- Email: [email protected]
Novel Spiral Phantom for Volumetric Modulated Arc Therapy Delivery Verification
Proceedings of the 52nd Annual ASTRO Meeting Materials/Methods: In our study, we made a phantom in which titanium implants were inserted in similar position to spinal posterior/posterolateral fusion. We compared the calculated dose of treatment planning systems and the measured dose in radiation therapy equipments. We used three kinds of CT (kilovoltage CT, extended-scaled kilovoltage CT and megavoltage CT) and three kinds of radiation therapy equipments (ARTISTE (Siemens, Berlin, Germany), Tomotherapy (TomoTherapy, Madison, WI) and Cyberknife (Accuray, Sunnyvale, CA). For measurement of dose, we used an ionization chamber and a Gafchromic EBT film. Results: The absolute dose which was measured using an ionization chamber at the isocenter in the titanium phantom was average 1.9% lower than in the reference phantom (p = 0.002). There was not statistically significant difference according to the kinds of CT images, radiation therapy equipments and the size of target. As the distance from the surface of the titanium implants becomes closer, the measured dose tends to be decreased (p \ 0.001). There was a statistically significant difference among the kinds of CT images. The effect of metallic implants is less in the megavoltage CT than in the kilovoltage CT or extended-scaled kilovoltage CT (p = 0.063). Conclusions: The range of errors due to the titanium implants is beyond clinically acceptable range. In order to reduce the error of dose calculation, we recommend that the megavoltage CT should be used for treatment planning. In addition, to prescribe the dose for the target and the dose constraint for the organ at risk, it is necessary to consider the distance between the titanium implants and the targets or the organ at risk. Author Disclosure: S. Son, None; Y. Kang, None; M. Ryu, None; B. Choi, None; H. Jang, None; I. Choi, None; H. Shin, None.
3263
Novel Spiral Phantom for Volumetric Modulated Arc Therapy Delivery Verification
M. Tanooka1, T. Sakai2, H. Inoue2, H. Doi1, Y. Takada1, M. Fujiwara1, K. Yasumasa2, K. Sakamoto2, N. Kamikonya1, S. Hirota1 Department of Radiology, Hyogo College of Medicine, Nishinomiya-city, Hyogo, Japan, 2Department of Clinical Radiology, The Hospital of Hyogo College of Medicine, Nishinomiya-city, Hyogo, Japan 1
Purpose/Objective(s): Volumetric modulated arc therapy (VMAT) method is a very novel treatment technique, but it is insufficient to make dose verification at only one point, profile, or in a single plane. Complete verification has to be volumetric including total coverage of the cross-section of the beam at all gantry angles. We have been conventionally using the films type in a configuration in which square blocks are used and multiple films are stacked either parallel or perpendicular to the beam. In our design we propose a novel spiral phantom (R-TEC. Inc., Tokyo, Japan) with radiochromic films for VMAT for complete delivery verification. Materials/Methods: A spiral phantom was machined to create a spiral cavity for placing radiochromic film, which was EBT2 gafchromic film (ISP Corp., Wayne, NJ, USA) in a spiral configuration. This spiral phantom was used to sample, calculate, and measure data in three-dimension (3D) subspace and water. The calculated data were obtained by projecting the patient plan data on the spiral phantom in the treatment planning system. The measured data were obtained by irradiating the spiral phantom as per the treatment plan. The calculated and measured data were converted to a two-dimensional (2D) matrix and plotted as a spiralogram with DD-System (R-TEC. Inc., Tokyo, Japan). Comparison of these calculated and measured spiralograms were performed using the gamma index analysis (with distance to agreement and dose thresholds set to 3 mm and 3%, respectively). Results: This method using DD-System in a spiral phantom was used to validate the dosimetric accuracy of the treatment plan. Comparison of the calculated and measured data provided a visually qualitative assessment of the treatment to be delivered. Comparison between calculated and measured dose distributions yielded a pass rate using gamma index (3%, 3 mm) of above 90% for EBT2 gafchromic film. Conclusions: A novel spiral phantom is a simple and cost-effective approach to sample 3D data from VMAT. Furthermore, the spiral phantom offers a completely innovative approach to rotational delivery QA (Quality Assurance) outside of familiar 2D conventions. Author Disclosure: M. Tanooka, None; T. Sakai, None; H. Inoue, None; H. Doi, None; Y. Takada, None; M. Fujiwara, None; K. Yasumasa, None; K. Sakamoto, None; N. Kamikonya, None; S. Hirota, None.
