PubMed Health⌕ Search

Biomedical subjects

H Valen

Publications and source records attributed to H Valen.

6 recordsLinked to original sources
PubMed2001

Dosimetric verification of a dedicated 3D treatment planning system for episcleral plaque therapy.

PURPOSE: Episcleral plaque therapy (EPT) is applied in the management of some malignant ocular tumors. A customized configuration of typically 4 to 20 radioactive seeds is fixed in a gold plaque, and the plaque is sutured to the scleral surface corresponding to the basis of the intraocular tumor, allowing for a localized radiation dose delivery to the tumor. Minimum target doses as high as 100 Gy are directed at malignant tumor sites close to critical normal tissues (e.g., optic disc and macula). Precise dosimetry is therefore fundamental for judging both the risk for normal tissue toxicity and tumor dose prescription. This paper describes the dosimetric verification of a commercially available dedicated treatment planning system (TPS) for EPT when realistic multiple-seed configurations are applied. MATERIALS AND METHODS: The TPS Bebig Plaque Simulator is used to plan EPT at our institution. Relative dose distributions in a water phantom, including central axis depth dose and off-axis dose profiles for three different plaques, the University of Southern California (USC) #9 and the Collaborative Ocular Melanoma Study (COMS) 12-mm and 20-mm plaques, were measured with a diode detector. Each plaque was arranged with realistic multiple 125I seed configurations. The measured dose distributions were compared to the corresponding dose profiles calculated with the TPS. All measurements were corrected for the angular sensitivity variation of the diode. RESULTS: Single-seed dose distributions measured with our dosimetry setup agreed with previously published data within 3%. For the three multiple-seed plaque configurations, the measured and calculated dose distributions were in good agreement. For the central axis depth doses, the agreement was within 4%, whereas deviations up to 11% were observed in single points far off-axis. CONCLUSIONS: The Bebig Plaque Simulator is a reliable TPS for calculating relative dose distributions around realistic multiple 125I seed configurations in EPT.

Brachytherapy↗
PubMed2000

Adapting the waterproof BMS-96 diode array for isodose determination of dynamic beams.

We report the application of the Schuster BMS-96 waterproof linear diode array for isodose determination of dynamic beams. The array recorded beam profiles correctly, while depth dose distributions of dynamic beams with large variations in dose rate were registered erroneously. The deviations could be eliminated by appropriate software modifications. Until the software is revised, true isodoses can be obtained by rescaling each individual profile to the depth dose curve as measured with a single ionization chamber. After the corrections presented in the paper, isodoses interpolated from these corrected data sets agreed with ionization chamber measurements within 1-2%.

Radiation Dosage↗
PubMed2000

Partially wedged radiation beams.

To increase dose homogeneity within certain radiotherapy targets, we defined a partially wedged radiation beam as a beam with wedge modification in one part of the field only. Partially wedged beams may be beneficial in cases with curved surfaces inside parts of the beam only, where they may compensate for missing tissue and/or for variations in depth to the target region. Possible sites suitable for partially wedged beams include urinary bladder and tangential breast irradiation. Customized partially wedged beams were delivered applying dynamic collimation techniques. Two different linear detector arrays, a semiconductor diode array and an ionization chamber array, were used independently in the same standard water tank to verify that the partially wedged beams were delivered according to the definition. Dose calculations of partial wedge fields were implemented in our treatment planning system and compared with the measured dose distributions. We re-planned a representative treatment plan for both advanced urinary bladder cancer and tangential breast irradiation using partially wedged beams. For both patients the target dose homogeneity was improved, and the doses to surrounding critical normal tissues were reduced.

Algorithms↗
PubMedDate not supplied

Effects of high and low single dose irradiation on glioma spheroid invasion into normal rat brain tissue in vitro.

The effects of radiation on direction on directional migration in monolayer cultures and brain tissue invasion by two glioblastoma cell lines (D-54 MG, D-247 MG) were investigated. The Leksell Gamma Unit was the radiation source and invasion was registered in an in vitro invasion assay developed in our laboratory. As tumor spheroids and brain tissue aggregates were treated simultaneously in cocultures; the effects of radiation on the interaction between the two tissues could be investigated. Tumor spheroids from both cell lines retained their ability to invade and destroy normal brain tissue, even after irradiation with 47.6 Gy. However, while the D-54 MG tumor spheroids showed a dose-dependent reduction of invasion, tumor spheroids from the D-247 MG cell line did not. In addition, radiation produced a dose dependent inhibition of directional migration of cells from D-54 MG spheroids. A similar significant inhibition of directional migration was found in D-247 MG, but it was not dose-dependent. Transmission electron microscopy revealed a loosening of the neuropil in the brain tissue of irradiated cocultures. However, this structural change did not seem to affect the invasiveness of the tumor. In this preliminary study, irradiation could not prevent invasion of two different glioblastoma cell lines into fetal rat brain tissue. Further studies using the same technique may help to understand the influence of ionizing radiation upon the invasion process in gliomas.

Animals↗