Tissue Compensators Research Articles

High-energy total body irradiation as preparation for bone marrow transplantation in leukemia patients: Treatment technique and related complications

Bone marrow transplantation with conditioning regimens that include total-body irradiation (TBI) is widely used in patients with acute lymphoblastic and acute myelocytic leukemias. The major causes of death in this population are relapse of leukemia, infection, and treatment related complications. Our purpose was to achieve a homogenous radiation dose distribution and to minimize the dose to the lungs, liver, and kidneys so that the incidence of organ injury was reduced. Dose to the bone marrow, midplane, and periphery was quantified by use of thermoluminescent detectors in a bone-equivalent tissue phantom. In an effort to reduce the risk of complications, we treated relapsed or refractory leukemia patients with TBI administered in fractionated, parallel opposed large fields with 24 MV photons, using tissue compensation and partial-transmission lung shielding. Tissue toxicities were then determined. Dose quantitation in bone-equivalent and tissue-equivalent phantoms demonstrated that backscatter and pair production interactions adjacent to bone increased the bone marrow dose by 6 to 11%. At an SSD of 400 cm and at patient diameters of 20 to 40 cm, the percent inhomogeneity across the phantom with 24 MV photons was 0 to 0.3%, compared to 4 to 6% for 6 MV photons. End-organ toxicities consisted of clinical interstitial pneumonitis in six patients, idiopathic interstitial pneumonitis in three patients, renal toxicity in seven patients, and veno-occlusive disease of the liver in one patient. Toxicities did not correlate with fractionation schedule. Total-body irradiation administered with 24 MV photons increases the dose deposition in bone marrow through pair production and backscatter interactions occurring in bone. Because percent depth dose increases with SSD, the 24 MV beam is more penetrating at a 400 cm distance than at 100 cm and dose homogeneity is improved with higher energies. Thus, the incidence of radiation-mediated injury to lung, liver, and kidney is reduced. This is an effective preparatory regimen for patients with high-risk leukemias requiring bone marrow transplantation.

Dose uniformity from a computerized three-dimensional tissue compensating system

A verification of dose uniformity of the Huestis Compu-Former, a three dimensional megavoltage tissue compensator, is presented. Tissue compensators were built for three different anthropomorphic phantoms; a head and neck, a mantle, and a breast. Film densitometry was used to evaluate the effectiveness of the tissue compensators by comparing isodose curves generated for a compensated and an uncompensated field. Evaluation of the isodose distributions for the three regions confirmed the use of the Compu-Former as a reliable tissue compensating system.

Clinical considerations in the use of missing tissue compensators for thoracic cases

The irregular shape or contour of the patient's surface in the treatment field can alter the dose distribution resulting in non-uniformity of dose in the treatment volume. Missing tissue compensators have been most commonly used to improve this non-uniformity, especially in head and neck, breast, lung and supraclavicular regions. Two or three dimensional compensators have been typically designed to make the dose uniform at a specific depth. This compensation shifts the dose distribution within the treatment volume so that some structures may be under or over compensated. This study will examine how various thoracic sites are affected by compensators, with and without heterogeneity corrections. We have also analyzed the uncertainty in compensated dose due to the daily variations in patient repositioning. Computer isodose plans using Cobalt-60 gamma rays and 6 and 18 MV X-rays were generated using sagittal contours. Results show that superficial sites, like the supraclavicular nodes and the spinal cord, do not receive as consistent a dose compensation as the midplane sites. In addition, the compensated lung dose increases. Finally, patient movement or positioning errors of ± 1.0 cm causes only a slight change in dose distribution.

Procedure for accurate fabrication of tissue compensators with high-density material

An accurate method for producing compensating filters using high-density material (Cerrobend) is described. The procedure consists of two cutting steps in a Styrofoam block: (i) levelling a surface of the block to a reference level; (ii) depth-modulated milling of the levelled block in accordance with pre-calculated thickness profiles of the compensator. The calculated thickness (generated by a dose planning system) can be reproduced within acceptable accuracy. The desired compensator thickness manufactured according to this procedure is reproduced to within 0.1 mm, corresponding to a 0.5% change in dose at a beam quality of 6 MV. The results of our quality control checks performed with the technique of stylus profiling measurements show an accuracy of 0.04 mm in the milling process over an arbitrary profile along the milled-out Styrofoam block.

