30 Influence of the effective size of the functional subunit (FSU) on rat spinal cord paralysis

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Proffered Papers was also significant in all IMRT cases and had a higher power than CI. Conclusion•: Better effectiveness in dose conformity for IMRT plans were confirmed in both CI formulas. Splitting the CI to IE and Ic enabled to perform more precise analysis of the PTV coverage, volume of healthy tissue received doses >95% and modeling the dose reduction in OAR.

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VpTV where VpTV>-95% - PTV volume received dose _>95% isodose level. M a t e r i a l s and m e t h o d • : 15 patients with prostate cancer were chosen for the analysis. PTV was defined as the prostate gland with 10 mm margins around. Bladder, whole rectum and femoral heads were outlined as organ at risk (OAR). 3, 4, 5, 6, 7 beam IMRT and 3DCRT plans were made for each patients. Identical beams geometry for IMRT and 3DCRT plans with the same number of beams were used. The same schemes for each plans divided by the number of beams and technique was executed. 150 plans divided into IMRT and 3DCRT groups and influence of number of beams on dose conformity were tested by current CI and components of modified CI (Ic, IE). Reduction of doses in OAR dependent on anatomical and plan parameters was examined by two multiple linear regression models. In the first following parameters were established as explanatory: CI, OAR volume, number of beams and in the second: CI was replaced by Ie. In each model dependant variables were: for bladder and rectum - maximum dose received in 30% of OAR volume, OAR volume received dose_>95%, mean dose; for femoral heads - mean and maximum dose. Results: Both formulas of CI show better dose conformity in PTV for IMRT than for 3DCRT (CI: 70.2% vs 57.4%; Ic: 99.5% vs 97.8%; IE: 44.5% vs 73.9%; p95% for IMRT (p=0.023). CI didn't show any specific information in both analysis. Graph shows evaluated conformity values for IMRT group. The volume was strongest parameter in bladder and rectum models describing the influence between received dose and anatomical or plan parameters. However, the IE 110 100

29 A p p l i c a t i o n of a Cauchy distribution model to q u a n t i f y d i f f e r e n t i a l dose v o l u m e

C. Van den Heuvel~ 7-. Depuydt, K. Haustermans University Hospital Leuven, Oncology and Experimental Radiation Oncology, Leuven, Belgium P u r p o s e / O b j e c t i v e : A model was developed using the Breit-Wigner (BW) expression of a Cauchy distribution to fit a differential dose volume histogram (dDVH). The purpose of this study is to assess the usefulness of this approach. The model allows to objectively quantify a dDVH and can be applied in automated analysis of large studies or eliminate dose point calculations in Monte Carlo studies. In this study of 20 patients the method is applied to the PTV and a critical organ. Material•/Methods: Twenty (20) patients treated for prostate carcinoma using IM-EBRT with a prescribed dose (average dose in the target volume) of 74Gy, were selected. The dDVH's for the P-IV and rectum were generated with a resolution of 0.1Gy and a superposition of BWcurves was fit to the data (BW-analysis). Dose to the organ was defined as the weighted mode of the BW-curves and heterogeneity was quantified as the weighted full width at half maximum (FWHM). For the PFV dose a correlation analysis was performed with the FWHM. Results: The absolute difference of the dose with the prescribed dose showed a mean of 4.8E- 03Gy and a standard deviation of 0.55Gy. A highly significant correlation (p=0.005) was found between absolute dose difference and FWHM using pearson's r. Applying a ranking method showed the correlation was not significant (p=0.53), which suggests a threshold effect. Dose to the rectum was more variable (1SD=6.47Gy) with a mean of 43.73Gy Conclusions: We have shown that the use of BW-analysis of dDVH's adequately predicts the dose and eliminates uncertainties in choosing dose levels in heterogeneous dose distributions. (a) P-I-V (b) rectum Figure 1: BW-analysis for the P-IV (Dose = 73.96Gy, FWHM = 1.05Gy and Volume = 130.15 cc) and for a rectum dDVH. (Dose = 53.62Gy, FWHM = 2.53Gy and V o lu m e ~ 54.86 cc).

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30 I n f l u e n c e of the effective size o f the functional subunit (FSU) on rat spinal cord paralysis

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M. Adamus-Gdrka, B. Lind, A. Brahme Karolinska Institute & Stockholm University, Medical Radiation Physics, Stockholm, Sweden To be able to estimate the clinical outcome of radiation therapy, normal tissue complication probability (NTCP)

Proffered Papers models are more and more commonly used. A good and reliable NTCP model should predict the sigmoid shape of the dose-response curve as well as possible and it should duly handle the volume effect. The work by P. van Luijk et al. suggests that no existing NTCP model is able to describe the volume effects present in the rat spinal cord during irradiation with small proton beams and that new models needs to be developed. Using the experimental data from P. van Luijk et al. we tried to explain the change in the fifty percent effective dose (EDso) for different field sizes. This study was initiated to evaluate whether the induction of white matter necrosis in rat spinal cord after irradiation with small proton beams could be explained independent of the NTCP model being used. We therefore introduced the concept of effective FSU dose, where we calculated the average doses in a functional subunit (FSU) as a convolution of the original dose distribution with a function describing the effective size of a single FSU. Such a procedure allows determining the fifty percent effective dose in an FSU of a certain size, within the irradiation field. Using the least square method a comparison of the effective doses for different sizes of functional subunits with the experimental data we observe the best fit for about 4 mm length. On the basis of this result it seems that this length could be understood as an apparent size of functional subunits in rat spinal cord, explaining what is otherwise interpreted as a volume effect.

