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Surface optimization for the mammosite applicator
S598
I. J. Radiation Oncology
● Biology ● Physics
Volume 60, Number 1, Supplement, 2004
mouthpiece and the accuracy of the set-up were estimated in this study. Intra-fractional as well as inter-fractional motion were also evaluated. Materials/Methods: Three 2-mm gold markers were implanted into a mouthpiece that had been specifically made for each patient before computed tomography (CT) for treatment planning. The coordinates of the gravity center of the three markers and its relationship to the gravity center of the planning target volume were transferred to the RTRT system before irradiation. The patient wore the mouthpiece, immobilized by a plastic shell positioned by laser localizers, and then two sets of orthogonal fluoroscopic images were taken. Translational error was corrected by remote-control of the patient couch after a comparison between the actual and planned positions of the three markers. Set-up errors in the conventional immobilization using a thermoplastic shell (manual set-up) and the RTRT system (RTRT set-up) were compared by measuring the discrepancy of coordinates of the gravity center of the three markers in this study. Rotational set-up error can be calculated using the three markers. The position of the markers in the mouth was monitored during irradiation by frequent fluoroscopic measurement. Six patients with pharyngeal tumors were used for this analysis. Results: Systematic set-up errors were 2.2, 3.1, and 1.5 mm in manual set-up and 0.8, 1.0, and 0.7 mm in RTRT set-up in right-left (RL), cranio-caudal (CC), antero-posterior (AP), respectively (n ⫽ 6). Random set-up errors were 2.3, 3.2, and 2.1 mm in manual set-up and 1.2, 1.5, and 1.0 mm in RTRT set-up (n ⫽ 125). Statistically significant differences (p ⬍ 0.001) were seen in random set-up errors between manual set-up and RTRT set-up. Using frequent fluoroscopic verification and modification of the patient couch during delivery of the treatment beam, intra-fractional random error was kept to 1.3, 2.3, and 1.2 mm (n ⫽ 654). Random rotational set-up errors were 3.9, 1.8, and 3.1 degrees around RL(␣), CC(), and AP(␥)-axes (n ⫽ 136). Conclusions: Set-up error was reduced by using an RTRT system with a mouthpiece and three gold markers in head and neck IMRT. The appropriate set-up margin can be significantly reduced though the use of a mouthpiece and an RTRT set-up technique to approximately half that with a manual set-up.
2436
Surface Optimization for the MammoSite Applicator 1
M. C. Kirk, A. Dickler,1 W. C. Hsi,1 J. C. Chu,1 K. Dowlatshahi,2 D. Francescatti,2 C. Nguyen1 Radiation Oncology, Rush University Medical Center, Chicago, IL, 2General Surgery, Rush University Medical Center, Chicago, IL
1
Purpose/Objective: We present a technique to optimize the dwell times and positions of a HDR Ir-192 source in brachytherapy procedures using the MammoSite applicator. Due to balloon deformation and the anisotropy of the Ir-192 source, the portion of the PTV along the source axis receives less dose compared to more central portions of the PTV if a single dwell position is used. The surface optimization method attempts to conform the 100% isodose line to the surface of the PTV. The surface optimization technique is described and the dosimetric characteristics of this method are compared to the single dwell position technique based on 20 patients treated at our institution.
