The Dosimetric Property of Tld2000 Thermoluminescent Dosimeter

Abstracts / Brachytherapy 14 (2015) S11–S106 Ir-192 HDR brachytherapy technique includes comparing orthogonal scouts and measuring skin-to-hub distance before every fraction. Although there are two different checks to verify that the device has not rotated nor collapsed, there is no set tolerance or guidance for what is considered an acceptable deviation in clinical practice. This study evaluates the potential dosimetric effects that are due to changes in patient anatomy or device positioning. These dosimetric effects substantiate the need to establish more accurate methods of pretreatment verification for APBI patients with SAVI implants. Materials and Methods: Two types of deviations introduced are explained in Cases 1 and 2 below. Case 1. Reduction in inflammation causing a change in patient anatomy with the cavity-skin surface distance shrinking by 5 mm. Twelve patient plans were analyzed. A 5 mm rind (inner wall) around the skin surface was contoured and named as ‘Skin Ring’. The initial planned dose to the Skin Ring was ~100% for all plans under study (Plan A). The Skin Ring was brought closer to the cavity by 5 mm (Plan B). No other change to plan A was introduced for this case study. The max dose to Skin Ring and the volume of Skin Ring receiving 100% of prescription dose (3.4 Gy) were obtained from both the plans (A and B) for all 12 patient plans and tabulated in Table 1a. Case 2. Change in device positioning - Introducing a known shift of 3 or 5 mm in catheter reconstruction. Four patient plans were studied. One plan was chosen where the cavity was well below the skin surface. Three plans were chosen where the cavity was !5 mm from the skin surface. The plan dwell positions and source activity were kept the same. Manual optimization on dwell times was chosen so that any changes to the catheter reconstruction would not alter the plan dwell times and source activity. The catheter reconstruction was adjusted to include 3 mm shifts on three plans and 5 mm shifts on two plans. DVH for Chestwall, Skin Ring and PTV_Eval is shown in Table 1b. Results: Case 1: When the Skin Ring was brought 5 mm closer to the cavity, dose to the Skin Ring increased from ~100% in plan A toO160% in plan B. Table 1a shows the results. Columns 2-3 show the increase in max dose to Skin Ring. Columns 4-5 show the volume of Skin Ring receiving 100% of the prescription dose in both plans 1 and 2. Only two cases Table 1a Change in skin dose due to change in anatomy (Case 1). Plan #

Plan A Max (%)

Plan B Max (%)

Plan A V100% (cc)

Plan B V100% (cc)

1 2 3 4 5 6 7 8 9 10 11 12

101.71 97.00 98.50 98.86 98.18 97.67 98.09 57.77 97.83 99.80 95.75 71.71

268 167 228 170 168 188 200 81 183 198 179 111

0.29 0.08 0.09 0.07 0.15 0.01 0.09 0.00 0.03 0.13 0.06 0.00

15.2 2.76 3.64 2.58 5.35 11.3 8.17 0.00 8.02 9.71 5.91 0.60

S93

(highlighted in green) would cause no complications when treated as initially planned because the cavity-skin surface distance was O 1 cm. However, with the reduction in tissue inflammation and the subsequent reduction in cavity-skin surface distance, the results indicate an overdose of 70% to the Skin Ring, as it would have been treated per the original plan. Case 2: Table 1b, Columns 3-5 correspond to Plan 1, and show the change in DVH for the first patient where the cavity-skin surface distance is greater (~ 1 cm) and thus, the negligibly small change in dose despite the large tissue deviation of 5 mm. Columns 6-11 show the change in dosimetric parameters for 3 mm or 5 mm shifts made for Plans 2, 3 and 4. Although the PTV coverage is the same, the max dose to Skin Ring and Chestwall are significantly different. Conclusion: The results indicate large dosimetric differences for even small shifts of just 3 mm in device position. These small deviations are difficult to identify with the current pretreatment verification processes. This substantiates the need for more elaborate checks such as fusion with a 3D scan (with !2 mm tolerance) prior to each delivered treatment in order to evaluate if the plan corresponds to the current treatment setup.

