S766 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 3, Supplement, 2010

Conclusions: Our 5-year experience shows that ExacTrac system performed extremely well with accuracy of better than 1 mm.However, it required careful CT simulation for perfect alignment of CT with BrainLab co-ordinates. We conclude that mask-basedcranial radiotherapy on a Classic Novalis requires image guidance.

Author Disclosure: S.K. Jani, None; G. Weinstein, None; N. Shah, None; P. Volpp, None; S. Glazebrook, None; J. Pothilat, None;K. Ott, None.

3255 Daily Evaluation of Prostate Position by Gold Marker Based Image Guided Radiotherapy (IGRT) in

Prostate Cancer Patients

K. R. Jani, D. Bhavsar, S. Gajjar, V. Patel, C. Krishnappan, T. Velmurugan, M. Bhonde

CBCC-Apollo Hospital, Ahmedabad, India

Purpose/Objective(s): The aim of the study was to evaluate the daily variation in prostate position by doing gold marker basedIGRT. The other objective was to evaluate the setup accuracy by offline bony matching.

Materials/Methods: Seventy-six patients of prostate cancer were evaluated with gold marker based IGRT (Image Guided IMRT)from August 2006 to December 2009. All the patients were planned for Image Guided IMRT by KV imaging and KV cone beamCT on Clinac ix. Under aseptic precautions 3 gold markers were inserted into prostate by transrectal route with ultrasound guidancein all patients. For immobilization of the patients orbit and knee lock were used in all patients. Twenty-one patients were givenpelvic radiotherapy + prostate boost. Forty-five patients were given prostate radiotherapy only. Daily AP and Lateral KV imagingwere done. KV cone beam CT was also done daily for all patients. Gold marker based matching were done for all patients online.Offline bony matching was done for all the patients to find out the setup error.

Results: After evaluation of the data mean error for marker matching and bony matching were calculated for all the fractions foreach patients. And from that mean error was calculated for 76 patients. The errors were in vertical, lateral and longitudinal axis. Themean error for marker matching (online matching) in vertical was 2.8 mm, lateral was 0.9 mm and longitudinal was 3.4. The meanerror for bony matching (offline matching) in vertical was 1.2 mm, lateral was 0.8 mm and longitudinal was 1.8 mm.

Conclusions: Proper immobilization with orbit and knee lock provides a good setup for the patients as shown in the offline bonymatching. The online error in all the three directions suggests the daily movement of prostate in the pelvic cavity. The marker basedIGRT improves the setup accuracy leading to a better outcome by dose escalation and less morbidity.

Author Disclosure: K.R. Jani, None; D. Bhavsar, None; S. Gajjar, None; V. Patel, None; C. Krishnappan, None; T. Velmurugan,None; M. Bhonde, None.

3256 Inverse Planning for Four-dimensional Volumetric Modulated ARC Therapy

Y. Ma L. Xing

Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA

Purpose/Objective(s): To develop a 4D volumetric modulated arc therapy (VMAT) inverse planning framework for 4D VMAT.

Materials/Methods: 4D VMAT inverse planning aims to derive a dynamic aperture shape and weight modulated arc therapy treat-ment plan that optimizes the accumulated fractional dose distribution with respect to both space and phase. In the current work, anassumption is made that the gantry rotation and patient breathing are synchronized such that there is a functional relationship be-tween the phase of the patient breathing cycle and the beam angle. For example, if the gantry rotates with constant rotational speed,the phase number depends on the gantry position in a deterministic manner provided that the initial phase of the breathing motion isknown. By using this information, the accumulated dose is obtained by summing the doses of all phases through image registrationof the CT images of different respiration phases. In the 4D VMAT inverse planning process, the accumulative dose distribution isoptimized to minimize the planning objective function by using a simulated annealing algorithm. Two clinical cases are employedto demonstrate the proposed technique. For comparison purposes, traditional 3D VMAT plan were also performed for the twocases.

Results: By considering the extra dimension of time in the VMAT inverse planning, it is shown that a rotational therapy is achievedwith a tight target margin and a full duty cycle otherwise not achievable by conventional 3D VMAT optimization.

Conclusions: The formulism proposed here provides useful insight on how the ‘time’ dimension can be exploited in rotational arctherapy to maximally compensate for the intrafraction organ motion.

Author Disclosure: Y. Ma, None; L. Xing, None.

3257 How Collimator Rotation Influence IMRT Dose Distribution and Dose Delivery: Assessment in Case of

Carcinoma Parotid

S. Sharma1, D. Manigandan1, V. Subramani1, S. Goyal1, P. Julka1, G. Rath2

1All India Institute of Medical Sciences (AIIMS), New Delhi, India, 2All India Institute of Medical Sciences (AIIMS), New Delhi,India

Purpose/Objective(s): Collimator rotation will give additional degree of freedom to optimization of intensity modulated radiationtherapy (IMRT) planning. However, planning and treatment delivery become complex. Hence, the purpose of this study is to eval-uate (1) how much improvement we can get in IMRT plan with collimator rotation in terms of target coverage and dose to criticalstructures (2) delivery efficiency of desired fluence with collimator rotation.

Materials/Methods: Five previously treated patients of carcinoma parotid were used for this study. Clinical Target Volume(CTV), Planning Target Volume (PTV) and other critical organs were delineated. IMRT (Dynamic, 60 Gy in 30 fractions) plans