PubMed Health⌕ Search

Biomedical subjects

A Olch

Publications and source records attributed to A Olch.

9 recordsLinked to original sources
PubMed1998

Range spectra in electron penetration problems.

The theory of electron penetration as predicted by the Fokker-Planck equation is first reviewed within a restricted context that considers the multiple scattering and transport of charged particles. We then broaden the context and show that range straggling effects also fit successfully into this framework, which completes an electron model initiated by Yang. We introduce those effects with a superposition of Fokker-Planck solutions, i.e., by using an incident beam that contains a spectrum of initial energies, or equivalently, a set of csda ranges. Straggling effects appear to be a beam property in this approach but are returned to the material when we use it. All the information needed to construct the spectrum is obtained from a measurement of the electron rest charge distribution in polystyrene. To illustrate the correctness of this procedure, we consider the case of a 20 MeV electron beam incident on water. We predict the absorbed dose distribution as a function of depth and also measure it with an ionization chamber in a water tank. We find nearly perfect agreement between calculation and experiment in this case where all the results derive and apply to a clinically operational machine.

Electrons↗
PubMed1997

The need for individualized dosimetry for tangential breast treatment.

We have examined the isodose distributions of 119 intact breast patients treated on a 6 MV linac to determine if a library of treatment plans could be used instead of individualized computer plans for patient treatments without compromising the quality of those treatments. The parameters studied were: field width, baseline separation, central axis separation, wedge angle, and isodose coverage. At least two wedges were used in the computer plans for each patient and the best plan was then chosen. In order to construct a library of plans, the choice of wedge, treatment isodose, and dose uniformity should be predictable. Our results show that for 90 out of 119 plans (76%), the 30 degrees wedge was best. In the other 29 cases, either the 15 degrees or the 45 degrees wedge yielded better plans. On average, the improvement in dose homogeneity due to choice of wedge was about 2% (range 0-7%) for these cases. Although grouping like-patient parameters generally restricted the isodose variation to +/- 2.5%, there were five patients for which up to a 7% underdosage would not have been predicted. For the set of plans using a 30 degrees wedge, a significant correlation was found for the ratio of the baseline to central axis separation vs. treatment isodose. The average isodose which covered the target area was 97% (range 90-100%) and 102 out of 107 patient plans using the 30 degrees wedge fell between 94 and 100%. We conclude from these results that the variation in dose distribution found with seemingly similar sized breasts is due to the variation in breast shape and symmetry. The use of a library plan with a single wedge and a standardized isodose line for tangential field treatment of intact breast could cause up to a 7% dose difference compared to the actual dosimetry for that patient.

Breast Neoplasms↗
PubMed1993

Application of the ICRU Report 38 reference volume concept to the radiotherapeutic management of recurrent endometrial and cervical carcinoma.

Radiation therapy was given to 25 patients presenting with pelvic recurrence of endometrial (14) and cervical (11) cancer. Of these patients, all but one had undergone hysterectomy following their original diagnosis. Two endometrial patients received preoperative intracavitary irradiation. The recurrence-free interval ranged from 5 to 71 months (mean = 21 months). External beam radiation therapy for pelvic recurrence ranged from 3000 to 5000 cGy. Additional central radiation was given to 18 patients with either external beam, intracavitary, interstitial, or transvaginal technique. Dose and dose rates from brachytherapy are documented with maximum values, along with the location of these dose points. Such specification is essential in obtaining a more accurate impression of the total dose delivered to the patient, especially when different techniques are employed to increase the dose to the center of the pelvis. Reference volume dimensions, similar to those specified by ICRU Report 38 for intracavitary treatments, are presented. Mean follow-up from completion to radiotherapy is 22 months; 16 patients are dead, 2 are alive with disease, and 7 are alive with no evidence of disease.

Adult↗
PubMed1990

The CT scanner as a therapy machine.

Many tumors in the brain and in other tissues can be delineated precisely in images obtained with a CT scanner. After the scan is obtained the patient is taken to another room for radiation therapy and is positioned in the beam with the aid of external markers, simulators or stereotactic devices. This procedure is time consuming and subject to error when precise localization of the beam is desired. The CT scanner itself, with the addition of a collimator, is capable of delivering radiation therapy with great precision without the need for external markers. The patient can be scanned and treated on the same table, the isocenter of the beam can be placed precisely in the center of the lesion, the beam can be restricted to just those planes in which the lesion appears several arcs can be obtained by simply tilting the gantry, and the position of the patient in the beam can be monitored continuously during therapy. We describe here the properties of the CTX, the CT scanner modified for therapy.

Animals↗
PubMed1989

Radiotherapy planning for simulation of prostate cancer: computerized tomographic scanning vs. conventional radiographic localization.

A computerized tomographic localization protocol for prostate cancer treatment planning is described. In 23 patients, this new method is compared to localization using conventional orthogonal radiographic simulation with contrast media in the rectum, bladder, and urethra. Advantages of the CT localization protocol include enhanced ability to delineate the tumor extension, particularly for superior, lateral, anterior, and posterior spread. Accurate CT localization of the inferior border of the target volume has also been demonstrated to be feasible, thereby avoiding the need for invasive urethral, bladder, and rectal manipulations.

Aged↗
PubMedDate not supplied

Conversion from Cs-137 to Ir-192 for high dose rate remote afterloading: practical considerations.

High dose rate (HDR) remote afterloading is increasingly being used to replace many conventional low dose rate (LDR) brachytherapy procedures. Implementation of the microSelectron-HDR with Ir-192 at our facility necessitated this study to obtain equivalent dosimetric distributions with those of our LDR Cs-137 techniques using our current treatment planning system. Three anatomical sites are presented: nasopharynx, esophagus, and uterine cervix. Attention must be given to the anisotropy of Cs-137 tubes when converting to Ir-192; for linear geometries, total equivalent activity may be preserved but the shapes of the resulting isodose curves for Ir-192 are longer than those of Cs-137. In the case of Fletcher-Suit intracavitary treatments of the uterine cervix, the longer contours for Ir-192 in the vaginal ovoids results in higher isodose levels reaching the bladder and rectum. Maintaining the traditional dose levels to these organs is accomplished by modifying the loading of the ovoids to approximately 85% of the corresponding Cs-137 activity. Computerized dosimetry is presented, along with a chart we have devised to easily convert a standard LDR treatment to HDR dwell times. Our results are especially suitable to those users who will continue to make use of their present computer treatment planning system.

Brachytherapy↗