PO32 The Dosimetric Property of Tld2000 Thermoluminescent Dosimeter Nan Zhao, MS, Ruijie Yang, PhD, Junjie Wang, MD. Radiation Oncology, Peking University Third Hospital, Beijing, China. Purpose: To study the dosimetric properties of TLD2000 thermoluminescent dosimeter (TLD), including repeatability, linearity of dose response, energy response and dose rate effect. Materials and Methods: 1300 TLD2000 TLDs were read out after exposure to a dose of 1 mGy of 65 keV x-ray, then were sorted out to have the same sensitivity within 3.0%. TLDs were irradiated to a dose of 120 MU using 6 MV x-ray then irradiated to the same dose after 24 h. TLDs were irradiated by two 125I seeds with the same activity for 24 h, and the interval time was 24 h, to study the repeatability of TLDs for 6 MV x-ray and 125I seed. TLDs were irradiated to different doses using 137Cs (662 keVg-ray), 125I seed and 6 MV x-ray, to study the dose response of the TLDs. TLDs were irradiated to a dose of 1 mGy using 137Cs, 48 keV, 65 keV, 83 keV, 118 keV and 250 keV x-rays, to study the energy response of the TLDs. TLDs were irradiated to a dose of 120 MU using 6 MV x-ray with different dose rates of 37 MU/min, 75 MU/min, 150 MU/min, 300 MU/min and 600 MU/min; TLDs were irradiated to the same dose using three 125I seeds with different activities of 0.739 mCi, 0.675 mCi and 0.559 mCi, and the irradiated time were 24 h, 26h 17 min and 31 h 48 min, respectively, to study the dose rate effect of TLDs for 6 MV x-ray and 125I seed. Results: 350 TLD2000 TLDs were selected with the sensitivity within 3.0%. The maximum deviations of the repeatability were 2.7% and 4.0% for 6 MV xray and 125I seed, respectively. The dose response of TLDs for 137Cs and 125I seed were linear. For 6 MV x-ray, the linear response range were 0.74 Gy-10.0 Gy, beyond 10.0 Gy the dose response became supralinear but proportional to the absorbed dose to TLD. The energy response for 48 keV, 65 keV, 83 keV, 118 keV and 250 keV x-rays, relative to the energy response of 137Cs, were 1.25, 1.08, 0.99, 0.91 and 0.96, respectively. There were no dose rate effects in the dose rate range of 37 MU/min to 600 MU/min for 6 MV x-ray and 0.66 cGy/h to 0.87 cGy/h for 125I seed.

Table 1b Change in DVH parameters for shift in device position (Case 2). Plan 1

Plan 2

Plan 3

Plan 4

Structure

Dose Parameter

No shift

3 mm shift

5 mm shift

No shift

5 mm shift

No shift

5 mm shift

No shift

3 mm shift

Chestwall Skin Ring PTV_Eval

Max (%) Max (%) V200 (cc) V150 (cc) V90 (%) V95 (%)

54.00 71.71 16.60 29.65 100.03 100.00

53.64 70.00 15.97 28.61 100.00 99.90

53.33 69.64 15.70 28.26 100.00 99.80

99.82 98.18 18.05 36.01 97.96 95.14

84.36 160.00 19.74 37.91 96.66 94.56

92.36 97.67 18.88 36.24 94.95 92.33

121.60 93.59 18.49 36.62 94.74 91.88

83.00 101.71 16.90 39.63 98.96 97.12

81.60 110.00 18.74 41.92 98.93 97.14

S94

Abstracts / Brachytherapy 14 (2015) S11–S106

Conclusions: TLD2000 TLD has good repeatability and linear dose response for 137Cs, 125I seed and 6 MV x-ray without dose rate effect, but the dose response is energy dependent.

PO33 Thermoluminescent Dosimetry of the Model Bt-125-1 125i Interstitial Brachytherapy Seed Nan Zhao, MS, Ruijie Yang, PhD, Junjie Wang, MD. Radiation Oncology, Peking University Third Hospital, Beijing, China. Purpose: To study the dosimetric parameters of dose rate constant, radial dose functions and anisotropy functions for the model BT-125-1 125I seed with thermoluminescent dosimeters. Materials and Methods: The preliminary experiment is to study repeatability, linearity of dose response, dose rate effect and energy response of the thermoluminescent dosimeters (TLD). The seed was placed perpendicularly in the center of the PMMA phantom, and 12 TLDs were placed parallel to the source long axis at radial distance of 1 cm with 30 increments, to study the dose rate constant of the model BT-125-1 125I seed; the TLDs were placed at radial distances of 0.5, 0.7, 1.0 to 10.0 cm with a 0.5 cm increment and a 5 step, to study the radial dose functions of the model BT-125-1 125I seed. The seed was placed horizontally in the center of the PMMA phantom, and the TLDs were placed vertically to the longitudinal axis of the seed at radial distances of 0.5, 1, 1.5, 2, 3 to 7 cm with a 1 cm increment, and polar angles in 20 increments at the radial distance of 0.5 cm, while polar angles in 10 increments at the other radial distances, to study the anisotropy functions of the model BT-125-1 125I seed. Results: For the model BT-125-1 125I seed, the maximum deviation of repeatability for TLDs was 4.0%. The TLDs had linear dose response without dose rate effect, but the dose response is energy dependent. The dose rate constant, radial dose functions and anisotropy functions were similar to the model 6711 125I presented in the TG43 U1 report. Conclusions: The thermoluminescent dosimetry presented the dose rate constant, radial dose functions and anisotropy functions of the model BT125-1 125I seed which were similar to those of model 6711 125I seed presented in the TG43U1 report.

