Investigation of the relative surface dose from Lipowitz-metal tissue compensators for 24- and 6-MV photon beams

Our radiation therapy department has acquired a tissue compensator system for construction of patient-specific Lipowitz-metal tissue compensators. Since the arrival of this apparatus its use has been limited due to the observance of marked skin reactions located directly beneath the area of the compensator. Although there has been extensive literature on the dose distribution of these metal compensators, there is little data on the effects to the skin for energies greater than 10 MV. Determination of the relative surface dose from Lipowitz-metal tissue compensators is herein investigated for 24- and 6-MV x-ray beams. Effects of field size, source-to-skin distance and thickness of compensator are evaluated as well as the effect of the Lexan tray which supports the compensator during treatment.

[Development and evaluation of a multi-applicator system of endocavitary radiothermotherapy of gynecologic tumors]

An applicator system for the radiothermotherapy of gynecological tumors has been developed. The concept is based on combining high-dose rate afterloading therapy with local hyperthermia (27.12 MHz). The main applicator, the rf-gamma applicator consists of a guide tube for the gamma source, an electrode system and a cooling system. Isodoses and isotherms can be adapted to the individual anatomical-pathological situation presenting. For therapy planning the interaction between the applicators and the perfused tissue was investigated in a theoretical FEM model. In the first step, the SAR-function in the vicinity of the rf-applicator was determined by means of a 2D FEM calculation. In a second step, the 3D temperature fields was determined using linear shape functions. The results of these calculations showed that in every case the hot spots shifted from the applicator surface into the depths of the tissue. With the aid of an infrared camera and a split phantom the calculations were examined in a homogeneous non-perfused tissue model. Thermometry confirmed the accuracy of the results obtained. The radiothermotherapy system described here was tested in animal experiments, and is presently being used in a clinical pilot study.

Improved portal film image quality in radiation therapy with high energy photons

Various metal screen-film combinations have been investigated in order to determine the best radiographic image. The quality of these different combinations has been evaluated by measuring the scattered to primary film dose ratio S/P. The S/P ratio increases with increasing atomic number of the front screen for 4 MV x-rays but shows no significant difference for 8 MV x-rays. For rear screens the S/P ratio is slightly increased for higher atomic numbers. A metal with an atomic number around 26-29 should be an optimal metal screen regarding quality aspects. A cassette of stainless steel has, in clinical use for portal and/or verification films, given very good images.

Computer-CT planning of the electron boost in definitive breast irradiation

Treatment planning of the electron boost in breast irradiation at this institution is performed by using information from CT scanning and from surgical clips used to define the tumor bed. A refinement of this technique, taking into account the surgical axis of approach, using computer-CT planning is now implemented. The location of the scar and the clips are digitized using a computer to define the tumor bed dimensions. With 3-D computer software a line, simulating the surgical axis and the central ray of the electron beam, is drawn between the deepest clip and the surgical scar. This beam's eye view along the surgical axis is projected onto a screen as a starting point. Appropriate gantry angle, treatment table position, beam energy, and precise shaping of the electron field borders with a 2 cm margin around the clips and the scar are determined. Simulation films comparing clinically set-up fields and computer-CT planned fields were reviewed. In only 5 of 17 patients did the clinically set-up field have adequate inclusion of the tumor bed within the treatment volume. Computer-CT planned fields ensured adequate inclusion of the tumor bed in all, including the remaining 12. In 7 patients obvious increased sparing of normal breast tissue was seen with computer-CT planned fields. This technique enables accurate placement of a shaped electron field and further refinement of electron boost treatment planning. This is especially true in situations in which the tumor bed is located at a site distant from the lumpectomy scar rather than directly beneath it and in cases where the tumor bed lies deep within the breast.

Stereotactic multiple arc radiotherapy

A radiotherapy technique is presented for delivering a concentrated pattern of absorbed dose to intracranial lesions. Treatment takes place on a conventional, isocentrically mounted linear accelerator, rotated in several planes around a single target site. A new, relocatable stereotactic frame is used which enables fractionated radiotherapy to be administered if required. Calculations of the absorbed dose distribution in three orthogonal planes are performed using specially prepared software on a computer used for standard treatment planning. In this way, the need for excessive computing power is avoided.

Treatment volume shaping with selective beam blocking using the Leksell gamma unit

The Leksell gamma unit at the University of Pittsburgh uses 201 highly focused 60Co beams arranged in a hemispherical array. Selective beam blocking can be used to modify the treatment volume into ellipsoid shapes oriented in different directions to match better the shape of the target volume. Dose distributions for different blocking patterns were calculated using specially developed computerized 3-D treatment planning software. The changes in dose distribution with different blocking patterns predicted by computer were verified by film densitometry. Techniques for using selective beam blocking to match more closely the treatment volume to the intended target volume have the potential of reducing the likelihood of complications for radiosurgery with the Leksell gamma unit and need to be further developed.

