1. AUTHORS (ALL): Huang, Xiaoyan 1, 2 ; Kuan, K M 2 ; Xiao, G L 2 ; Tsao, S Y 3, 2 ; Qiu, X B 2 ; Ng, K 2. INSTITUTIONS (ALL): 1. Radiation Oncology, Sun Yat-sen University Cancer Center, Guangzhou, Guangdong, China. 2. Radiation Therapy Center, Kiang Wu Hospital, Macau SAR, China. 3. Radiation Therapy & Oncology, The Baptist Hospital, Hong Kong SAR, China. Materials/Methods: At the Kiang Wu Hospital, indicated NPC patients were immobilized in thermoplastic shells, and were applied on a Varian Clinac linear accelerator equipped with a 120-leaf “Millennium” MLC pairing up with the “Eclipse” TPS. PTV and OAR were delineated with appropriate cross-sectional imaging of the head and neck. Isocenter 1 for the lower neck static field (6 MV photons) was placed – the field had 3 segments with identical isocenters and equal weighting, forming an overlapping "dose-buffering zone". Resembling "step & shoot", the upper border had 3 steps: "Neck 1": at the lower margin of the PTV for the head region as viewed with beam's eye view (BEV); "Neck 2 & 3": 0.5 cm and 1.0 cm respectively above "Neck 1" - creating a multi-matchline zone. (After May-2004, the sliding window technique was used for the neck field. The upper border of MLC, the same as “Neck 1”, was sliding through 1.5cm toward the head during the treatment(Fig.1).) Isocenter 2 was in the head region for IMRT with sliding window, 7 or 9 fields, 6 MV photons(Fig.2). Field size was PTV + 0.5 cm. Inverse IMRT optimization yielded a uniform composite dose distribution in the PTV and matchline which was an "intelligent" junctional zone, with due consideration of all dose contributions from the neck fields. Prior to treatment, dosimetric verification was done by ionization chamber and KODAK EDR2 films. Fig.1 The Neck field Fig.2 The NP IMRT field Conclusions: With multiisocentric techniques, field-matching methods, e.g., a gap between the head and the neck fields or, IMRT optimization for a simple overlapping zone, are prone to disproportionately large dosage variations even for minor matchline parameter errors on setup. Monoisocentric non-divergent "halfbeam" techniques are less prone but still cannot tailor adequately for the very unique anatomy of the head - highly complex yet rigid enough for good reproducibility. Utilizing fully the new Base Plan optimization feature of Eclipse for enhanced matchline dosimetry, we have initial success to obviate such unpredictable setup errors - by our "dose-buffering zone". The thinnest (5 mm) leaves of the MLC were also fully utilized by placing them directly over the head region as far as feasible. Moreover, with matchlines away from critical structures, safety was also enhanced. After the modification was done to the neck field, the homogeneity of the matching zone was improved. Thus, dedicated isocenters for the head and the neck are now more practicable - a step in the right direction towards the ultimate goal of a "seamless matchline" for NPC treatment. As the strictest QA is paramount for IMRT, actual documentation of dose variation is mandatory regardless of the method employed. Attempted "Seamless Matchline" of Intensity-Modulated Radiation Therapy Head & Static Neck Fields for Nasopharyngeal Cancer: Treatment Planning & Dosimetric Verification Corresponding author: gzhxyan@hotmail.com Purpose/Objective: For nasopharyngeal cancers (NPC), if the PTV covers also the lower neck, intensity-modulated radiation therapy (IMRT) with a monoisocentric technique is commonly employed. However, dedicated isocenters for the vastly different anatomies of the head and the neck are, in fact, more preferable, especially with the advent of thin-leaf multileaf collimators (MLC) and more versatile treatment planning systems (TPS), provided that there is satisfactory progress on the arduous task for field-matching. Working on the ultimate goal of a "seamless matchline" for IMRT fields for the head and static fields for the neck regions, we made an initial attempt. Results: No significant "hot spots" nor "cold spots" were detected. After fluence mapping with ionization chamber on 138 fields, less than 6% had a variation of the calculated versus measured value of over +/-3%; no single field had any variation of over +/-5%; average variation was 0.1%; all resultant absolute dose value as measured by ionization chamber had a variation of less than +/-3%. The isodoses as measured by film matched well with that calculated by TPS. At the matchline, the variation was within 5% for combined fields (Fig.3). Our final composite dose distribution showed definite improvements on matchline dose homogeneity (Fig.4). Fig.3 The isodoses as measured by film matched well with that calculated by TPS. At the matchline, the variation was within 5% for combined fields match zone Fig. 4 The composite dose distribution showed that the dose was homogeneity at the match zone, no significant "hot spots" nor "cold spots were detected. CoronalSaggital