Double-plane Implants Research Articles

Evaluation of Geometrically Optimized Single- and Double-plane Interstitial High Dose Rate Implants with Respect to Conformality and Homogeneity

The use of a stepping source in high dose rate brachytherapy supported with dwell-time optimization makes it possible to deviate from the classical dosimetry systems. Dose distributions of single- and double-plane implants were analysed for conformality and homogeneity at idealized target volumes. The Paris system was used for catheter positioning and target volume determination. Geometric optimization and individual dose prescription were applied. Volumetric indices and dose parameters were calculated at optimal active length, which was found to be equal to target volume length. The mean conformality, homogeneity, external volume and overdose volume indices were 0.78, 0.67, 0.22 and 0.13, respectively. The average minimum target and reference doses were 69% and 86%, respectively. Comparisons between the volumetric indices of geometrical optimized and non-optimized implants were also performed, and a significant difference was found regarding any index. The geometrical optimization resulted in superior conformality and slightly inferior homogeneity. At geometrically optimized implants, the active length can be reduced compared to non-optimized implants. Volumetric parameters and dose-volume histogram-based individual dose prescription are recommended for quantitative assessment of interstitial implants.

Conformality and homogeneity of dose distributions in interstitial implants at idealized target volumes: a comparison between the Paris and dose-point optimized systems

Background and purpose: The use of high dose rate stepping source in interstitial brachytherapy provides more possibility to conform the dose distribution to the target volume compared to the classical systems. The purpose of this study was to evaluate implants made according to the Paris, the stepping source and the conformal dosimetry system with respect to dose homogeneity and conformality, and to compare these systems using volumetric parameters.Materials and methods: Single-plane and double-plane implants with catheters arranged in square and triangle pattern were used in the analysis. Twenty-seven idealized planning target volumes (PTV) were generated. They formed slabs with rectangular or trapezoidal cross-section. The lengths were 3, 5 and 7cm, the widths and heights were determined according to the Paris system for catheter separation of 1, 1.5 and 2cm. The dose specification was selected such that the coverage index was 0.95 for each implant. Optimal active lengths were determined according to the best conformality at the optimized implants. From the dose–volume histogram (DVH) the following indices were calculated for every implant: conformal (COIN), external volume (EI), relative dose homogeneity (HI) and overdose volume (OI). Furthermore, the mean central dose (MCD) and minimum target dose (MTD) was also determined. The dosimetry systems were compared through the mean values of these parameters and the volumetric indices were analyzed according to the geometry of the PTV.Results: For the optimized systems the optimal active length was 0.5–1.0cm shorter than the target volume length, depending on the catheter separation and geometry of the PTV. For the Paris, the stepping source and the conformal dosimetry system, the mean COIN was 0.66, 0.82 and 0.82; the mean HI was 0.71, 0.68 and 0.68; the mean EI was 0.44, 0.17 and 0.17; the mean OI was 0.11, 0.13 and 0.12, respectively. The statistical analysis showed that the Paris system differed from the optimized systems significantly. For the Paris, the stepping source and the conformal dosimetry system, the mean reference isodose was 85, 90 and 95%, the MCD was 100, 100 and 109%, the MTD was 67, 71 and 73%, respectively. Regarding geometry of the PTV, the most conformal and homogeneous dose distributions occurred when the catheter separation was small, the target volume was long and its shape was a thick rectangular slab.Conclusions: Positioning the catheters according to the rules of the Paris system, but applying optimization on dose points placed either between the catheters in the whole target volume or on the surface of the target volume, and selecting the reference isodose by DVH, can provide highly conformal dose distribution to the target volume, with only a slightly worsened dose homogeneity compared to the Paris system.

A dosimetry system for 192IR interstitial breast implants performed at the time of lumpectomy

Purpose : 192Ir interstitial breast implants performed at the time of lumpectomy present a unique problem because they cannot be preplanned, and yet they are expected to produce a treatment dose rate (TDR) from 0.3 to 0.5 Gy/h using sources already procured. The purpose of this work is to describe a system of dosimetry that works within these constraints and has been used to perform more than 600 such implants. Methods and Materials : The underlying principle is to fix the ribbon spacing, the interplaner separation, and the linear activity (1 mCi/cm) so that the TDR will depend only on the area (L × W) implanted. The ribbons are spaced 1.5 cm and 2.0 cm apart in single plane and double implants, respectively. Idealized implants were used to study the TDR as a function of the implant dimensions, and to study the effects of varying the ribbon spacing and interplanar separation. Volume-dose histograms were generated to study the homogeneity of dose. Results : The TDRs of single plane implants range from 0.3 Gy/h for small 4 × 4 cm 2 implants to 0.4 Gy/h for large 10 × 10 cm 2 implants. The TDRs for double plane implants are similar for the same range of dimensions. Conclusions : Implants with a TDR between 0.3 and 0.5 Gy/h can be performed for a wide range of geometries without preplanning using fixed ribbons spacings of 1.5 and 2.0 cm for single and double plane implants, respectively, and a linear activity of 1 mCi/cm.