Treatment planning for primary breast cancer: A patterns of care study

Purpose : The 1989 Patterns of Care Study included treatment planning or early breast cancer. A Consensus Committee of radiation physicists and oncologists determined current guidelines and developed questionnaires to determined treatment planning and delivery processes used by the participating institutions (e.g., use of portal films). This article present adn analyzes the results of that survey. Methods and Materials : The survey included 449 respondents, distributed as follows: 136 (30%) from Strata I (academic facilities); 169 (38%) from Strata II (hospital based facilities); and 144 (32%) from Strata III (freestanding facilities). The treatment planning procedures surveyed included: whether individualized tissue compensators are used, whether inhomogeneity corrections are used in dose calculations, the use of computerized tomography, whether isodose distributions for external beam tangents and interstitial implants are generated, the use of lymphoscintigraphy, immobilization devices, simulations, portal films, etc. Results : The survey results demonstrated that out of 305 patients from Strata I and II institutions, 237 (78%) had simulated tangential fields. Consistent with this finding is that 76% of patients from Strata I and II institutions were immobilized, while only 51% of Strata III patients were. Moreover, only 18 out of the 449 (4%) of cases did not have any type of external beam dose distribution calculated—presumably, in these cases missing tissue compensation would be unlikely. On the other hand, 41% of the Strata II, 27% of Strata III, but only 19% of Strata I ( p < 0.0002) cases received CT. Surprisingly, 19% of the Strata I, 35% of the Strata II, and 25% of the Strata III ( p = 0.0011) patients received lymphoscintigraphy, perhaps reflecting the use of wide tangents to encompass the internal mammary nodes in these patients. In terms of optmizing treatment, 74% of Strata I, 70% of Strata II, and 78% of Strata III patients had wedges used on both tangential fields, although in 5, 12, and 14%, respectively, no beam modification of any sort was used. Furthermore, it should be noted that in 7% of the Strata I, 23% of Strata II, and 37% of Strata III cases there was no attempt to reduced the divergence of the tangential fields into the lung. On the other hand, if one considers the 135 (of 449) patients where matching of the tangential and supraclavicular fields was applicable, 41% of Strata I, 22% of Strata II and 46% of Strata III patients had those fields matched in a vertical plane, which would involve sophisticated alignment procedures. Quality control of treatment delivery was high: 97% of all surveyed received portal films at least once. The use of thermoluminescent dosimetry (TLD) to measure the dose to the contralateral breast was of little interest: only 4 of the 305 Strata I and II patients received invivo measurements. Conclusions : This national survey has established the patterns of treatment planning for early breast cancer. It shows a generally consistent approach—although a number of statistically significant variations have been identified.

Evaluation of a pencil-beam dose calculation technique for charged particle radiotherapy

Purpose : The purpose of this article is to evaluate a pencil-beam dose calculation algorithm for protons and heavier charged particles in complex patient geometries defined by computed tomography (CT) data and to compare isodose distributions calculated with the new technique to those calculated with conventional algorithms in selected patients with skull-base tumors. Methods and Materials : Monte Carlo calculations were performed to evaluate the pencil-beam algorithm in patient geometries for a modulated 150-MeV proton beam. A modified version of a Mont Carlo code described in a previous publication (18) was used for these comparisons. Tissue densities were inferred from patient CT data on a voxel-by-voxel basis, and calculations were peformed with an without tissue compensators. A dose calculation module using the new algorithm was written, and treatment plans using the new algorithm were compared to plans using standard ay-tracing techniques for 10 patients with clival chordoma and three patients with nasopharyngeal carcinoma who were treated with helium ions at Lawrence Berkeley National Laboratory (LBL). Results : Pencil beam calculations agreed well with Monte Carlo calculations in the patient geometries. The pencil-beam algorithm predicted several multiple-scattering effects that are not modeled by conventional ray-tracing calculations. These includes (a) the widening of the penumbra as a function of beam penetration, (b) the degradation in the sharpness of the dose gradient at the end of the particle range in hgihly heterogeneous regions, and (c) the appearance of hot and cold dose regions in the shadow of cmplex heterogeneities. In particular, pencil-beam calculations indicated that the dose distribution within the target was not a homogeneous as expected on the basis of ray-tracing calculations. on average, for the 13 patients considered, only about 72% of the cone down target volume received at least 99% of the prescribed dose, whereas, 93% of the conedwn volume was contained within the 95% isodose surface. This may e significant because in standard charged particle dose calculations, the dose across the spread-Bragg peak is assumed to be uniform and equal to the maximum or prescribed dose. Conclusions : Dose distributions computed with the pencil-beam model are more accurate than ray-tracing calculations, providing additional information to clinicians, which may influence the doses they prescribe. In particular, these calculations indicate that for some patients with skull-base tumors, it may be advantageous to prescribe proton doses to a lower isodose level than is commonly done.