3264
The Impact of Daily Tumor Shift on Normal Tissue Sparing and Dose Coverage for Breath Hold Stereotactic Body Radiotherapy (SBRT) of Lung Tumor
Y. Peng, R. Sadagopan, S. Vedam, S. Gao, J. Chang, Z. Liao, R. Komaki, M. Jeter, R. Mohan, P. Balter M. D. Anderson Cancer Center, Houston, TX Purpose/Objective(s): To investigate the effect of inter-fraction changes in BH level, hence tumor position, on normal tissue sparing and planning target volume (PTV) coverage in breath hold (BH) gated SBRT for lung tumors. Materials/Methods: We evaluated 13 early stage lung cancer patients who were treated with BH gated SBRT due to large, .10 mm (20.0 ± 7.3 mm), tumor motion under normal respiration. Patients received 50 Gy in 4 fractions using five to nine 6 MV Xray beams on consecutive days with BH gating. Simulation consisted of multiple deep inspiration BH CTs using visual feedback to assist the patient in maintaining a constant, intra-fraction, BH level. One of the BH CT data sets where the GTV was near the center of the range of the tumor positions was chosen as the reference CT for treatment planning. A structure, created to encompass the GTvs. on all intra-fraction BH CTs, was expanded by 8 mm for subclinical disease to obtain an ITV (CTV + IM) and then by 3 mm for setup uncertainties to obtain a PTV. Daily CBCTs were acquired under 2-4 BHs to account for inter-fraction differences in BH level. The required shifts to cover the target were determined. These shifts were compared to those from bony alignment to determine if they caused differences in the delivered dose vs. the planned dose to target and critical structures. We
S769
3263
Novel Spiral Phantom for Volumetric Modulated Arc Therapy Delivery Verification
M. Tanooka1, T. Sakai2, H. Inoue2, H. Doi1, Y. Takada1, M. Fujiwara1, K. Yasumasa2, K. Sakamoto2, N. Kamikonya1, S. Hirota1 Department of Radiology, Hyogo College of Medicine, Nishinomiya-city, Hyogo, Japan, 2Department of Clinical Radiology, The Hospital of Hyogo College of Medicine, Nishinomiya-city, Hyogo, Japan 1
Purpose/Objective(s): Volumetric modulated arc therapy (VMAT) method is a very novel treatment technique, but it is insufficient to make dose verification at only one point, profile, or in a single plane. Complete verification has to be volumetric including total coverage of the cross-section of the beam at all gantry angles. We have been conventionally using the films type in a configuration in which square blocks are used and multiple films are stacked either parallel or perpendicular to the beam. In our design we propose a novel spiral phantom (R-TEC. Inc., Tokyo, Japan) with radiochromic films for VMAT for complete delivery verification. Materials/Methods: A spiral phantom was machined to create a spiral cavity for placing radiochromic film, which was EBT2 gafchromic film (ISP Corp., Wayne, NJ, USA) in a spiral configuration. This spiral phantom was used to sample, calculate, and measure data in three-dimension (3D) subspace and water. The calculated data were obtained by projecting the patient plan data on the spiral phantom in the treatment planning system. The measured data were obtained by irradiating the spiral phantom as per the treatment plan. The calculated and measured data were converted to a two-dimensional (2D) matrix and plotted as a spiralogram with DD-System (R-TEC. Inc., Tokyo, Japan). Comparison of these calculated and measured spiralograms were performed using the gamma index analysis (with distance to agreement and dose thresholds set to 3 mm and 3%, respectively). Results: This method using DD-System in a spiral phantom was used to validate the dosimetric accuracy of the treatment plan. Comparison of the calculated and measured data provided a visually qualitative assessment of the treatment to be delivered. Comparison between calculated and measured dose distributions yielded a pass rate using gamma index (3%, 3 mm) of above 90% for EBT2 gafchromic film. Conclusions: A novel spiral phantom is a simple and cost-effective approach to sample 3D data from VMAT. Furthermore, the spiral phantom offers a completely innovative approach to rotational delivery QA (Quality Assurance) outside of familiar 2D conventions. Author Disclosure: M. Tanooka, None; T. Sakai, None; H. Inoue, None; H. Doi, None; Y. Takada, None; M. Fujiwara, None; K. Yasumasa, None; K. Sakamoto, None; N. Kamikonya, None; S. Hirota, None.
3264
The Impact of Daily Tumor Shift on Normal Tissue Sparing and Dose Coverage for Breath Hold Stereotactic Body Radiotherapy (SBRT) of Lung Tumor
Y. Peng, R. Sadagopan, S. Vedam, S. Gao, J. Chang, Z. Liao, R. Komaki, M. Jeter, R. Mohan, P. Balter M. D. Anderson Cancer Center, Houston, TX Purpose/Objective(s): To investigate the effect of inter-fraction changes in BH level, hence tumor position, on normal tissue sparing and planning target volume (PTV) coverage in breath hold (BH) gated SBRT for lung tumors. Materials/Methods: We evaluated 13 early stage lung cancer patients who were treated with BH gated SBRT due to large, .10 mm (20.0 ± 7.3 mm), tumor motion under normal respiration. Patients received 50 Gy in 4 fractions using five to nine 6 MV Xray beams on consecutive days with BH gating. Simulation consisted of multiple deep inspiration BH CTs using visual feedback to assist the patient in maintaining a constant, intra-fraction, BH level. One of the BH CT data sets where the GTV was near the center of the range of the tumor positions was chosen as the reference CT for treatment planning. A structure, created to encompass the GTvs. on all intra-fraction BH CTs, was expanded by 8 mm for subclinical disease to obtain an ITV (CTV + IM) and then by 3 mm for setup uncertainties to obtain a PTV. Daily CBCTs were acquired under 2-4 BHs to account for inter-fraction differences in BH level. The required shifts to cover the target were determined. These shifts were compared to those from bony alignment to determine if they caused differences in the delivered dose vs. the planned dose to target and critical structures. We
S769