2890 A Virtual Environment for the Training and Development of Radiotherapy Techniques

A. W. Beavis1, J. W. Ward2, R. M. Appleyard3, R. Phillips4

1Princess Royal Hospital, Hull, United Kingdom, 2University of Hull, Hull, United Kingdom, 3Sheffield-Hallam University,Sheffield, United Kingdom, 4University of Hull, Hull, United Kingdom

Purpose/Objective(s): Radiotherapy techniques have rapidly evolved in recent years, partly due to the availability of newtreatment machines. One consequence is that more demands are placed on limited resources in clinics. Many training centershave identified that reduced access to such clinical resources and an increased complexity in techniques have resulted in aserious problem for the training of new and existing staff groups. In this work we have developed a set of virtual reality toolswhich have been implemented to alleviate this problem in our training center.

Materials/Methods: Our training tools are developed and implemented within an Immersive Visualisation Environment (IVE)in order to add to the users sense of reality. This includes the use of 3D-stereoscopic visualisation, interactive control, soundand position tracking of the user. At the Hull IVE we are able to project our graphics display on a 5.3 m by 2.5 m ‘Power Wall’with Stereoscopic visualisation enabled via LCD shutter glasses.

We have created a faithful representation of a bunker containing a Varian 2100CD treatment machine. The Linac and couchare controlled with an actual Hand Pendant, which, with realistic sound production and the scale of the stereoscopic 3Dprojection, adds to the sense of immersion. The ‘position tracking system’ allows individual users to view the scene from anynumber of directions again adding to the sense of reality.

We have used the IVE to demonstrate very simple concepts such as the isocentre and exit dose to first year students. Toenable this we developed an interface to read patient plans from the CMS XiO treatment planning system. The patient(contoured structures and the planning CT images) is shown ‘fixed’ to the couch, mirroring reality, computed dose is alsodisplayed within the patient. Furthermore, we have implemented tools to visualize the effect of patient mis-alignment ormis-positioning on the treatment couch.

In a pilot study we have developed a specific environment to enable students to gain experience setting up a ‘direct’ electronfield. This commonly used technique often proves problematic for students and difficult to quickly master. It is therefore a goodexample to test whether the use of virtual reality software/ training environments may prove useful in the training ofradiotherapy personnel.

Results: In the general studies we have found that the Linac IVE is universally accepted by those given access to it. Allcomment on the benefits from being able to learn from mistakes without inconveniencing clinical colleagues or patients, orworse, creating dangerous situations. In the direct-field pilot study we found all the participating students reported the trainingenvironment to be realistic. 93% reported that their understanding of this clinical technique was improved by using the virtualreality software and 69% found the control system easy to master.

Conclusions: We believe our training environment to be a useful experience for students. It is not intended to replace theirclinical experience, rather support and enhance it; Visualization of CT data, contoured structures and dose information are notreadily available in the treatment bunker! Given prior exposure and practice on a Virtual Reality Linac (and patient) the studentscan concentrate on developing their ‘patient-skills’ when later gaining experience in the real environment.

Author Disclosure: A.W. Beavis, None; J.W. Ward, None; R.M. Appleyard, None; R. Phillips, None.

2891 A TLD and Film Dosimetry Study of Surface Dose for Head and Neck IMRT

M. J. Mitchell, C. R. Ramsey

Thompson Cancer Survival Center, Knoxville, TN

Purpose/Objective(s): To compare planned and measured surface doses for a series of head and neck IMRT treatments anddetermine optimal PTV configurations for cases where skin sparing or high skin dose is desired.

Materials/Methods: A primary PTV was defined on an anthropomorphic head phantom for a typical parotid-sparing head &neck treatment. The PTV extended from the base of skull to below the cricoid cartilage. Treatment plans were created with 0,1, 2, 3, 4, and 5 mm separation between the skin surface and PTV boundary. Helical tomotherapy treatments were planned usingthe TomoTherapy HI-ART treatment planning system and delivered using a TomoTherapy HI-ART treatment delivery system.IMRT treatments were planned using the Pinnacle treatment planning system and delivered on a Varian 21EX with a 120-leafmultileaf collimator. Inverse planning was performed using Direct Machine Parameter Optimization (DMPO) for step and shootIMRT. Surface dose was evaluated for each type of treatment delivery using thermoluminescent dosimeter (TLD) chips placedon the surface of the phantom and film placed transversely between phantom slices. Surface doses were measured from digitizedfilm images by averaging over a 2 mm path spanning the phantom’s surface and passing through one of two predefined referencepoints at the left or right lateral surfaces of the phantom, just below the ears. Planned surface doses were determined in ananalogous manor; values were averaged over a 2 mm range spanning the surface and passing through a reference point. TLDmeasurements were averaged from locations on the surface nearest each reference point. Error was determined by computingthe standard deviation for each measurement set.

