$322 Friday/Saturday, 20-21 September 2002 Posters

and dynamic wedged symmetric field sizes of 10x10, 10x20 cm 2 and Y- stainless steel prosthesis. The close variation was between 15.2-13.9 % for asymmetric fields (Y1=10 cm and Y2=5 and 0 cm) were used. SSD of 100 Co60, 7.5-6.0 % for 6 MV, 2.8-1.2 % for 15 MV in titanium and 31.7-29.8 cm, measurement depths of 5 and 10 cm on-axis and 5 cm off-set points % for Co60, 15-15.3 % for 6 MV, 4.2-3.4 % for 15 MV in stainless steel were measured. The calculated absorbed doses were verified using an ion prosthesis. These dose decreases detected by TLD chips were comparable chamber system (NE Farmer type 0.6cc chamber), calibrated in terms of with the doses which had been calculated by treatment planning computer absorbed dose to water, except in two points which were in front and behind the head of the pros- Results: The results showed systematical shift of measured data in corn- thesis. parison with the calculations. The in-vivo measurement always gave us Conclusions: During the irradiation with femoral head prosthesis clue to higher dose then the calculations. In details, the shift was dependent on the dose decrease, radiotherapy should be performed by higher energy pho- degree of field asymmetry, depth of measurement and degree of wedge, tons other than Co60 preferably. Partly asymmetric field with point of measurement at 5 cm depth presented 10-20% overdose according to the wedge size, at 10 cm depth the differ- 1113 Poster ences were 10-23%. Half- beam blocked field showed 17-55% and 20-50% The matter of small c i rcu la r f ie lds in electron beam therapy differences accordingly. This result indicates that phantom and collimator A. Toy, N. Tunnel, A.O. Kizildag, H. Vidinlioglu, H.H. Orhun, M. Altun scatter dose contribution is underestimated by the TP system and should be Akdeniz University School of Medicine, Radiation Oncology, Antalya, Turk- corrected on basis of in-vivo dosimetry ey

1111 Poster Aim: To investigate the percent depth dose, profile and isodose curves for Dose calculation and error estimation in small photon f ie lds different Source-Skin Distance (SSD) and different electron energies in J.M. Jensen 1, E./hnen 2, K. Ei/f 1 order to detect the effect of small circular fields in clinical applications.

Material and methods: Percent depth dose and profile measurements were 1UniversityofKie/Medica/Schoo/, Dept. of Radiotherapy, Kie/, Germany done using marcus and 0,125cc ion chambers, for 6,8,10,12 and 15 MeV 2University of LObeck Medica/School, Dept. of Radiotherapy &Nuc/. Med., electron energies of Sli 25 Linear Accelerator in the PTW-MP3 water phan- LObeck, Germany tom. Measurements were performed on reference field (10xlOcm), the

