Tissue Compensators Research Articles

Modeling of asymmetric compensator geometries

Dose distributions arising from the use of near unit density retracted missing tissue compensators in symmetric geometries have been successfully modeled on the basis of primary and first-order scattered radiation. This method of analysis has been extended to both low- and high-density materials in asymmetric geometries. Good agreement is achieved between theory and experiment.

Three-dimensional photon treatment planning of the intact breast

Three-dimensional treatment planning for the intact breast was performed on two patients who had undergone CT scanning. A total of 38 treatment plans were evaluated. Multiple plans were evaluated for each patient including plans with and without inhomogeneity corrections, plans using varying photon energies of 60Co, 4 MV, 6 MV, 10 MV, and 15 MV, and three-dimensionally unconstrained plans. Increased hot spots were appreciated in the central axis plane when lung inhomogeneity corrections were used. Additional hot spots were appreciated in off-axis planes towards the cephalad and caudad aspects of the target volume because of lung inhomogeneity corrections and changes in the breast contour. The use of 60Co was associated with an increase in the magnitude and volume of hot spots, whereas the use of higher energy photons such as 10 MV and 15 MV was associated with an unacceptable target coverage at shallow depths. Therefore, for the two patients studied, the use of a medium energy photon beam (such as from a 6 MV linear accelerator) appeared to be the energy of choice for treatment of the intact breast. The three-dimensionally unconstrained plans were able to improve slightly upon the standard plans, particularly with relationship of dose to normal tissue structures. Areas for future research were identified, including the use of tissue compensators.

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.

The Trilogy of the “Triad of O'Donoghue” in the Knee and Its Analogy to the TMJ Derangement

An attempt has been made to describe the similarities between the pathological injury "Triad of O'Donoghue" in the knee and its analogous equivalent in the internal derangements of the temporomandibular joint (internal derangements). Because of the similarities between these internal derangements and the treatment they require, TM joint internal derangements should be reimbursed as a medical problem by insurance carriers as internal derangements in the knee are reimbursed. A description of the anatomical, physiological, and arthro-kinomatics of these two synovial joints has been done to further point out that they both operate by the same physiological principles, and therefore should be treated as similar dysfunctions. Treatment should be rendered to the TM joint, as in the knee, from an orthopedic viewpoint. This requires that treatment to the TM joint include a cooperative rehabilitative team approach. This team approach encompasses medical and dental cooperative care to stabilize the joint and secondarily control joint related soft tissue compensation and pain.

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.

Optimized tissue compensators

The fabrication of retracted missing tissue compensators which account only for missing primary attenuation produces compensation which is less than optimum. Experimental results can be accurately reproduced with theoretical calculations based on primary and first-order scattered radiation. This method of analysis may be extended to predict the shape of a compensator which will produce the desired dose distribution at a given depth in phantom. An optimized compensator is constructed based on these theoretical considerations and the analysis of its performance is presented.

Peripheral doses of two linear accelerators employing universal wedges

The peripheral dose (PD) is defined as the dose delivered to tissues outside the collimated radiotherapy beam. This absorbed dose will be of clinical concern if any uninvolved radiosensitive or critical structures outside the beam were to receive dosages approaching their tolerance levels. Sources of the PD include head leakage (transmission through the collimation system), collimator scatter, scatter and secondary particle production from a beam modifier (tissue compensator, wedge filter or shielding) present in the beam and scatter arising from within tissue. A fifth source, that due to radioactivity induced within the patient by a high-energy beam, is not considered in this work. In the past, several studies have been undertaken to characterize the peripheral dose as a function of beam modality and quality, field dimension, the separation between the radiation source and the patient and the presence and composition of a beam modifier (Keller et al, 1974; Scrimger & Kolitsi, 1979; Nusslin & Hassenstein, 1980; Fraass & van de Geijn, 1983; Greene et al, 1983; Kase et al, 1983; Starkschall et al, 1983; Greene et al, 1985; Sherazi & Kase, 1985; Francois et al, 1988). Peripheral doses at specific clinical sites, such as the contralateral breast (Fraass et al, 1985) and the gonads (Yau et al, 1981), have also been measured on patients and within anthropomorphic phantoms. In general, these studies indicate that the PD decreases approximately exponentially with distance from the beam edge.