94 An optimized distance-strength relationship for planning Ir-192 double-plane implants

94 An optimized distance-strength relationship for planning Ir-192 double-plane implants

Method of localization and implantation of the lumpectomy site for high dose rate brachytherapy after conservative surgery for T1 and T2 breast cancer

Purpose : This article describes our technique of localization and implantation of the lumpectomy site of patients with T1 and T2 breast cancer. Our method was developed as part of our Phase I/II pilot study of high dose rate (HDR) brachytherapy alone after conservative surgery for early breast cancer. Methods and Materials : In March 1992, we started a pilot study of HDR brachytherapy to the lumpectomy site as the sole radiotherapy after conservative surgery for clinical T1 or T2 invasive breast cancer. Initially, the protocol required intraoperative placement of the interstitial needles at the time of definitive surgery to the breast. The protocol was then generalized to allow the implantation of the lumpectomy site after definitive surgery to the breast, either at the time of subsequent axillary nodal dissection or postoperatively. To date, five patients have been implanted intraoperatively at the time of definitive breast surgery. Twelve patients were implanted after definitive breast surgery, with 7 patients being done at the time of axillary nodal dissection and 5 patients postoperatively. We devised a method of accurately localizing and implanting the lumpectomy site after definitive breast surgery. The method relies on the previous placement of surgical clips by the referring surgeon to mark the lumpectomy site. For each patient, a breast mold is made with radio-opaque angiocatheters taped onto the mold in the supero-inferior direction. A planning CT scan is then obtained through the lumpectomy site. The volume of the lumpectomy site, the number of implant planes necessary, and the orientation of the implants are then determined from the CT scan. The angiocatheters provide a reference grid on the CT films to locate the entry and exit points of the interstitial needles on the plastic mold. The entry and exit points for reference needles are then transferred onto the patient's skin enabling implantation of the lumpectomy site. Needle positions with respect to the lumpectomy site are then verified using simulator radiographs. Results : Eight double plane implants and four single plane implants have been done using this method. Five implants were done using direct visualization. It has not been necessary to reorient the implant in any of the patients. If not for the presence of surgical clips, the size of the lumpectomy site cannot be separated from the surrounding normal breast tissue. Conclusion : This technique is an accurate way to localize the lumpectomy site for HDR brachytherapy.

Dose uniformity in a planar interstitial implant system

Purpose : This work makes of a volume-ratio technique to examine dose uniformity in a planar interstitial implant system based entirely on geometrical constraints. The rationale for determining an upper limit for acceptable dose variation is examined and discussed. Variation of ribbon spacing and interplanar separation is evaluated in terms of its effect on dose homogeneity. Methods and Materials : Volume-dose were generated for a range of planar implant dimensions. The volume inside the target region and enclosed between the reference isodose and a higher isodose surface was calculated as a measure of dose uniformity. Studies of homogeneity, target coverage, and external tissue irradiation were carried out to evaluate the importance of flexible interplanar spacing in optimizing implants. New dose tables were generated to accomodate the frequent clinical need to minimize the number of cathether insertions. Results : Implants carried out in accordance with specified geometric constraints were found also to provide optimal dose homogeneity as determined using the volume ratio method with a flexible high dose limit. For two-plane implants, the interplanar spacing should be determined specifically in each case to ensure accurate target coverage. Calculations for specific cases showed that the tissue volume treated to unnecessarily high dose levels can be reduced by a large factor by careful positioning of the implant planes. A smaller ribbon and seed spacing will, in general, lead to better dose uniformity when this is evaluated in terms of the volumes treated to very high dose levels. Conclusions : Our studies showed that implants carried out using simple and useful geometric guidelines will also provide an acceptably uniform dose distrbution. For double plane implants, the separation of the implant planes should be optimized for each target thickness.

Concept of dose nonuniformity in interstitial brachytherapy

Purpose : Evaluation of the 3-dimensional dose distributions of interstitial implants using the dose nonuniformity ratio. Methods and Materials : Single source, two sources, three and four sources arranged both linearly and in the form of a triangle or a square, ribbons with different seed spacings, a single-plane and double-plane implants were evaluated. The evaluations involved the use of differential dose volume histograms and the dose nonuniformity ratio defined as the ratio of the high dose volume to the reference volume. Results : For a single source, the dose nonuniformity is the same regardless which dose rate is selected as the treatment dose rate. For any multi-source implant, the dose nonuniformity is altered depending on the selection of the reference dose rate. In addition, the dose nonuniformity curve exhibited three characteristics zones. Conclusion : The dose nonuniformity ratio can be a useful tool in assessing and optimizing interstitial implants.