P. van Luijk et al. 2005 Data on dose-volume effects in the rat spinal cord do not support existing NTCP models. IJROBP 61(3):892-900

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oxygenated compartment was reduced correspondingly. The optimum ratio of the mean dose to the hypoxic region and to the well-oxygenated region was determined as the dose ratio giving maximum TCP. The optimum dose ratio was used as input in the IMRT planning of a spontaneous canine osteosarcoma, for which the oxygenation status had been assessed from pretreatment contrast-enhanced MR images. The expected TCP for this specific case was then calculated from the resulting dose distribution. Results: The maximum TCP was found to decrease with increasing standard deviation in dose for each tumor compartment. The optimum dose ratio depended on both the standard deviation in dose and the degree of reoxygenation between treatment fractions, and ranged typically from 1.6 to 1.8. The corresponding maximum TCPs increased considerably compared to the TCP resulting from a uniform dose distribution. In the case of the canine osteosarcoma, the non-uniform IMRT dose plan showed good conformality to the subregions of the tumor and an increase in TCP was predicted. However, creating sufficiently steep dose gradients between the two regions, in order to reach maximum TCP, proved a challenge. The effect of MLC leaf size on the IMRT dose distribution and the implications of dividing the tumor into further subregions of varying degrees of hypoxia will be discussed. Conclusion: Non-uniform, biologically optimized dose distributions may result in an improved treatment outcome for hypoxic tumors compared to uniform dose distributions of equal mean dose, and IMRT can be used to create such dose distributions.

Brachytherapy 32 M R I versus TRUS d e l i n e a t i o n of the p r o s t a t e a f t e r 11 2 5 seed i m p l a n t a t i o n

M. Steqqerda I, J. Duppen 1, H. Van der PoeP, L. Moonen 1, L.

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Dep endence of EDs0 on the field width. The d i a m o n d s are t he ex perimental data from P. van Luijk e t al. and the solid lines effective FSU doses for different sizes of functional subunits. 31 Predicting t r e a t m e n t o u t c o m e of biologically o p t i m i z e d I M R T for hypoxic t u m o r s

A. Soevik 1, E. Malinen 2, D. OIsen 2 IThe Norwegian Radium Hospital, Medical Physics and Technology, Oslo, Norway 2The Norwegian Radium Hospital, Department of Radiation Biology, Oslo, Norway Purpose: To establish radiobiologically optimal dose distributions for hypoxic tumors, and to evaluate realistic dose distributions created by IMRT with respect to tumor control probability (TCP). M e t h o d s and materials: A two-compartment theoretical model for tumor hypoxia was developed, in which the tumor was divided into a well-oxygenated and a hypoxic compartment. Reoxygenation, acute and chronic hypoxia, and normally distributed variations in dose within each compartment were included in the model. The mean dose to the tumor was kept constant, while the dose to the hypoxic region was increased. Hence, the dose to the well-

Zijp I, C. Schneider1 1The Netherlands Cancer Institute - Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands 2The Netherlands Cancer Institute - Antoni van Leeuwenhoek Ziekenhuis, Urology, Amsterdam, The Netherlands I n t r o d u c t i o n : Often T2-weighed MRI has been mentioned as the optimal image modality for segmentation of the prostate. However, most frequently used for implantation planning and image guidance during implantation is TRUS imaging, although seldomly used for post-implant evaluations. The purpose of this study is to compare delineation of the prostate on MRI and TRUS after implantation of 1-125 seeds for prostate cancer. M e t h o d s and Materials: The prostate of thirteen patients was delineated by three experienced observers on both a T2weighted MRI scan and a TRUS scan, acquired one month after implantation of the seeds. The TRUS scan was realized by a 3D transducer with an internal scanning mechanism. To allow geometrical comparisons, both MRI and TRUS scan were matched with a CT scan and consequently both scans were registered to the same coordinate system. The contours of the prostate were drawn on 3 mm thick transversal MRI and TRUS slices with an increment of 3 mm. The delineations were inspected on sagittal and coronal slices and corrected if necessary. From the six delineated structures per patient a median surface was determined. This median surface was then divided into five regions: anterior, posterior, lateral, apex and base. To compare the different delineations, the mean perpendicular distance of the median surface to each individual surface was calculated for the defined regions. Results: For each patient and observer the distances of the MRI delineation to the median surface were averaged per region. The same procedure was carried out for the TRUS