181 Measurement of respiratory induced motion of implanted gold seeds in the prostate using digital fluoroscopy

CARO 2004

for the first part of phase 1 and wedged fields in the superiorinferior direction for the remaining portion were used to deliver the resultant summed phase 1 plan. Increased dosimetric planning and physicist checking time has resulted from our change. Linear accelerator pre-treatment verification time has increased but actual patient treatment time is not different from previous. Conclusions: Using multi-segment MLC virtual compensation for the lateral phase 1 fields in H&N radiotherapy has allowed us to use our new technology and develop skills useful for pursuit of intensity modulated radiotherapy (IMRT). Hopefully, normal tissue complications will reduce as a result of a homogenous biologic radiotherapydose in phase-1. As we progress up the learning curve for this new technique planning time should decrease. Patient treatment time is not significantly different, an important point for a busy radiotherapy department. 180 Time-lntregrated DVH for Clinical Post-Implant Dosimetric Assesments of 1-125 Permanent Prostate Brachytherapy in the Presence of Edema. G. Leclerc I, M.C. Lavall~ 2, A.G. Martin 3, E. Vigneault ~, L. Beaulieu2. ICHUQ, HSteI-Dieu de Quebec, Quebec, CHUS, Sherbrooke; 2CHUQ, H6teI-Dieu de Quebec and Universit~ Laval, Qu6bec; 3CHUQ, HSteI-Dieu de Quebec, Quebec

Purpose: This study is proposing and validating a simple method to obtain clinical time-integrated (dynamical) DVHs from which more accurate dosimetric parameters can be extracted. Methods and Materials: Apart from the initial TRUS volume reconstruction and pre-plan seed distributions, CT exams (contouring) and fluoroscopic images (precise seed positions) are acquired a few hours after the implant and 40 days (average) post-implant for 57 patients. Prostate volume changes were,calculated from the contoured CT exams and also based on the inter-seed distances. An exponential volume resorption scenario was implemented. The prostate contours and seed positions were allowed to dynamically change from t=0 to 244 days (over 6 edema half-life) in steps of 1 day. DVHs were generated at each step and convoluted with a weight equal to the fraction of the total dose delivered in that particular step to obtain a time-integrated (TI) DVH. Results: The maximum edema relative to the pre-planning volume (PPV) is 1.51 (SD=0.44). By 40 days (average), the prostate volume was still 20% higher than the PPV. The extracted mean edema half-life is 34 days (SD=20), which is higher than previously published estimate. The TI dosimetry yield an optimun timing for post-plan dosimetry of 45 days for 1125 implants. TI D90 was on average the same than that of the regular clinical post-plan. However, there are variations ranging from -15 to +15 Gy depending of each patient specific edema half-life and time to post-implant CT exam. The same effect is seen for D10, D50, V100, V150 and V200. Conclusions: The magnitude and half-life of the edema can not be predicted based on PPV, number of seeds or needles. As such, the dosimetric impact of the edema can only be accounted for if a TI dosimetry is performed. TI dosimetry might be necessary to obtain meaningful correlations to biological responses and toxicities. 181 Measurement of Respiratory Induced Motion of Implanted Gold Seeds in the Prostate Using Digital Fluoroscopy. Ft. Tirona, K. Sixel, P. Cheung. Toronto Sunnybrook Regional Cancer Centre, Toronto, Ontario

Introduction: The goal of daily on-line targeting during radiotherapy is to minimize interfraction variability of the target volume position. Although the planning target volume (PTV)

September 9-12 $55

margins can theoretically be reduced significantly with on-line targeting, intrafraction motion remains a source of error. Respiratory induced motion is one of the sources of intrafraction motion that must be accounted for. A study was conducted to quantify the intrafraction motion of the prostate using digital fluoroscopy. Methods and Materials: Thirty-two patients underwent transperineal gold seed insertions into their prostate and were imaged one week later dudng their CT planning appointment with a digital fluoroscopy unit (FACTS- Philips, Cleveland, OH). Orthogonal movie loops (anterior-posterior and lateral) of twenty seconds duration were captured for each patient. The patients were instructed to breathe normally and then on a verbal cue provide a deep inspiration in order to observe its effects. An in-house program enabled playback and seed motion measurement capabilities for each movie loop in all three principle directions. Results: Means and standard deviations for Superior-Inferior, Lateral and Anterior-Posterior prostate motions were 0.5 mm + 0.2 ram, 0 mm _+0.1 mm and 0.2 mm ~ 0.2 mm, respectively, during quiet respiration. Results for deep inspiration for the Superior-Inferior, Lateral and Anterior-Posterior prostate motions were 2.1 _+ 1.6 mm, 0.6 _+0.6 mm and 1.7 _+0.5 mm, respectively, with observed extremes of motion in the superiorinferior direction of 6.3 mm, 2.5 mm in the lateral direction and 6.5 mm in the anterior-posterior direction. Conclusions: Intrafraction motion of the prostate due to quiet respiration has minimal impact on PTV calculations. Although the effect of a deep inspiration can be significant, coaching patients to quiet respiration would minimize the potential for geographic miss. 182 A Dosimetric Feasibility Study of Conformal Radiotherapy (CRT) vs. Intensity Modulated Radiotherapy (IMR'r) vs. Simultaneously Integrated Hypofractionated Boost IMRT (SIHB) in Prostate Adenocareinoma. A. Karvat, C. Yu, M. Liu, S. Kristensen, N. Ka/ach. British Columbia Cancer Agency - Fraser Valley Centre, University of British Columbia, Surrey, British Columbia

Purpose: To dosimetrically evaluate 3 different dose escalation methods for prostate radiotherapy to assess the feasibility of combining dose escalation, hypofractionation, whole pelvic radiotherapy, and simultaneously integrated boost. Methods: CT simulation data from five patients with localized prostate cancer receiving CRT including the whole pelvis were used. The target volumes were: Phase I - whole pelvis, and Phase II - prostate + 0.5 cm. For CRT and IMRT: Phase I was treated to 44 Gy, and Phase II to 78 Gy (2 Gy/fr). For SIHB: 68.75 Gy in 25 fractions was prescribed (2.75 Gy/fraction); phase II PTV was covered by the 95% isodose, and the phase I PTV by the 62.2% isodose (95% of 45 Gy/25, 1.8 Gy/fr). The CRT technique was a 4 field box for Phase I and a 4 field technique (RAO, LAO, Rt Lat, Lt Lat)for Phase I1. For IMRT and SIHB, five fields (AP, RAO, LAO, RPO, LPO) were used with dynamic MLC creating the desired photon fluences. Comparisons of dose to the rectum, bladder, hips, small bowel, and PTVfs were done for all plans. Dose volume histograms, normal tissue complication probabilities, tumor control probabilities, minimum, maximum, and mean doses were considered. All doses were converted to biologically equivalent doses (BED) as per the a/13formalism. Results: IMRT and SIHB provided superior rectal sparing results compared to CRT. Rectal dose constraints were achievable with SIHB. PTV coverage was comparable in all plans. Although differences in dose to the bladder and hips were noted, these were not felt to be clinically significant. Conclusions: SIHB appears to be dosimetrically feasible, with results superior to CRT and comparable to conventionally