Separation of dose-gradient effect from beam-hardening effect on wedge factors in photon fields

It is known experimentally that a wedge transmission factor depends upon the field size and depth of measurement in particular. Dependence of the transmission upon depth has been attributed to a hardening of the incident beam through the filter, which preferentially absorbs the low-energy photon of the bremsstrahlung component of that beam. We have attempted to separate this hardening effect from that of increased phantom scatter due to dose gradient induced by the wedge filter. Using an experimental wedge machined from cerrobend, the filter transmission at depth is measured and redefined relative to an "equally hardened" beam, obtained by filtering through a flat slab of equal thickness at the center of the wedge. Results of the Co-60, 4-, and 8-MV wedged beams indicate that nearly half of the increase in the transmission at depth is due to the effect of dose-gradient scatter in polystyrene phantom. Based on a simple relationship between primary and scattering radiation, an algebraic presentation is indeed in support of the dose gradient resulting in apparent increase in the wedge factors, at depth.

Effects of electron cutouts on absorbed dose in and outside Varian-20 electron fields

Electron beam outputs in the presence of beam shaping cutouts were measured for 6, 9, 12, 16, and 20 MeV electrons from a Varian-20 linear accelerator. This machine was provided with a new model of electron cones, for which photon jaw sizes were set differently from those of old cones. The effects of different photon jaw sizes on the electron outputs were studied. Also measured was the absorbed dose under a cutout, which was presumably contributed by scattered electrons and bremsstrahlung photons.

Shielding considerations for an operating room based intraoperative electron radiotherapy unit

The leakage radiation characteristics of a dedicated intraoperative radiotherapy linear accelerator have been measured on a machine designed to minimize the shielding required to allow it to be placed in an operating room suite. The scattering foil design was optimized to produce a flat beam for the field sizes employed while generating minimal bremsstrahlung contamination over the available energy range. More lead shielding was used in the treatment head than is used in conventional accelerators. A small amount of borated polyethylene shielding was also employed since neutron production was present at measurable levels. The room shielding installed in the operating room was demonstrated to be adequate to treat at least 20 patients each month to an average dose of 20 Gy. The worst case exposure was found to be 73% maximum permissible exposure. Administrative control was required for adjoining areas when calibrations and maintenance were performed.

Tissue compensators with use of vinyl lead sheets for head and neck portals on 4-MV x rays

Tissue compensators made with a high atomic number material are compact and retain the skin-sparing effect of megavoltage beams. In this note, we have described the use of a commercially available vinyl lead as a material for tissue compensators for use with 4-MV beams on a Clinac-4/80. The dosimetric data obtained included the sheet's lead equivalence, thickness ratios, surface dose and buildup region, and transmission factors. The presence of a compensator did not alter the beam's skin-sparing effects. It was concluded that the vinyl lead sheets allowed an easy and rapid fabrication of a tissue compensator for head and neck portals of up to 15 X 15 cm.

A dose-per-pulse monitor for a dual-mode medical accelerator

On a radiotherapy accelerator, the dose monitoring system is the last level of protection between the patient and the extremely high dose rate which all accelerators are capable of producing. The risk of losing this level of protection is substantially reduced if two or more dose monitoring systems are used which are mechanically and electrically independent in design. This paper describes the installation of an independent radiation monitor in a dual-mode, computer-controlled accelerator with a moveable monitor chamber. The added device is fixed in the beam path, is capable of monitoring each beam pulse, and is capable of terminating irradiation within the pulse repetition period if any measured pulse is unacceptably high.

[Characterization of ultra high energy neutron beam generated by 500 MeV proton beam]

An ultra high energy neutron facility was constructed at PARMS, University of Tsukuba, to produce a neutron beam superior to an X-ray beam generated by a modern linac in terms of dose distribution. This has been achieved using the reaction on a thick uranium target struck by 500 MeV proton beam from the booster-synchrotron of High Energy Physics Laboratory. The percentage depth dose of this neutron beam is nearly equivalent to that of X-rays at around 20 MV and the dose rate of 15 cGy per minute. Relative biological effectiveness of this neutron beam has been estimated on the cell killing effect by the use of HMV-I cell line. Resultant survival curve of cells after the neutron irradiation shows the shoulder with n and Dq of 8 and 2.3 Gy, respectively. RBE value at 10(-2) survival level for the present neutron, compared with 137Cs gamma-rays is 1.24. The result suggests that the biological effects of high energy neutrons are not practically large enough whenever the depth dose distribution of neutrons becomes superior to high energy linac X-rays.

