10MV_FFFB. RA plans were generated using two full arcs.All RA plans were generated to deliver a dose of 50.4Gy in 28 fractions for PTV and ALARA for OAR’s. All plans were analysed for PTV Coverage, conformity Index, homogeneity index, dose to OAR’s, integral dose to normal tissue and total monitor units were studied. Results: DVH was used to evaluate RA plans for both 10MV_FFB and 10MV_FFFB photon beam. Planning results show a comparable PTV coverage for both energies. Results shows volume of PTV receiving prescription dose were 95.10+ 0.09% and 95.09 +0.11%, and volume of PTV receiving a dose of 107% is 0.45+0.96% and 5.25+8.9%, homogeneity index (HI) were 1.051+0.007 and 1.066+0.008, Conformity Index(CI) were 1.003+0.019 and 1.012+0.013, Mean Integral dose were 2.65+0.34 and 2.60+0.33(*10−5Gy.cm3) for 10MV_FFB and 10MV_FFFB respectively. 10MV_FB shows statistically significant (p<0.05) improvement in mean doses to bladder, rectum, bowel and mean total number of MU’s and also shows remarkable decrease in mean total no. of MU’s by 43.7% in comparison to 10MV_FFFB. There is statistically significant (p<0.05) difference found in CI and HI for 10MV_FB in comparison to 10MV _FFF beam. 10MV_FFFB shows statistically significant (p<0.05) for mean NTID and delivers 1.65 % less NTID in comparison to 10 MV_ FB. Conclusion: 10MV_FB is superior to 10MV_FFFB for rapid arc planning in case of Cervix carcinomas, it offers better target coverage and OAR’s sparing, comparable mean Integral dose to normal tissues and 10 MV_ FB also produced highly conformal and homogeneous dose distribution in comparison to 10MV_FFFB.

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.