Tissue Compensation With Aluminium For Co-60 Therapy

Tissue Compensation With Aluminium For Co-60 Therapy

Comparison of dose distribution by means of bolus versus water bath phantom techniques in treatment of cutaneous sarcoma of an extremity

Cutaneous involvement by Kaposi's sarcoma (KS) is frequently diffuse, requiring large field irradiation. Selective tissue compensation techniques, accomplished with either a 1.5 cm bolus cast or water bath phantom, are compared. Dosimetric determinations were performed using 6 MVX with bone heterogeneity corrections. Application of 1.5 cm bolus effectively eliminated dose buildup and provided tissue compensation equivalent to water bath phantom technique. Treatment of other cutaneous malignancies, such as angiosarcoma, requiring higher total doses, should be accompanied by dosimetric evaluation, because cutaneous and midline dose inhomogeneity may result in a significant total dose differential over the target volume. Application of 1.5 cm bolus improved accuracy in treatment positioning, eliminated concerns regarding infection control in AIDS-related KS, and provided dose homogeneity comparable to the more standard water bath phantom technique.

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.

Genucom Knee Analysis System: An evaluation of the soft tissue compensation procedure

Genucom Knee Analysis System: An evaluation of the soft tissue compensation procedure

An Analytic Approach to Optimized Retracted Missing Tissue Compensators

The introduction of an attenuating medium into a photon beam serves both to reduce the intensity of the primary beam and to create secondary radiation due to scatter. When retracted missing tissue compensators are employed to compensate for irregular surface contour or internal inhomogeneities, they are often fabricated without regard to the scattered radiation that they introduce into the system. The study of a mathematically describable conical geometry has clearly demonstrated the need for improved compensator design1. Experimental results obtained with this geometry can be reproduced with good agreement, using theoretical calculations based on primary and first order scattered radiation. This method of analysis may be extended to predict the shape of a compensator which will produce an optimized dose distribution at a given depth in a phantom, equivalent to that which would be obtained when an irregular surface is filled with unit density material (bolus), producing a flat surface. An optimized compensator was constructed based on these theoretical considerations and excellent agreement was observed between theory and experiment. Dramatic improvement in the restoration of bolus dose is obtained with this optimized compensator. Finally, an anthropomorphic phantom of the neck region has been constructed and the performance of a compensator designed according to current clinical methods for this geometry has been evaluated. The performance of an optimized compensator specific to this geometry is presented and good agreement between theoretical predictions and experimental results is observed. Dramatic improvement in bolus dose restoration over that obtained with the clinically designed compensator is realized.

Limitations of Retracted Missing Tissue Compensators: An Experimental Analysis

The introduction of an attenuating medium into a photon beam serves both to reduce the intensity of the primary beam and to create secondary radiation due to scatter. When retracted missing tissue compensators are employed to compensate for irregular surface geometry or internal inhomogeneities they are almost always fabricated without regard to the scattered radiation that they introduce to the system. Analysis of such a retracted compensator designed for use with an anthropomorphic phantom reveals an inability to provide true compensation. Subsequent analysis of a mathematically describable conical geometry demonstrates the need for improved compensator design.

Tissue Compensators Made of Solid Water or Lead for Megavoltage X-ray Radiotherapy

Curvature of the body within the treatment field of a patient undergoing radiotherapy can result in a non-uniformity of the dose distribution within the tumor volume which, for a 20 cm × 20 cm field and 6 MV X-rays, could exceed 30% of the prescribed tumor dose. The use of bolus is usually not satisfactory as skin sparing is lost. The use of a compensator at the position of the block holder assembly attenuates the primary beam while not significantly effecting the scattered radiation in the patient’s body. Consequently, the necessary compensator thickness depends on the type of material used, the field size, beam energy, and depth of the point of interest. Using a compensator with thickness equal to that of the missing tissue would result in significant underdosing of the tumor. In this paper, measurements of the necessary thickness of solid water and lead required to reduce the non-uniformity of dose on the central axis to within 3% of the prescribed dose are presented for 6 MV and 10 MV X-rays.