Electron-photon field matching in the treatment of paranasal sinus tumors: A case report

The treatment planning of paranasal sinus tumors is technically demanding due to the compact anatomy of the region and the close proximity of critical structures. In the majority of cases, conventional approaches utilizing 2 or 3 photon fields are adequate. However, in patients with locally advanced disease, these standard techniques may result in the unnecessary treatment of surrounding structures. We present here a case in which, because of the complex tumor geometry, conventional techniques would result in treating through both orbits. A novel treatment approach has been devised in which opposed lateral photon fields are matched to an anterior electron field (both in depth and in profile) to provide a uniform dose distribution to the target volume, while minimizing the dose to certain critical structures. The treatment design, in particular the methodology of electron-photon field matching as well as the specification of tissue compensation and customized blocking, is discussed.

Evaluation of electronic portal imaging device for missing tissue compensator design and verification.

The purpose of this investigation is to determine if electronic portal imaging devices (EPIDs) can be used for the design and verification of compensating filters. In order to do this, we investigated the operating characteristics of a commercially available EPID and the variation in transmitted dose for various measurement situations. We performed four initial tests to determine the EPID response specific to compensator situations. The tests determined EPID response to variable patient SSDs, different gantry angles, positions of an inhomogeneity within a phantom, and the sensitivity variation of different parts of the imager. After these tests, we determined the attenuation functions relating EPID response to phantom thickness for various phantom materials. With these functions, we tested simple compensation situations to demonstrate that missing tissue compensators can both be designed and verified using EPIDs.

Tangential breast irradiation

We agree with Professor Rodger that the success of breast conserving surgery and radiotherapy depends on surgical technique and radiotherapy dose/fractionation as well as dosimetry. We question his prediction that a randomized controlled trial to explore the importance of dosimetry is not feasible. Our expectation is that in large-breasted women, patient self-assessment questionnaires will clearly discriminate between patients treated with conventional wedge techniques and those treated with 3D tissue compensators. Breast shrinkage, induration and breast pain/tenderness are all expected to be significantly affected. Our prediction is that the improvements will be clinically worthwhile for a significant sub-group of patients.

Tissue compensation using dynamic collimation on a linear accelerator

The availability of computer-controlled collimators on some accelerators has led to techniques for dynamic beam modification, mainly to simulate beam wedge filters. This work addresses the practical aspects of dynamic tissue compensation in one dimension using available treatment-planning software. Data derived from the treatment-planning program is used with an iterative calculational routine to determine the monitor unit settings needed for the collimator-controlling computer. The method was first tested by simulating a 60 degrees physical wedge. Further studies were carried out on a specially fabricated plastic phantom that modeled the sagittal contour of the upper torso, neck, and lower head regions. Dynamic wedge point doses generated by the planning program agreed within 1% with the values directly measured in a polystyrene phantom. In the patient phantom, dynamic collimation achieved calculated dose uniformity within 0.5% in a reference plane near the phantom midline. A comparison of computer-generated and measured point doses in this case showed agreement within 3%. Dynamic collimation can provide effective compensation for contours that vary primarily along one direction. A conventional treatment-planning program can be used to plan dynamic collimation and deliver a prescribed dose with reliable accuracy.

2064 Clinical considerations in the use of missing tissue compensators for head &amp; neck cases

2064 Clinical considerations in the use of missing tissue compensators for head &amp; neck cases

Accuracy of dose calculation methods for retraced tissue compensators

Uncertainties arise in dose calculations involving retracted tissue compensators due to the effects of the compensator upon the scatter component of the dose. Many commercial treatment-planning systems cannot allow directly for the presence of a compensator in isodose calculation or else use simple 2D methods. The authors present data to test calculation accuracy for a wax compensation system by comparing retraction factors measured along central-axis and off-axis raylines for a variety of compensator shapes, with those derived using effective attenuation coefficient and 3D analytical calculations. The accuracy of using measured uniform-thickness retraction factors for non-uniform shapes and the dose uniformity achievable using a simple compensation system are also discussed. The authors conclude that the accuracy of simple calculation methods is shape dependent and that calculation errors and dose variations can exceed +or-5% where missing-tissue thickness variations are large. The analytical method is shown to give good agreement with experiment and indicates that it should be possible to adapt algorithms that calculate scatter from the patient for use with compensators.

Clinical considerations in the use of missing tissue compensators

Clinical considerations in the use of missing tissue compensators

A technique for the evaluation of a missing tissue compensator system.

A two dimensional approach to the evaluation of a missing tissue compensator system has been developed and implemented. Quantitative descriptors of the dose area histograms generated by scanning films irradiated under a variety of compensated and uncompensated conditions are proposed. The method has been found to be useful during the acceptance testing of a commercial compensator system and the results of this exercise are presented.