[The use of absorbers to optimize the surface dosage in treatment using ultrahard x-rays]

The surface dose in megavoltage x-ray therapy can be modified using two different techniques: either varying the thickness of an absorber positioned directly on the surface of the patient or varying the distance to the surface of an absorber with constant thickness. Surface dose as a function of distance and absorber thickness has been measured at 4 MV and 8 MV x-rays. Both methods yield sufficiently and well-defined dose enhancements at the surface of the patient as desired clinically. The variation of the absorber-surface distance however has been proved to be the more simple and practicable method, offering the additional advantage of no contacting the patients skin.

The calculation of transmission through a photon beam attenuator using sector integration

A calculational scheme is presented for the prediction of the transmitted fraction (TR) through an attenuator of known material and physical dimensions, at any point in a photon beam, for a beam of any shape or size. The method considers the total TR to be composed of scatter and primary components and computes the scatter component by sector integration. The input for the calculations consists of measured narrow- and broad-beam transmitted fractions through lead in air for various circular fields, thicknesses of the attenuator, and angles from the central axis, in a geometry approximating typical treatment conditions. The method has been tested for the case of a uniform half slab and a 45 degree wedge in a 4-MV photon beam. It was found that the use of TR values obtained by the above method reduced the maximum absorbed dose computation error from 8% computed with a commercially available algorithm to 3%, in a typical treatment setup. This method is generally applicable to any shaped attenuator such as a wedge or compensator covering whole or part of a radiation field.

[Development of special electron applicators for intraoperative radiotherapy at the Grosshadern Clinic]

A specialized applicator system for intraoperative radiation therapy using high energy electrons of a linear accelerator has been developed and manufactured, regarding the specific situation that there is no dedicated linear accelerator available at the surgery facility. Additionally, long lasting interruptions of the daily routine irradiations are hardly tolerable. A significant improvement of the procedure could be achieved developing applicators, which are divided into two halves. Positioning the lower part of the applicator into the patient at the operating room already, the transportation and the irradiation of the closed patient is possible. Adapted to the linear accelerator in use, a Siemens Mevatron KD having electron energies in the range from 6 to 21 MeV, the dosimetric properties of the system have been optimized by iteration. Excess dose values near the applicator walls could be avoided completely. A rapid decrease of the leakage dose could be realized.

[Intraoperative radiotherapy (IORT) at the Graz University Clinics: development and dosimetry of an applicator system]

The use of high energy electron beams for intraoperative radiotherapy necessitates special technical adjustments of the conventional megavolt equipment. At the University Clinic of Radiology, Division of Radiotherapy in Graz, an applicator system for electron energies up to 20 MeV and cone diameters up to 14 cm was developed. A self modified commercial operating table was used instead of the treatment couch to guarantee optimum mobility for the docking maneuver. The technical set-up of this adaptor system as well as the dosimetric considerations and calculations will be presented in detail.

Design and production of customized field shaping devices for electron arc therapy

A key element in the implementation of electron arc therapy is the use of customized field shaping devices on or near the patient's surface to protect normal tissue surrounding the treatment surface. Techniques for design and production of field shaping devices have evolved to meet the requirements of improved efficiency, patient comfort and protection, and reproducibility of patient set-up. Techniques in current use at the University of Utah are described and illustrated.

Treatment planning and delivery in neutron radiotherapy of soft tissue sarcomas

Well differentiated soft tissue sarcomas may benefit from fast neutron radiotherapy, in particular inoperable and recurrent tumors and tumors with residual disease after non-radical surgery. Treatment planning in a multidisciplinary pretherapeutic approach has to be based on tumor size and site and histopathology. Target volume definitions for potential microscopic spread and for the high risk region of local recurrence have to consider the preoperative tumor localization (imaging), the biological behavior and the extent of surgery. GI/II, T1-3 tumors after intralesional or marginal resection are indications for neutron therapy. Treatment planning and delivery has to take into account the narrow therapeutic range of fast neutrons and include individual immobilization devices, manufacturing of bolus and CT based computed dose calculations. Neutron radiotherapy techniques at the d, T generator are comparable to photon techniques except individual beam shaping, field size and portal verification. Total dose is 16 Gy neutrons in the high risk region and 12 to 13 Gy in the region of potential microscopic spread.

A study of effective attenuation coefficient for calculating tissue compensator thickness

Dose uniformity throughout the treatment volume is essential to precision radiation therapy. Tissue compensators are often used as a means to eliminate dose nonuniformity resulting from surface contour irregularities. This paper evaluates the accuracy of using an effective attenuation coefficient for calculating the thickness of missing tissue. This coefficient is found to vary strongly with thickness of missing tissue when the initial depth is situated in the buildup region. The use of a single attenuation coefficient produces errors as high as 54% in the calculated compensator thickness when 10-MV x rays are used. At depths greater than the depth of maximum dose, the attenuation coefficient remains a function of field size, not the initial depth.