Proceedings of the 46th Annual ASTRO Meeting
Materials/Methods: The study population consists of 20 patients treated using the MammoSite device between October 18, 2002 and February 13, 2004. Treatment was delivered in 10 fractions of 3.4 Gy, b.i.d. Each patient was planned using two techniques based solely on CT images. To improve the accuracy of the balloon catheter reconstruction and placement of the prescription point, the original CT dataset was resliced such that the entire catheter is visible on a single slice. The single dwell position technique uses a single source in the center of the balloon to deliver 3.4 Gy to the prescription point 1cm from the balloon surface along a line perpendicular to the source axis. The surface optimization technique uses optimization points on the surface of the PTV and optimizes the dwell times to equalize the dose at the optimization points. The resulting dwell times are then scaled to deliver 340 cGy to the single dwell position prescription point Results: The surface optimization technique increased the percentage of the PTV covered by the prescription dose compared to the single dwell position technique from a mean of 85% to 94%. The mean percentage of the PTV receiving 150% of the prescription dose was 37% for the surface optimization technique and 29% for the single dwell position technique. The dose homogeneity index, DHI, and full width at half maximum (FWHM) of the differential PTV dose volume histogram were used to evaluate dose uniformity. The surface optimization had a mean DHI ⫽ 0.61 and the single dwell position had a mean DHI ⫽ 0.67. The surface optimization method had a mean FWHM ⫽ 204 and the single dwell position technique had a mean FWHM ⫽ 202. Conclusions: Compared to the single dwell position technique, the surface technique corrects for the balloon elongation and source anisotropy and provides greater coverage of the PTV. The DHI indicates that the single dwell position technique had a more homogenous plan. However, DHI fails to account for the portion of the PTV receiving less then 100% coverage and the single dwell position method had a larger DHI because it is normalized to a smaller V100. The FWHM values are equivalent for the two techniques and thus indicate equivalent uniformity(TABLE 1).
2437
Skin Dose Determination for Hypo-Fractionated Breast Treatment Using Mixed Photon and Electron Beams
S. Stathakis, J. Li, K. Paskalev, L. Wang, J. Yang, C. Ma Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA Purpose/Objective: Skin complications have been an important factor in the dose prescription for breast radiotherapy. However, most treatment planning systems fail to accurately predict the dose at the surface for breast treatment planning. Recent advances in radiotherapy treatment techniques such as intensity modulated radiation therapy (IMRT) and energy- and intensity-modulated electron radiotherapy (MERT), and new treatment schemes, such as hypo-fractionated breast therapy have made the precise determination of the surface dose necessary. Detailed information of the dose at various depths of the skin is critical in designing new treatment strategies using these new techniques and in the selection of beam modalities, energies, boluses and beam spoilers. Monte Carlo methods provide a tool that can be used to accurately calculate the dose at the skin surface of the patient at very shallow depths. The purpose of this work is to establish the correlation between the calculated values and measurements with thin TLDs and thin-walled ion chambers for skin dose determination.
S599
I. J. Radiation Oncology
● Biology ● Physics
Volume 60, Number 1, Supplement, 2004
mouthpiece and the accuracy of the set-up were estimated in this study. Intra-fractional as well as inter-fractional motion were also evaluated. Materials/Methods: Three 2-mm gold markers were implanted into a mouthpiece that had been specifically made for each patient before computed tomography (CT) for treatment planning. The coordinates of the gravity center of the three markers and its relationship to the gravity center of the planning target volume were transferred to the RTRT system before irradiation. The patient wore the mouthpiece, immobilized by a plastic shell positioned by laser localizers, and then two sets of orthogonal fluoroscopic images were taken. Translational error was corrected by remote-control of the patient couch after a comparison between the actual and planned positions of the three markers. Set-up errors in the conventional immobilization using a thermoplastic shell (manual set-up) and the RTRT system (RTRT set-up) were compared by measuring the discrepancy of coordinates of the gravity center of the three markers in this study. Rotational set-up error can be calculated using the three markers. The position of the markers in the mouth was monitored during irradiation by frequent fluoroscopic measurement. Six patients with pharyngeal tumors were used for this analysis. Results: Systematic set-up errors were 2.2, 3.1, and 1.5 mm in manual set-up and 0.8, 1.0, and 0.7 mm in RTRT set-up in right-left (RL), cranio-caudal (CC), antero-posterior (AP), respectively (n ⫽ 6). Random set-up errors were 2.3, 3.2, and 2.1 mm in manual set-up and 1.2, 1.5, and 1.0 mm in RTRT set-up (n ⫽ 125). Statistically significant differences (p ⬍ 0.001) were seen in random set-up errors between manual set-up and RTRT set-up. Using frequent fluoroscopic verification and modification of the patient couch during delivery of the treatment beam, intra-fractional random error was kept to 1.3, 2.3, and 1.2 mm (n ⫽ 654). Random rotational set-up errors were 3.9, 1.8, and 3.1 degrees around RL(␣), CC(), and AP(␥)-axes (n ⫽ 136). Conclusions: Set-up error was reduced by using an RTRT system with a mouthpiece and three gold markers in head and neck IMRT. The appropriate set-up margin can be significantly reduced though the use of a mouthpiece and an RTRT set-up technique to approximately half that with a manual set-up.
