[Prostatic carcinoma: problems in the interpretation of rectal dose-volume histograms]. / Prostatakarzinom: Zur Problematik bei der Interpretation rektaler Dosis-Volumen-Histogramme.

BACKGROUND: Dose-volume histograms (DVHs) are used for the prediction and calculation of late radiation side effects. In literature the predictive value of rectal DVHs is controversially discussed. Differences in contouring might contribute to the contradicting results. In particular the cranial and caudal border of the contoured organ are not uniformly defined. PATIENTS AND METHODS: The DVHs of 12 patients who were treated with conformal radiotherapy for prostate cancer were investigated. Six of the patients suffered from mild rectal bleeding as a late side effect of radiotherapy. Six patients without rectal bleeding (minimal follow-up 30 months) matched for age, concomitant disease and treatment concept served as controls. Four different DVHs with 4 different definitions of the cranial and caudal rectal border were generated for each patient. For each of the 48 DVHs the percent volume fractions (V50, V80, V95) and absolute volume fractions (aV50, aV80, aV95) were calculated that received more than 50%, 80% and 95% of the reference dose. RESULTS: For every patient there were considerable variations in the volume fractions depending on the definition of the rectum borders (Table 1). The mean and median values of the percent and absolute volume fractions of the bleeding patients were higher than those of the non-bleeders no matter how the rectum borders were defined. None of the volume fractions could totally separate bleeding from non-bleeding patients. CONCLUSION: There is a high variability of absolute and percent volume fractions of rectal DVHs depending on how the rectal borders were defined. For the comparison and for the interpretation of rectal DVHs a uniform definition would be helpful.

Volume-based dose optimization in brachytherapy.

PURPOSE: We address the question of how to optimize the dwell time distribution in brachytherapy with a stepping source if a minimal tumor dose is prescribed within the planning target volume (PTV). METHODS AND MATERIALS: For a given PTV, reference points inside and at the surface of the PTV are generated and dose constraints are prescribed. The dose at these reference points can be calculated if the positions of the sources are known. We determine a set of dwell times such that the dose constraints are fulfilled, and at the same time, the total irradiation time is minimized. The simplex algorithm allows us to find a solution (if any exists) for this problem. RESULTS: The performance of this method has been tested for a geometrically simple PTV. This method gives better results than conventionally used algorithms for dwell time optimization. CONCLUSION: The method described in this paper allows a volume-oriented optimization for brachytherapy dose distribution. The algorithm guarantees finding a dwell time distribution which fulfills the prescribed dose constraints, if any solution exists.

[3-Dimensional irradiation planning in brain tumors. The advantages of the method and the clinical results]. / Dreidimensionale Bestrahlungsplanung bei Hirntumoren. Vorteile der Methode und klinische Ergebnisse.

PURPOSE: Radiotherapy became an important component in the treatment of brain gliomas. The aim of this study is to analyse several advantages of the three-dimensional conformal radiation therapy in comparison with a two-dimensional conventional technique and to present the clinical results of 43 patients with brain gliomas treated according to a three-dimensional planning. PATIENTS AND METHOD: Between January 1994 and December 1995, 43 patients with malignant brain gliomas (WHO III and IV) were treated in our department according to a three-dimensional treatment planning. The patients received a total irradiation dose of 60 Gy, 2 Gy/day, 5 days/week. The rate of survival was analysed in relation with the known prognostical factors: histology, Karnofsky index, age, resection status. In 10 patients a three-dimensional treatment planning was compared with a conventional two-dimensional planning: the volume of the normal brain tissue irradiated to high dose levels (95% isodose) and the normal tissue complication probability (NTCP) for the brain by Kutcher and Lyman were comparatively analysed. RESULTS: The survival rate for the whole group was 14 months. The histology of the tumor, age, Karnofsky index and resection status were important prognostical factors. The three-dimensional planning allows a 15 to 20% reduction in the volume of normal brain tissue irradiated to high dose levels (95% isodose). The NTCP is significantly lower using the three-dimensional technique (range 0.03% to 13%), in comparison with the two-dimensional conventional technique (range 0.1% to 26%). The value of NTCP increases with tumor volume. CONCLUSIONS: Concerning the tumor control and survival rate, the three-dimensional treatment planning shows no advantages compared to the standard conventional methods. The main advantage of the three-dimensional treatment planning is the possibility to spare normal brain tissue. The possibility to integrate mathematical models in the evaluation of the therapy could give this technique new dimensions.

[The flab method of intraoperative radiotherapy]. / Die Flabmethode zur intraoperativen Bestrahlung.

PURPOSE: Improvement of the relapse rate by locally increasing the tumor dose. METHODS: To increase the dose in the tumor bed a method for intraoperative radiation therapy has been developed. The radiation is applied using a high-dose-rate afterloading system. Flexible plastic flabs are used as applicators. Each flab contains tubes for the alterloading source. The size of the applicator is chosen to correspond to the size of the target volume. In the dosage system we use the dwell times at all source positions are equal. The dwell time has been precalculated to give the reference dose at the reference position which is located at the surface of the flab in the center of the target volume. RESULTS: The dose distributions around the flab have been calculated as a function of the thickness of the flab. If the source is not placed on a regular grid within the applicator due to a non ideal positioning of the tubes within the applicator, the dose distribution is not altered too much, if the positioning error is not larger than +/- 2 mm. The influence of a curvature of the flab has been evaluated and methods to decrease the dose at critical organs are discussed. CONCLUSION: The flab methods for IORT is safe and easy and has been demonstrated for more than 150 cases. This method allows the increase in tumor dose even in regions which are not easily treated by electron IORT due to the rigid electron applicators. Because of the rapid dose fall off, the flab method can only be used for flat target volumes like the tumor bed.

Optimization of activity distribution in brachytherapy.

We investigated the problem of how to minimize the dose rate outside a spherical target volume for a given minimal dose rate inside the sphere for 1/r2 photon emitters. An integral equation for an activity distribution is derived from the demand that the dose rate is constant inside the sphere. The solution of this equation is given and it is proven that the resulting activity distribution gives the smallest dose rate at every point outside the target volume. Some other activity distributions are discussed; they are assessed by the required integral activity, which is proportional to the dose rate at some distance from the target volume. Compared with a single point source at the center of the sphere, one can save 50% of the total activity when using the optimal activity distribution. The benefit reduces to about 30% for the other continuous activity distributions and to a few % for symmetrical arrangements of a few point sources.

Structural changes of human and monkey trabecular meshwork following in vitro cultivation.

The entire chamber angle tissue of ten monkey eyes and nine normal human eyes was cultivated in organ cultures for 1-10 days and then investigated by electron microscopy. We found that the uveal and corneoscleral trabecular cells often degenerate as early as 2-3 days after explantation, whereas the cells of the cribriform region proliferate and show an increasing number of cell organelles (mitochondria, endoplasmic reticulum and Golgi complexes). Regarding the behaviour in vitro, we distinguished three different cell populations in the trabecular meshwork with probably different functions: (1) the endothelial cells of Schlemm's canal, (2) the trabecular cells of the cribriform region and (3) the uveal or corneoscleral trabecular cells.