brachytherapy, and low-dose-rate (LDR) permanent seeds implant treatment of localized prostate cancer. Materials and Methods: Ten patients with localized prostate cancer were retrospectively selected for this study. Volumetric modulated arc therapy, high-dose-rate brachytherapy and low-dose-rate permanent seeds implant plans were created for each patient. For volumetric modulated arc therapy, planning target volume (PTV) was created by adding a margin of 5 mm to the clinical target volume. Bladder, rectum, femoral heads, urethra, and pelvic tissue were considered as organs at risk. 78 Gy in 39 fractions were prescribed for PTV. The dose prescription was D90 of 34 Gy in 8.5 Gy per fraction and 145 Gy to clinical target volume for HDR (192Ir) and LDR (125I), respectively. The dose and dose volume parameters were evaluated for target and organs at risk. Physical dose were converted to dose based on 2-Gy fractions (equivalent dose in 2 Gy per fraction, EQD2) for comparison of three techniques. Results: All three techniques could achieve the prescribed dose to the target and clinically satisfactory plans. The volume of rectum and bladder exposed to 30-70 Gy (EQD2) was reduced for LDR and HDR when compared with VMAT. The sparing of normal tissue and femoral heads was also significantly improved in LDR and HDR compared with VMAT. The mean dose to urethra was lower in HDR than LDR. Conclusions: Brachytherapy techniques were clearly superior in terms of bladder, rectum, femoral heads and normal tissue sparing compared with VMAT. HDR could further improve the sparing of urethra compared with LDR.

PO35 Uniformity Investigation of Iridium and Ytterbium Brachytherapy Sources Norbert Hugger, BS Physics. Nuclear Science and Engineering, Worcester Polytechnic Institute, Worcester, MA, USA. Purpose: As part of an ongoing research project with Source Production Equipment Co., the activity uniformity of neutron activated Ir-192 and Yb-169 brachytherapy sources were studied. Because of their high thermal neutron cross sections (~950 B and 2300 B respectively), an irradiated batch of brachytherapy sources, or more importantly, a single brachytherapy source could have a non-uniform activity distribution. Materials, Methods and Results: MCNP is being used to conduct simulations of the neutron activations of brachytherapy seeds. The model HDR 4140 ytterbium-169 HDR brachytherapy source and the M-19 NDR brachytherapy source will be used. The arrangement consist of a single seed and an arrangement of a typical reactor activation canister. The simulations are in the process of running. In the radiography research, MCNP was used to simulate the neutron activation of the sources. They were irradiated on the surface of an aluminum canister in a reactor. Both elemental and enriched compositions of iridium were simulated. A FMESH tally data was collect across the individual sources and entire canister. The radiography sources had less activity in the center. The flux was decreased more in enriched iridium then in elemental. Conclusions: The iridium radiography disk did not activate evenly do to the high cross sections. Enriched has a higher cross section then elemental, causing the depression to be higher. This phenomena will most likely be more prevalent in ytterbium which has an even higher cross section.

PO36 PO34 Dosimetric and Radiobiological Comparison of Volumetric Modulated Arc Therapy, High-dose-rate Brachytherapy and Low-dose-rate Permanent Seeds Implant for Localized Prostate Cancer Ruijie Yang, PhD, Anyan Liao, MD, Haitao Sun, MS, Xile Zhang, MS, Lu Liu, MS, Yuliang Jiang, MD, Ping Jiang, MD, Hao Wang, MD, Junjie Wanag, MD, PhD. Department of Radiation Oncology, Peking University Third Hospital, Beijing, China.

Dosimetric Consequences From Minimal Displacements in ABPI With Savi Applicators Silvia Pella, PhD1,2, Mikko Hyvarinen, MS2, Nicolae Dumitru, MS2,3, Shereen Chandrasekara, MS4. 1Radiation Oncology, SFRO, Boca Raton, FL, USA; 2Physics/Medical Physics Graduate Program, Florida Atlantic University, Boca Raton, FL, USA; 3Physics/Medical Physics Graduate Program, University of Bucharest, Faculty of Physics, Bucahrest, Romania; 4Florida Atlantic University, Boca Raton, FL, USA.

Purpose: To assess the dosimetric and radiobiological differences among volumetric modulated arc therapy (VMAT), high-dose-rate (HDR)

Purpose: To highlight the importance in scanning each patient before every treatment in ABPI with brachytherapy. The initiative led by Florida Atlantic