A dose homogeneity index for evaluating 192Ir interstitial breast implants.

To evaluate and optimize dose homogeneity of 192Ir interstitial breast implants, we define a quantity, the dose homogeneity index (DHI), as follows: DHI = [V(TDR)--V(HDR)]/V(TDR), where V(TDR) denotes the total treatment volume enclosed by the prescribed treatment dose rate (TDR) and V(HDR) denotes the volume enclosed by high-dose rate (HDR), which is 1.5 X TDR or greater. We have used the DHI to examine and compare 192Ir double-plane implants of various sizes planned by the Memorial system or the Tufts system. Criteria have been suggested for the number of planes required for implants in a given treatment volume. Anderson's volume-dose histogram with inverse square suppression is adopted for illustration.

Planning of Ir-192 seed implants for boost irradiation to the breast.

The conservative management of early stage breast cancer with tumor excision and irradiation of the breast is becoming increasingly accepted as an alternative to modified radical mastectomy. The radiotherapy typically consists of 45 to 50 Gy delivered with external beam irradiation, followed by boost irradiation of 15 to 20 Gy to the tumor bed using electron beams or interstitial implantation. Pathological evaluation of the excised tumor, clinical assessment, and mammography are used to determine the tissue volume potentially containing a residual tumor burden and therefore requiring boost irradiation. In this paper we describe planning and implantation procedures for Quimby-type breast implants using Ir-192 seeds encapsulated in nylon tubing. This system deviates in several important respects from the requirements of the standard brachytherapy systems. For double-plane implants, optimized values of the interplanar spacing are given for a range of implant sizes, along with the corresponding target dose rates for 1.0 mCi seeds. We also describe a modification of the angiocatheter implantation technique, which allows the radioactive sources to be secured in place by a magnetic cap and washer, thus greatly facilitating the removal of the sources at the end of treatment.

Dosimetry tables for standard iridium-192 wire planar implants.

The Paris brachytherapy technique (Pierquin & Dutreix, 1967; Pierquin et al, 1978) involves the implantation of a block of tissue with an equally-spaced array of straight wires of the same activity per unit length. The activity of the wires and the duration of the treatment are chosen to deliver a prescribed dose to some reference point or contour, such as 85% of the mean of the doses at points midway between the centres of the sources. Figure 1 shows a typical implant and illustrates the various parameters used to define the treatment volume. Dosage and treatment volume parameters are often calculated (usually by computer) for each individual implant. An alternative approach involving minimal calculation in use is described below for regularly spaced single or double plane implants. The tables produced cover a wide range of implant length/width ratios for up to 10 wires per plane and so are applicable to virtually all the regular planar implants likely to be encountered in practice.

Single and double plane implants: A comparison of the Manchester System with the Paris System

A comparison between the Manchester System and the Paris System of interstitial dosimetry has been made in the case of single and double plane implants. The rules of both systems are reviewed. A brief description of the Paris System is presented in an appendix. Dose distributions for two different examples are presented in two orthogonal planes. The Paris System uses considerably fewer sources than the Manchester System. It results in a larger volume of high dose than the Manchester System. The use of Iridium-192 sources whose source strength and source length can be adjusted represents a significant advantage. The Paris System attempts to adapt the implant configuration to the clinical situation as the target thickness is used to define the source separation and the target length is used to define the source length. The differences in the dose definition are discussed.

Single- and double-plane iridium-192 interstitial implants: implantation guidelines and dosimetry.

Computerized dosimetric studies of single- and double-plane iridium-192 (Ir-192) planar implants were performed. With respect to dose homogeneity, we found that the optimal source and ribbon separation for single-plane implants was 1.0 cm. For double-plane implants, the preferred ribbon and plane separation was 1.5 cm, maintaining a 1-cm separation for the sources. Using these separations, standard dose rate curves for single- and double-plane Ir-192 implants were generated by computer calculations. These standard curves are useful for quickly and fairly accurately estimating the dose from any size planar implant, without requiring more time-consuming individual computer dosimetry. We believe that the curves will prove to be of practical clinical value to physicists and radiotherapists.

Dosimetry of gold-198 seed implants.

For single plane implants with gold seeds, assuming a matrix distribution for the seeds, the optimum separation of the matrix elements and the optimum ratio of the activity of the seeds in the area to that of those in the periphery were calculated, so that the uniformity of dose distribution is within plus or minus 10%. Area dosages were also computed and are tabulated. The dose variations in double plane implants and volume implants also were studied and the results are presented. Dosage graphs are given for both cases. The theoretical dose distributions were verified for a few implants experimentally using LiF thermoluminescence dosimeters. (auth)