Achieving Uniform Dose with the Use of a Custom Tissue Compensator and a Leveled Beam for Tangential Breast Fields

In order to achieve uniform dose distribution in intact breast treatments, wedges can be employed. This paper will describe a custom compensator made from brass chips used in conjunction with a leveled beam and a custom “cast” to treat breast shapes that are less suited to a standard wedge set up. Materials and design, dosimetry, criteria, efficacy and results will be described.

Computer aided design and verification of megavoltage tissue compensators for oblique beams.

A computer based radiotherapy dose compensator system is presented. A plane of uniform dose (pud) is chosen which passes through the central axis at a reference depth in the patient. The pud is oriented at an arbitrary angle. Tissues thicknesses are the ray line distances from the patient surface to the pud. Relative dose estimates on the pud are derived from tissue-air ratio, tissue-maximum ratio, or tissue-phantom ratio tables interpolated for field size and tissue thickness. An inverse square law correction is also applied. Compensator thicknesses are calculated from measured effective attenuation coefficients to attenuate the beam by a factor proportional to the estimated dose on the pud. Water phantom measurements for 60Co for both oblique incidence and wedge geometries demonstrate dose uniformity to better than +/- 4%. Dose measurement with radiographic films sandwiched between the two halves of a wax phantom verify adequate compensation for neck irradiation by 60Co opposed lateral beams and 6-MV x-ray inferiorly angled lateral beams. Compensator factors used for absolute dose determination are accurate to better than +/- 3%. The dose modeling routine, QCBEAM, is found to be sufficiently accurate for routine compensator verification for these geometries.

Skin dose near compensating filters in radiotherapy

In radiotherapy treatments with MV beams, the use of tissue compensators affects the dose to the skin. Methods of calculating the relative skin dose (RSD) are described and a formula is derived to predict the contribution of a tissue compensator to the RSD. Measurements of RSD for various field sizes and distances from the compensator are presented.

Incorporation of Patient Immobilization, Tissue Compensation and Matchline Junction Technique for Three-Field Breast Treatment

A protocol for the treatment of the intact breast was developed to maximize dose homogeneity and reproducibility. This protocol uses patient and breast immobilization, three-dimensional tissue compensators, and a technique for geometric matching of fields when the supraclavicular area is treated. A series of phantom measurements and analysis of patient port films was performed to evaluate dose homogeneity and reproducibility using this technique, and the potential adverse effect of loss of skin sparing from the immobilization device was investigated. Dose homogeneity throughtout the phantom breast was within ±6% of the prescribed central axis dose, and homogeneity at the supraclavicular match line was ±10%. This represented a significant improvement over techniques not using tissue compensation or geometrically matched fields. Reproducibility of patient treatments was not significantly improved from previous non-immobilized treatment techniques, but there was no loss of skin sparing from the device, and other advantages of immobilization were observed. Details of the protocol are discussed together with changes that are currently being made to improve the results obtained thus far.

Polyethylene-Lead Tissue Compensators for Megavoltage Radiotherapy

Tissue compensators afford one the opportunity of producing idealized dose distributions in radiotherapy. We have developed a technique which may be used to produce fast, accurate and inexpensive compensators within a few minutes and requires minimal patient involvement. Three dimensional contour data is acquired using a moire fringe photograph taken at the time of simulation. The photograph is projected to the size needed for the actual compensator and a sketch of the compensator is made. The sketch is placed in a specially designed small portable pantographic unit and the design is traced while the unit cuts the compensator from an indexed polyethylene-lead blank which is premounted on an acrylic tray. The polyethylene-lead material proves to have an ideal combination of properties for compensator construction including relatively high density, ease of machining, good handling characteristics and low cost.

The Use of Posterior Wedge Portals as an Alternative to Tissue Compensators

In a regional oncology center, where the patient census makes routine construction of tissue compensators prohibitive, a simple alternative can be successfully implemented. In this instance, a posterior wedge is used to alter isodose levels. This allows us to minimize critical structure doses, and optimize tumor dose levels. The following chest isodose distribution is presented for discussion.