2436
Surface Optimization for the MammoSite Applicator 1
M. C. Kirk, A. Dickler,1 W. C. Hsi,1 J. C. Chu,1 K. Dowlatshahi,2 D. Francescatti,2 C. Nguyen1 Radiation Oncology, Rush University Medical Center, Chicago, IL, 2General Surgery, Rush University Medical Center, Chicago, IL
1
Purpose/Objective: We present a technique to optimize the dwell times and positions of a HDR Ir-192 source in brachytherapy procedures using the MammoSite applicator. Due to balloon deformation and the anisotropy of the Ir-192 source, the portion of the PTV along the source axis receives less dose compared to more central portions of the PTV if a single dwell position is used. The surface optimization method attempts to conform the 100% isodose line to the surface of the PTV. The surface optimization technique is described and the dosimetric characteristics of this method are compared to the single dwell position technique based on 20 patients treated at our institution.
Proceedings of the 46th Annual ASTRO Meeting
Materials/Methods: The study population consists of 20 patients treated using the MammoSite device between October 18, 2002 and February 13, 2004. Treatment was delivered in 10 fractions of 3.4 Gy, b.i.d. Each patient was planned using two techniques based solely on CT images. To improve the accuracy of the balloon catheter reconstruction and placement of the prescription point, the original CT dataset was resliced such that the entire catheter is visible on a single slice. The single dwell position technique uses a single source in the center of the balloon to deliver 3.4 Gy to the prescription point 1cm from the balloon surface along a line perpendicular to the source axis. The surface optimization technique uses optimization points on the surface of the PTV and optimizes the dwell times to equalize the dose at the optimization points. The resulting dwell times are then scaled to deliver 340 cGy to the single dwell position prescription point Results: The surface optimization technique increased the percentage of the PTV covered by the prescription dose compared to the single dwell position technique from a mean of 85% to 94%. The mean percentage of the PTV receiving 150% of the prescription dose was 37% for the surface optimization technique and 29% for the single dwell position technique. The dose homogeneity index, DHI, and full width at half maximum (FWHM) of the differential PTV dose volume histogram were used to evaluate dose uniformity. The surface optimization had a mean DHI ⫽ 0.61 and the single dwell position had a mean DHI ⫽ 0.67. The surface optimization method had a mean FWHM ⫽ 204 and the single dwell position technique had a mean FWHM ⫽ 202. Conclusions: Compared to the single dwell position technique, the surface technique corrects for the balloon elongation and source anisotropy and provides greater coverage of the PTV. The DHI indicates that the single dwell position technique had a more homogenous plan. However, DHI fails to account for the portion of the PTV receiving less then 100% coverage and the single dwell position method had a larger DHI because it is normalized to a smaller V100. The FWHM values are equivalent for the two techniques and thus indicate equivalent uniformity(TABLE 1).
2437
Skin Dose Determination for Hypo-Fractionated Breast Treatment Using Mixed Photon and Electron Beams
S. Stathakis, J. Li, K. Paskalev, L. Wang, J. Yang, C. Ma Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA Purpose/Objective: Skin complications have been an important factor in the dose prescription for breast radiotherapy. However, most treatment planning systems fail to accurately predict the dose at the surface for breast treatment planning. Recent advances in radiotherapy treatment techniques such as intensity modulated radiation therapy (IMRT) and energy- and intensity-modulated electron radiotherapy (MERT), and new treatment schemes, such as hypo-fractionated breast therapy have made the precise determination of the surface dose necessary. Detailed information of the dose at various depths of the skin is critical in designing new treatment strategies using these new techniques and in the selection of beam modalities, energies, boluses and beam spoilers. Monte Carlo methods provide a tool that can be used to accurately calculate the dose at the skin surface of the patient at very shallow depths. The purpose of this work is to establish the correlation between the calculated values and measurements with thin TLDs and thin-walled ion chambers for skin dose determination.
S599