179 Virtual compensation in head and neck radiotherapy

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inversely planned IMRT treatment, we used 8 unequally spaced 6 MV fields (275, 160, 355,220, 270 310, 100, 80 degree). The HT plan was generated using the HT treatment planning system (Tomotherapy Inc., Madison, Wisconsin) and employed a modulation factor of 2.25, pitch of 0.3, jaw width of 1 cm. Results: All plans allowed sparing of the left optic apparatus. For 3DCRT and IMRT, number of beams and their directions had a large impact on the degree of sparing between the brain and the left optic nerve. HT and IMAT generated more conformal plans that spared more brain tissue at higher (>20 Gy) dose levels. Point dose phantom measurement of the IMAT plan agreed with plan within 1%. Film dosimetry agreed with the plan and the maximum deviation was within 5% or 5 mm isodose shift. Conclusions: For this case of a large base of skull tumor we found that HT and iMAT best met clinical constraints. In this instance, more beam directions (and hence more beam's eye view shaping opportunities) provided improved conformality compared to the use of intensity modulation along a smaller number of beam trajectories. 177 Acute and Subacute Toxicities and Response Rates in a Pilot Study of Intensity Modulated Radiotherapy for Locoregionally Advanced Head and Neck Squamous Cell Cancer. S. Larsson 1, F. Wong2, J. Wu3, S. Kader~, E. Berthelet ~, A. Karvat2, D. Wells ~, C. Duzenlf, E. Vollans 3. I Vancouver Island Centre, British Columbia Cancer Agency, Vancouver, British Columbia; 2Fraser Valley Centre, British Columbia Cancer Agency; 3Vancouver Centre, British Columbia Cancer Agency, Vancouver, British Columbia

Methods: Between September 2003 and March 2004, twenty patients were entered into a study of intensity-modulated radiotherapy (IMRT) for Iocoregionally advanced head and neck squamous cancer (T1-2 N+, or T3-4 N0-3). Treatment used 7 fields to a dose of 60 Gy/25 fractions to gross disease, and 50 Gy to at-risk nodal areas. Constraints were set intending that 50% of each parotid gland receive less than 20 Gy, and the spinal cord receive less than 47 Gy. Toxicities were recorded weekly during treatment using RTOG scores (scale 0-4) for skin, mucosa, salivary, pharynx/oesophagus and larynx function. Toxicities were recorded at 2, 4, 8 and 12 weeks posttreatment. Clinical response rates for patients over 4 weeks post-treatment is reported. Results: 12 patients had completed treatment as of this abstract. Median toxicity scores after weeks 1, 3 and 5 of treatment were: skin 0.1, 0.9, 1.4, mucosa 0.3, 1.6, 2.2, salivary 0.6, 1.4, 1.5, pharynx 0.1, 0.6, 1.6, larynx 0.1, 0.2, 1.1. No skin scores greater than 2 were seen. Transient grade 3 mucosal toxicity was seen in two patients. No grade 4 toxicities were seen. Seven patients have toxicities scored to 12 weeks posttreatment. Median scores at 2, 8 and 12 weeks were: skin 1.1, 0.3, 0.0, mucosa 0.6, 0.2. 0.0, salivary 1.0, 1.0, 1.1, pharynx 1.1, 1.0, 0.6, larynx 0,3, 0.0, 0.0. Of 9 patients who are more than 4 weeks from completion of IMRT, 1 has progressive disease, and 8 have no clinical evidence of disease. Conclusions: IMRT is a feasible treatment option for patients with Iocoregionally advanced head and neck cancer. Treatment toxicities are generally less than with conventional radiotherapy. Response rates in this study are encouraging.

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178 Impact of Geometric Variations on Dose Distribution During Intensity-Modulated Radiotherapy (IMRT) of Head and Neck Cancer Patients. O. Ballivy, H. Patrocinio, T. Vuong, G. Shenouda, W. Parker. Department of Oncology, Division of Radiation Oncology and Medical Physics, McGfll University, Montreal, Quebec

Objectives: To assess the effect of geometric variations on target coverage and dose to organs at risk (OAR) during IMRT treatment and to determine appropriate planning target volume (PTV) and planning OAR volume (PRV) margins. Methods: Eight head and neck cancer patients had repeat CT scans (median=3) during treatment. Target volumes and OAR were outlined on all CT scans. For each patient, three IMRT plans were generated from the treatment planning CT scans: one without any PTV or PRV margins, one with 2.5 mm and one with 5 mm margins. The beam intensities of the IMRT plans were then applied on the repeat CT scans and dose distributions were recalculated. Results: Without PTV margins, target coverage was considered adequate (V100 > 95%) for only 12.5% of the GTVs, 54.3% of the upper neck CTVs and 27.4% of the lower neck CTVs. The use of 2.5 and 5 mm PTV margins resulted in adequate coverage for 87.5% and 97.5% of the GTVs, and for 97.1% and 100% of the upper neck CTVs respectively. Only 72.7% of the lower neck CTVs were adequately covered with both the 2.5 and 5 mm margins. The average mean dose to the parotids was 24.9, 26.6 and 29.1 Gy for the plans without PTV margins, 2.5 and 5 mm margins, respectively. Maximum dose to the spinal cord was > 45 Gy in 57.7% of the plans when no PRV margins were used, as compared to 34.6% and 15.4% with 2.5 and 5 mm margins, respectively. Conclusions: Based on our findings, 5 mm PTV margins for GTVs and 2.5 mm margins for CTVs represent an adequate compromise between target coverage and OAR sparing. In addition, PRV margins decrease the risk of overdosing critical structures. Coverage of the lower neck may improve with better shoulder immobilization. 179 Virtual Compensation in Head and Neck Radiotherapy. J. Wilson, W. Webb, D. Mason, C. Araujo. Centre for the Southern Interior, British Columbia Cancer Agency, Kelowna, British Columbia; University of British Columbia, Vancouver, British Columbia

Objectives: We wished to improve the delivery and outcome of radiotherapy for patients to the head and neck (H&N) region at our centre using new technology. Design: We replaced our previous wedge compensated large lateral fields with multi-segment, multi-leaf collimator (MLC) virtually compensated fields. The poster describes the process for our first patient. Materials and Methods: Three dimensional (3D) computed tomography (CT) data of the patient in the treatment position was acquired. Tumour volumes and field borders were identified. Cadplan was used to calculate the isodose distribution of the large phase 1 lateral fields. Using a forward, iterative approach hot and cold areas were corrected by designing field segments with MLC. All the planning information was electronically transferred to the linear accelerator workstation. Three segments per large lateral phase 1 field per day were treated. Results: Improved tumour isodose homogeneity and avoidance of hot areas in normal tissues were better achieved with our new virtual compensation technique compared with our previous wedge compensation method. All segments per field were treated each day, whereas in our older method a combination of fields wedged in the anterior-posterior direction

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)

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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