Tissue compensation and verification of dose uniformity

The problem of nonuniform dose distribution due to irregular and sloping surfaces is frequently encountered during treatment planning whose ultimate goal is to deliver uniform dose to a specific target volume. In order to overcome this problem, various tissue compensation systems have often been employed. The purpose of the present study was to evaluate the computerized tissue compensation system, Autocomp, manufactured by Nuclear Associates, for its ability to obtain dose uniformity both in phantoms and in clinical use. For the phantom studies, a film was placed below a tissue-equivalent phantom with a 45° sloping surface and exposed to either a compensated or an uncompensated beam of 60Co. The field profiles scanned by a computerized film scanning densitometer showed significant improvement in dose uniformity when the compensator was used. For the clinical study, a left buccogingival sulcus carcinoma treated with 60Co gamma ray was chosen to verify the dose uniformity at a depth over the entire field of irradiation. A comparison of two isodose distributions, obtained with and without tissue compensation, indicated the elimination of the nonuniform dose distribution.

94. 寝台移動法 TBI における区間対応変速機能による厚み補償(治療-1 全身照射)

94. 寝台移動法 TBI における区間対応変速機能による厚み補償(治療-1 全身照射)

94. 寝台移動法 TBI における区間対応変速機能による厚み補償(第 49 回総会学術大会一般研究発表予稿)

94. 寝台移動法 TBI における区間対応変速機能による厚み補償(第 49 回総会学術大会一般研究発表予稿)

Missing tissue compensators: evaluation and optimization of a commercial system.

A commercial system for producing retracted compensators has been adapted to suit local needs, and is evaluated here. It comprises a magnetic field surface digitizer and computer-driven milling machine. Improvements in dose distributions, resulting in standard deviations of the mean dose between 2% and 3%, have been achieved for treatment fields in wax phantoms simulating the head and neck regions. Optimization of compensator shape to allow for changes in the amount of scattered radiation has resulted in a further improvement in dose uniformity, particularly near the field borders; for these compensators the standard deviation was as low as 1.6%. The system using the basic algorithm has been in clinical use since July.

Improved dose homogeneity in the breast using tissue compensators

The improvements achievable by introducing individual tissue compensators in photon therapy of the breast were assessed. In 37 patients the dose ranged from + 15% to − 10% of the mid target dose using combinations of wedge filters and beam weights alone. With a tissue compensator the dose ranged from + 4% to − 11% provided that allowance was made for lung attenuation. A megavoltage imaging system is a potential source of the X-ray transmission data which can provide a basis for the calculation of thickness of the compensator.

A comparison of postoperative techniques for carcinomas of the larynx and hypopharynx using 3-D dose distributions

If a head and neck cancer originates low in the neck with a primary site below the shoulders, a technical challenge to the radiation oncologist exists in that the entire neck needs treatment while avoiding overlap of multiple fields on the spinal cord. No standard solution to this problem exists. We have developed a 3-D treatment planning tool that can be used to develop and compare 3-D treatment plans and dose distributions. Usig this tool, we have studied the following techniques for the postoperative treatment of carcinomas of the larynx and hypopharynx, tumors that often embody the problems discussed above: (a) the mini-mantle technique used at the Massachussetts General Hospital, (b) a 3-field technique used at the University of Florida at Gainesville (UF 3-field), (c) a 3-field technique used at our institution and at many others (standard 3-field), and (d) the kicked out lateral technique used at our institution and at others. The 3-D dose distributions from these plans are compared. With 100% delivered just anterior to the vertebral body at mid-neck, the mini-mantle technique results in large 120% hot spots laterally and anteriorly in the neck. Near the mastoid tips, however, the dose falls to 100%. The upper neck nodes may be underdosed since this is 20% cooler than the lateral-anterior neck dose (where a large 120% hot spot exists). The spinal cord is adequately blocked. The two 3-field techniques result in small hot spots at the junction of the lateral and anterior fields. Because different methods are used to prevent overlap at the spinal cord, these hot spots occur anteriorly in the standard 3-field technique and laterally in the OF 3-field technique. The spinal cord block results in untreated neck tissue which can be supplemented with electrons in the standard 3-field technique, but is left untreated in the OF 3-field technique. Both techniques result in a generous length of spinal cord which does not receive full dose. The kicked out lateral technique treats the entire neck and reconstructed pharynx without matching fields at midneck. The upper mid mediastinum is underdosed 10–20% despite being within the posterior inferior portion of the beam. This could be minimized by using a tissue compensator. Unless there is significant subglottic extension or significant risk of disease in the upper mediastinum, we favor treating these malignancies with the kicked' out lateral technique, which avoids the problem of junctioning lateral and anterior fields and provides a fairly homogeneous dose distribution.