667 Poster Contralateral breast doses in tangential breast radiotherapy

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WednesdayFl'hursday, 18-19 September 2002 $203

rection was required. This resulted in a reproducibility better than 0.5% over a period of three months, Conclusions: Accurate EPID dosimetry during patient treatment using liquid-filled EPIDs requires a quality assurance protocol which should include a temperature correction. The first resu.lts of the reproducibility for the a-Si imager are very promising. 665 Poster F u l l y a u t o m a t e d in r i v e d o s i m e t r y : f r o m m e a s u r e m e n t t o statistics

B. De Ost, E. Messens, P. Meijnders, D. Van den Weyngaert

account. Otherwise the energy of the electrons striking the target will be underestimated. Also for OAF measurements in IMRT the variation of the chamber response with off-axis distance should be considered. 667

Poster

Contralateral breast doses in tangential breast radiotherapy S. Maqqi 1, G. lacoviello2, M. Giacometti 2, F. Grillo Ruggieri3, L. Fabbietti3 1Azienda Ospedaliera "Umberto I" Medical Physics, Ancona, Italy 2Azienda Ospedaliera , Medical Physics, Ancona, Italy 4Azienda Ospedafiera , Radiotherapy, Ancona, Italy

~,.Z. Middelheim, Radiotherapy, Antwerp, Belgium Introduction: An automatic in vivo dosimetry (IVD) system was developed to check the actual entrance dose in routine for all patients, Method: Beam data from our pianning system (ADAC Pinnacle 6.0m) are transferred with DICOM to the Elekta RTD system. After every treatment, snapshots are made of the treatment parameters and transferred to ONCIS (ONCology Information System 6.6, the in house developed information system). These snapshots contain most of the treatment parameters but lack e.g. SSD. For an tVD measurement, the IVD detector (EDP20 for 6MV and 18MV, EDE5 for Co60) is placed on the central axis of the field. In the ONCIS system the patient is marked for tVD and the treatment can be started. ONCIS controls the electrometer and displays the corrected reading (corrected for energy, fieldsize, SSD ; these factor are different for wedged fields). The expected value is calculated from the snapshot information (MU, fieldsize, wedge, tray,...). After the treatment is completed the magnirude of the deviation is visualised immediately by ONCIS. Results: Until March 2002, more than 3000 automatic measurements were performed. For the measurements during the first treatment session: 86% of the deviations were within 5%, more than 96% within the 10% deviation limit. The large deviations are mainly caused by variation in SSD, by the fact that no missing tissue correction is used when calculating the expected value for tangential beams, that the shadow tray (not checked by our verification system) is forgotten in the Cobalt beam. More details on the results will be presented, Conclusion: The automated system reduces the workload and makes IVD easy to handle. Since the results are stored in ONCIS they can be used for administration and statistics, 666

Poster

Variation of off-axis response of ionisation chambers in photon beam dosimetry O.S. Dohm. M. Fippel, G. Christ, F. Haryanto, F. Nuesslin Universit#tsklinik fuer Radioonkologie, Abt. Medizinische Physik, Tuebingen, Germany Introduction: Measurement of beam profiles and off-axis-factors (OAF) is essential to provide an accurate data base for most treatment planning systerns, to validate accelerator head models for dose calculation, and in particular to verify fluence modulated beams like in IMRT. Normally profiles and OAF are measured using small volume ionisation chambers. It has been observed that ionisation chambers are oversensitive to low-energy photons. Since photon energy spectra of linacs change with off-axis dislance the ionisation chamber response is expected to vary. The clinical relevance of this effect however is not well known. The purpose of this investigation is. to quantify the variation of off-axis response of ionisation chambers. Methods: Profiles and OAF at 6 MV photon energy from an ELEKTA Sli linac were measured using a 0.125 ccm ionisation chamber (type 31002, PTW Freiburg). Measured data was compared to Monte Carlo simulations with the BEAMnrc code. The complete radiation head was modelled and a phase-space file of a 40 cm x 40 cm field at 100 cm target distance was generated. This file was then used to score the OAF at critically distances to the central axis. For scoring the OAF the BEAM modules dosxyznrc and dosrznrc were used which allowed to perform dose calculations in narow grids comparable to the geometry of small ionisation chambers. These geometries could be filled with different materials, so we could score the energy deposited in water or the energy deposited in an ionisation chamber, which was surrounded by either water or a brass buildup-cap and air. Results: A 6 MV X-ray beam at 40 cm x 40 cm field size showed a variation of about 25 % ir) the mean energy between the central axis and a 15 cm offaxis position, which results in a stopping-power-ratio water to air variation. The final chamber response is a combination of the variations in stoppingpower-ratio and perturbation effects. A net response variation of the chambet over that distance was estimated to be +1.1%. Conclusions: For verifications of Monte Carlo treatment head models the behavior of ionisation chambers in measured profiles should be taken into