Results: For tomotherapy delivery of the 0mm plan (PTV extended to the skin surface), the measured and planned surface dosesat the right-hand reference point were: TLD 169.4�14.2, film 168.0�3.0, and planned 184.4�0.2 cGy. Tomotherapy deliveryof the skin-sparing 5mm plan (5mm separation between skin surface and PTV boundary) showed greater difference betweenplanned and measured doses; surface doses at the right-hand reference point were: TLD 123.2�10.2, film 124.0�2.0, andplanned 151.5�0.2 cGy.

Conclusions: Both film and TLD measurements of surface dose were consistently lower than planned for all cases studied. Thetomotherapy planning system was observed to overestimate surface dose by up to 28 percent. For the high skin dose test case,the surface dose was measured to be 7% lower than planned–a difference of 15 cGy per fraction. In cases where high surfacedose is required, routine planning may be insufficient to ensure the skin is receiving the prescribed dose.

S714 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006

Author Disclosure: M.J. Mitchell, None; C.R. Ramsey, TomoTherapy, Inc., D. Speakers Bureau/Honoraria; TomoTherapy, Inc.,F. Consultant/Advisory Board.

2892 Interim Report of A Randomized Trial Comparing Two Forms of Immobilization of the Head forFractionated Stereotactic Radiotherapy

G. Bednarz1, M. Machtay1, M. Werner-Wasik1, B. Downes2, J. Bogner3, W. Curran1, A. Dicker1, R. Valicenti1, J. Evans2,D. Andrews2

1Department of Radiation Oncology, Kimmel Cancer Center of the Jefferson Medical College, Philadelphia, PA,2Department of Neurosurgery, Thomas Jefferson University, Philadelphia, PA, 3Department of Radiotherapy andRadiobiology, Medical University of Vienna, Vienna, Austria

Purpose/Objective(s): Fractionated stereotactic radiotherapy (SRT) requires extremely precise and reproducible immobiliza-tion of the patient’s head. This study compared the efficacy of two commonly used forms of immobilization used for SRT.

Materials/Methods: Two routinely used methods of immobilization, which differ in their approach to reproduce head positionfrom day-to-day are the Gill-Thomas-Cosman (GTC) frame and the BrainLab thermoplastic mask. The GTC frame fixates onthe patient upper dentition and thus is in direct mechanical contact with the cranium. The BrainLab mask is a two-part maskingsystem custom fitted to the front and back of patient’s head. After patients signed an IRB-approved informed consent form,eligible patients were randomized to either GTC frame or mask for their course of SRT. Patients were treated as per standardprocedure; however, prior to each treatment a set of orthogonal digital kV images (ExacTrac, BrainLabAB, Germany) wastaken. These images were fused with reference digitally reconstructed radiographs obtained from treatment planning CT to yieldlateral, longitudinal and vertical deviations of isocenter and head rotations about respective axes. The primary endpoint of thestudy was to compare the two systems with respect to mean and standard deviations (SDs) using the distance to isocentermeasure.

Results: A total of 68 pts have been enrolled; positioning data for 37 patients (19 frame patients and 18 mask patients) arecurrently analyzable. The means, the distance measures and their SDs (in mm) for both immobilization systems are shown inTable 1.

Table 1. The mean lateral (lat), longitudinal (long) and vertical (vert) shifts and distances [mm], and rotations [deg] with theirrespective SDs for frame and mask. Positive mean shifts indicate shifts to the patient left, superior and anterior.

A mixed-effects linear regression and two-tiled t-test were used to compare the distance measure for both the systems. Therewas a statistically significant (p�0.004) difference between mean distances for these systems, suggesting that the GTC framewas more accurate.

Conclusions: Both immobilization techniques were highly effective, but the GTC frame was a more accurate immobilizationsystem as determined by analyzing distance measurements obtained using ExacTrac system, particularly in the longitudinaldimension. To optimize the accuracy of SRT, daily kV image guidance is recommended with either immobilization system.

Author Disclosure: G. Bednarz, None; M. Machtay, None; M. Werner-Wasik, None; B. Downes, None; J. Bogner, None; W.Curran, None; A. Dicker, None; R. Valicenti, None; J. Evans, None; D. Andrews, None.

Lat Long Vert DistanceLat

AngleLongAngle

VertAngle

GTC Frame Mean 0.34 0.55 0.45 1.93 0.26 0.27 0.01SD 1.08 1.28 1.12 0.98 0.74 0.99 0.69

BrainLab Mask Mean 0.02 0.41 0.00 3.19 0.24 0.07 0.18SD 1.16 3.32 1.37 2.06 2.37 1.26 1.57

S715Proceedings of the 48th Annual ASTRO Meeting