Posters S445

Treatment planning and dosimetry 1059 poster

Bladder as a crit ical volume. Comparison with dose- volume histogram study in prostate cancer radiotherapy M. Lacruz ~'2, F. Ferrer ~, P. Fore ~, D. GSmez ~, J. Quera ~'2, A. - R e ~ S a n z 1, C. Au~On', N. Rodriguez 1, M. Algara 1'2

~Hospital de I'Esperan~a-IMAS, Radiation Oncology dept. IOR, Barcelona, Spain 2Universitat Pompeu Fabra, UPF, Barcelona, Spain Introduction: Urologic toxicity in prostate cancer irradiation is mainly caused by damage on bladder wall tissue.

Purpose. To study the relevance on dose-volume histograms between bladder volume and wall bladder volume dose distributions.

<B: Thirty consecutive prostate cancer patients were included. Initial virtual simulation with 3-D planning (Helax- TMS 5.1B) was performed. PTV was defined as GTV with 0.7 cm of margin. Bladder volume (V1) was defined as external bladder contour and bladder volume 2 (V2) was defined as bladder wall contour. Dose-volume histograms for bladder were calculated and comparisons between area under histogram curve; mean, median and maximum doses in prostate volume and doses for 33%, 66% and 100% volumes were done. Paired sample t-student test were done to compare variable means.

Results:

;~ ......................................................... T .................................... ~ ~ i i ~~ .............. ~ ............ i {Bladder (Vl) ;t e

i I(v2) i " ................................................ ; ................................. 4 : .................. 4 !Volume (cm 3) !113,99 }23 27i3,48 0 002

'Maximum dose. (Gy) 68,16 ~68,11 i l ,42~ns

i - 6 ; ~ i i a ' ~ , ose iG~; i - i '~5~i 2 . . . . . . . 4 5 j 2 6 0,~;-i~nsO'~- i

V33% 50,87 52~ 70 i2125' "d~032! ~37 77 0 84~ns V66% i38 27

Conclusions: Knowing that V33% represents volume receiving the higher dose and that represents an increasing in damage tissue risk wall bladder volume (V2) is considered the appropriate critical volume for bladder in prostate cancer 3-D radiation planning.

1060 poster

Monitoring of diode sensitivity during thermal transient (in vivo dosimetry)

C. Caldera Aalborg Hospital, Medical Physics, Aalborg, Denmark Introduction: Semiconductor diodes precision and accuracy in the determination of the dose to the patient are limited by the sensitivity variation with temperature (SVWT).

The measurement is typically performed during the thermal transient between room and equilibrium temperature, which means that the sensitivity is time dependent.

This corresponds to a significant uncertainty and inaccuracy in the determination of the dose.

Aim: Accuracy and precision improvement of dose to the patient measurements.

Method: The problem is addressed here by a new rigorous method, based on real time monitoring of the diode sensitivity during treatment and on an exact calculation of the

temperature correction to be applied. Volt-ampere metric measurements are combined with a mathematical model of the thermal transient.

Results: Significant reduction of uncertainty and up to 4 % improvement in measurement accuracy by elimination of systematic errors in the temperature correction.

Conclusion: The feasibility of a clinical protocol for temperature independent in-vivo-dosimetry is demonstrated. For the first time the role of the thermal transient is analyzed with a rigorous mathematical model. An exact calculation of the correction factors is performed and a significant improvement in precision and accuracy is thereby achieved.

1061 poster

Development of a microdensitiometer system for reading radiochromic films K.Y. Lee 1, C.S. Kwok 2, K.L. Fung I

1 The Hong Kong Polytechnic University, Optometry and Radiography, Hong Kong, China (HongKong) 2City of Hope National Medical Center, The Department of Radioimmunotherapy, CA, USA Radiation dose deposited on a radiochromic film (RCF) is considered as a dose image. A precise image extraction system with commensurate capabilities is required to measure the transmittance of the dose image and correlate it with the radiation dose. Currently, densitiometers used for reading radiochromic films are not primarily designed for it. These densitiometers either use a light source with an emission spectrum that does not correspond to the absorption spectrum of the RCFs or offer a spatial resolution inferior to what the RCFs can provide. This poster describes the development of a microdensitiometer system which has been designed to achieve this goal according to the unique characteristics of the radiochromic films, namely (a) the linearity and sensitivity of the dose response of the radiochromic films being highly dependent on the wavelength of the analysing light; and (b) the inherently high spatial resolution of the RCFs. The microdensitiometer system consists of a monochromator which provides analysing light of variable wavelength, a film holder on a high-precision scanning stage, a CCD-dedicated microscope in conjunction with a thermoelectrically cooled CCD camera, corresponding computer interfaces and a microcomputer. The fine resolution achieved is based on the principle of microscope photometry together with the measure-and-step technique to cover the whole area of interest. The measurement of the transmittance of the RCF is done at the two absorption peaks to acquire the maximum sensitivity according to the dose magnitude and this is only made possible by the developed system where the wavelength of the analysing light source is selectable. The features unique to this microdensitiometer were also evaluated and the results show that the system can serve the intended purposes of providing a spatial resolution on the order of five micrometers. The developed microdensitiometer system will definitely increase the film applicability of the RCFs in the fields where fine resolution are always required such as in intensity modulated radiosurgery treatment, intravascular brachytherapy and hot- particle dosimetry.