A possible link between breast cancer radiotherapy and a subsequent development of a controlateral breast cancer is a controversial matter in the literature, but in order to follow the ALARA principle and to minimize the dose to healthy tissues, the authors have investigated about the contribulion of scattered radiation dose to the contralateral breast of women undergoing radiation therapy and about the method to reduce it. The total scatter dose is attribuited to internal and external scattered radiation. The contribute to controlateral breast doses of internal radiation was estimated for 40 patients treated at 50Gy with opposed tangential beams (6MV), using the 3D "Collapsed Cone Convolution Superposition" algorithm (TPS Pinnacle3-Adac). To evaluate the contribute of external component, a series of in vivo skin measures were carried out for 4 patients and an antropomorphic phantom ABDS, using TL dosimeters (LiF TLD 100) callbrated at 60Co. To find a suitable shield of external scattered radiation, the same measures were performed, in the phantom, using Pb shield with different thicknesses. To verify our results,the measures have been repeated in other 4 patients with a lead shield of 0.5 mm. In our study group, there were no differences in controlateral breast doses between patients treated at left or right breast, while a significative differences arose using a 15° or a 30 ° wedge, in fact,only for internal scatter radiation, the average glandular dose increased of 45% using a 30 ° wedge. In our evaluation on TPS, the minimum, the average and the maximum doses, referred to glandular breast volume, were: 3.0_+1.6cGy, 47 _+20cGy, 3.2__+1.1Gy. The following table shows the results with TLD on breast skin. BREAST PATIENT PHANTOM medial 4.17 7.00 3.74 4.44 3.82 superior 2.23 4.37 2.40 2.99 2.75 lateral 1.51 2.45 1.63 1.83 1.84 inferior 1.97 4.13 2.29 2.60 2.31 The measures performed on phantom with different thickness lead shields showed a valid reduction of the external scattered radiation with 0.5 mm lead thickness, and the measures on patients skin confirmed this result, intact the average dose reduction in medial point of controlateral breast was 48%-+5%, and in external point of 39%+6 %. We believe that it's possible, giving the prescribed dose to PTV, to minimize the dose to the contralateral breast already during treatment planning. It is also useful to shield the contralateral breast, at least in young women. 668 Poster C l i n i c a l a p p l i c a t i o n o f transmission dosimetry in head and

neck cancer radiation treatment S. Kim 1, S.N. Huh 1,2, H.K. Yun3, H.K. Wu 1,2, H.K. Lee4, S. Ha 1,2 1Seoul National University College of Medicine, Therapeutic Radiology, Seoul, Korea 2Medical Research Center, Seoul National University, Institute of Radiafion Medicine, Seoul, Korea 3College of Medicine, Dankuk University, Radiation Oncology, Cheonan, Korea 4College of Medicine, Catholic University, Biomedical Engineering, Seoul, Korea Purpose : To assess the usefulness and accuracy of transmission dose measurement as a method of i n vivo dosimetry for QA purposes in radiation treatment of head and neck cancer patierits. Methods : Parallel plate type ion chamber and electrometer Were developed and used to measure the transmission dose during the course of daily radiation treatment. Algorithms for estimation of transmission dose for open radiation field, irregularly shaped field and tissue deficit were developed. Correction for tissue inhomogeneity was performed. Transmission dose was measured in 35 patients treated to head and neck area and brain. Results : Phantom studies indicated that the total accuracy of the algorithms were within ± 2.0 %. The differences between estimated and measured transmission doses were bigger in clinical application. The differences were