shaped circular field in this applicator (2-5cm diameter) for SSD=100cm and For dose calculation in general basic beam parameters have to be mea- 2-5cm diameter (D) cones for SSD=100 and 95cm. Measurements were sured, i. e. TMR-values, output-factors, depth-dose-profiles, etc. Especially applied on different SSD for cones because in practice, depend on treat- for small fields the output-factor is of importance because the accuracy of ment region SSD=95cm always has not been carried out. Our Dept. has got this data is transferred directly to the dose calculation result: D = C x TMR 2-5cm D cones, then the shaped fields with the same diameter were cho- x OUT x MOD. sen for dosimetric analysis. In large-field-techniques (4 cm < F < 40 cm) the output-factor OUT can be Results:For all energies and SSD's, the depth of maximum dose (dmax) is determined by only 3 single measurements (!) for all square and rectangu- more superficial, in both shaped fields and cones according to dmax of ref- lar field sizes with a precision of ± 0.5 %. For stereotactic and in some cases erence field. The depth of maximum dose increased with rising of circular also for IMRT (s&s; dyn.) field sizes smaller than above mentioned are nec- size. But, this value couldn't reach the maximum dose of reference field essary, and subsequently highly accurate output-factors are needed (super- depth. As an example; in 6 MeV energy the dmax value for shaped circu- position of errors according to Gauss). lar fields was found 0mm for 2cm and 6ram for 5cm, while in 15MeV ener- By a simple measuring set up, a standard ionisation chamber, and a non- gy the dmax was found 2mm and 14mm, respectively. With increasing ener- standard build-up-cap, full lateral electron equilibrium (LEE) is already gy, dmax value was found approximately the same value in cones and achievable for field sizes as small as 0.3 x 0.3 cm 2. But it can be shown that shaped fields. For all energies and SSD=100cm, the 90% isodose curve for field sizes smaller than ca. 4 x diameter of focal spot the focus is partly width at dmax was obtained closely on both shaped fields and cones. In masked by applicators or movable collimators or jaws. As consequence 6MeV energy this width at dmax for 2cm diameters was found 9,7mm in dOUT/dF > 0.05/mm, in detail the dose calculation error depending on gee- shaped fields and cones. While in 15MeV was found 12,9mm and 12mm in metrical uncertainties, such as collimator position, migrating and diameter shaped fields and cones, respectively. This width increased with increasing of focal spot, isocenter movement, etc., increases rapidly, exceptionally up energy for all SSD values. The results for 2cm D cone in SSD=95cm, this to -,- 20 %. In combination with other geometrical variances (CT-scanning; width was 11,6ram for 6MeV and 14mm for 15MeV. This width value was dose planning; patient positioning) the mean dose-volume-error for a 1.0 x found 32,4mm and 42mm respectively for 5cm D cone. With increasing 1.0 cm 2 field comes up to about 28 %, when total uncertainties amount to diameter, this width increased in all energies. For all circular field sizes and 1.1 mm. energies this width was chanced depending to SSD. With decreasing SSD Consequently special attention has to be directed to output-factors as well this width approximately increased about 10%. Conclusion: irradiation field as geometrical instabilities, when field sizes at normal treatment distances have to be prescribed according to the change of this width at dmax depend are to small to achieve complete LEE in ionization measurement, on SSD and energy. Criteria have to be considered in clinical applications

for low energy and small diameter fields such as 90% isodose curve width 1112 Poster at dmax and out-put values. Effect of femoral head prosthesis on megavoltage beam radiotherapy 1114 Poster B. Demir 1, F.(9. Dincbas 2, S. Karacam 2, A. Koca 2, B. Gunhan 2, S. Koca 2 The application of asymetrical beams. When, why and how ?

1Radiation Onc., Physics, istanbul, Turkey L. Frenc/1,2, P. Cemehuby 1, Z. Poulova 1, K. Dedeckova 1, J. Kvech 1,2, 2Radiation Onc., Radiation Onc., istanbul, Turkey F. Trebicky 1, M. Vosmik 1

1institute of Radiation Oncology, 1st Faculty of Medicine, Charles Universi- Purpose: Femoral head prosthesis are frequently replaced instead of femur ty & University Hospital Na Bulovce, Prague, CZ after pathological fracture of the femur and during soft tissue or bone tumor 2institute for Postgraduate Medical Education, Prague, CZ operations. However after operation this prosthesis cause close variations around it. In this study, dose variations were assessed due to these femoral There are many practical situations in radical radiotherapy where the use of head prosthesis. the combination of asymmetrical and symmetrical beams is necessary. The Materials and methods: Fresh femoral bone of a cow is used in this study, reasons are: Femoral head prosthesis made of titanium and stainless steel are fixed into - the better coincidence of CTV and dose distribution" the femur of cow. Both of them are putted into a box filled with rice in order - impossibility of more isocenters to create a leg phantom. This phantom was irradiated isocentrically with two - shielding of organs at risk parallel opposed fields with Co60 , 6MV and 15 MV photon beams of lineer - limitation of large fields with wedge filters. accelarator with a 10x20cm field. The dose was 2 Gy in the middle axis. When, why and how to realize these techniques is presented for linacs Totally 5 TLD chips were used. Three of them were put around the head Orion 5, Orion 6, Saturne, 43 and simulator Verasim II, TPS Target, verifi- and 2 chips were put 2 cm away from the head of prosthesis were put into cation system Sincer. the phantom. It was irradiated after taking out the metal prosthesis in the The techniques with 5 or 6 fields (3 or 4 symmetrical and 2 asymmetrical same conditions. The dose variations before and after prosthesis were corn- fields) are used for prostate cancer and pelvic. The techniques of 2 sym- pared. Furthermore these results were compared with the treatment plan- metrical and 1 asymmetrical fields are applicated for mediastinum and hilus ning computer calculation results also. where protection of spinal cord is asked. The techniques of 2 symmetrical Results: There were dose decrease both in front and behind of titanium and and 2 asymmetrical fields are used for large volume in sarcomas where