Implementing a tantalum wire mesh to increase the skin dose in low-energy electron irradiation of the chest wall.

Radiation treatment of the post-mastectomy chest wall is performed in our institution by straight-on electron irradiation. The chest-wall thickness is measured and the beam energy is chosen so that the chest wall is treated to therapeutic doses, while sparing the underlying lung tissue. The most commonly chosen energies are 6 and 9 MeV. The skin dose should be 90% of the dose prescribed to the chest wall, which is higher than can be achieved with 6- and 9-MeV beams because of the low surface dose. The addition of a bolus slab during part of the treatment can correct for this; however, the added depth means that a higher energy has to be chosen, which will increase the lung dose (the higher the electron energy, the slower the falloff of the electron depth-dose curve). A mesh of a high-Z material above the skin gives rise to obliquely scattered and low-energy electrons that effectively spoil the buildup zone. Dosimetric measurements of a Tantalum (Ta) mesh were performed using a dose scanner in a water tank and a film inserted in a humanoid phantom during a simulated treatment. Measurements were also done for the clinically relevant cases of oblique beam incidence and with the mesh placed 1 cm above the surface. The measurements demonstrate the spoiling of the buildup zone, while having only a moderate influence on the dose distribution beyond the dose maximum. The mesh also changes the absolute dose. In a fractionated regime, the first part of the treatment would be without the mesh, adding it only during the latter fractions. The total dose distribution gives 90% to the skin, while leaving the depth-dose characteristics beyond the dose maximum virtually unchanged.

Spreadsheet calculations of absorbed dose to water for photons and electrons according to established dosimetry protocols.

The calculation of absorbed dose to water according to a Code of Practice demands a strict adherence to the rules and data of the protocol. To ease the calculations and to avoid computational and methodological errors, we have developed a number of spreadsheets to perform the calculations in accordance with an established dosimetry protocol-in our case those of the International Atomic Energy Agency (IAEA) and the Institution of Physics and Engineering in Medicine and Biology (IPEMB). The spreadsheets are implemented as Microsoft Excel V5.0 worksheets. Only a limited selection of dosimetry equipment is used for calibration, which is performed according to only one of the methods allowed by the protocol. This voluntary limitation of equipment and methods is reflected in a spreadsheet that is beam-specific, compact, focused, and very practical. There are four main spreadsheets: high-energy photons (IAEA), high-energy electrons (IAEA), medium energy X rays (IPEMB), and low-energy X rays (IPEMB). The sheets allow the input of setup and measured data, but tabulated data and formulas are protected. Parameter values are copied from the protocols, and the relevant value is found by linear interpolation. Once the spreadsheets are drawn up correctly and thoroughly checked, protocol calculations are performed easily and accurately. The spreadsheets presented are tailored to suit our specific needs but can easily be modified to conform to the practices of any other institution. They are not intended as "cookbooks" but need to be filled in by a radiation physicist with the input data checked by a second professional. The same method is also used for calculating the Reference Air Kerma Rate of brachytherapy sources.

Total skin electron irradiation in mycosis fungoides dose and fractionation considerations.

This study was undertaken to analyze the influence of total skin dose and dose-fractionation schedules on the response rate, survival and skin toxicity of patients with mycosis fungoides [MF] treated with total skin electron irradiation [TSEI]. From 1979 to 1992, 40 patients with MF were treated with TSEI using a modified Christie Hospital technique. Mean follow-up time was 48 months [median 20 months]. 37/40 patients completed TSEI; three died due to non-treatment-related conditions during therapy. 34/37 [92%] treated patients achieved complete remission [CR] and 16/40 [40%] are alive with no evidence of disease. Over the years, changes in dose-fractionation schedules were made and correlated with the pattern of CR and skin toxicity. The 5-year actuarial survival [Stanford staging] was 84% in Stages IA-IB [all Stage IA patients are alive] and 59% in Stage II. The probability of survival of Stage III-IV patients was 30% at 30 months. Late skin toxicity was mild to moderate in 60% and severe in 25% of patients. A reduction of the total dose and dose-per-fraction resulted in an acceptable CR rate and a significantly lower toxicity. TSEI is effective in early stage MF. Skin control and late skin toxicity seem to be dose-fractionation-schedule related. For the early stages, the optimal treatment schedule seems to be 24-30 Gy to the whole skin surface in 2.4-3.0 Gy fractions, given twice weekly over a period of four to six weeks. Total doses of 24-30 Gy at 2.4-3.0 Gy per fraction yielded comparable skin control rates with lower skin toxicity.

A simple oral cone attachment for the Varian linear accelerator.

Lesions in the oral cavity are often treated with two opposed lateral fields. These include a significant amount of normal healthy tissue whose radiation tolerance is dose-limiting. The tumor dose can be boosted to tumorcidal levels by brachytherapy or by small electron fields directed straight on the lesion. We have developed a simple attachment to the standard electron applicator of the Varian Clinac 1800 that allows irradiation of small electron fields through acrylic tubes-the oral cones. These tubes have been evaluated in terms of depth dose and field profiles for 6, 9, 12, 16, and 20 MeV electrons using film for relative dosimetry. At these small field sizes there are significant changes in output factors, in the depth dose as well as in the effective size of the field, and a thorough dosimetric evaluation is imperative prior to treatment. The attachment can be manufactured locally at low cost. For reasons of patient safety the assembly is collapsible. In clinical practice the cone is directed directly on the tumor. For deep-seated lesions we use a penlight and a mirror for positioning.

Dose perturbation due to the presence of a prostatic urethral stent in patients receiving pelvic radiotherapy: an in vitro study.

Temporary metallic intraprostatic stent is a new alternative treatment for patients with urinary obstructive syndrome caused by prostate cancer. Definitive radiotherapy is a treatment of choice for localized prostate cancer. This study evaluates in vitro the effect of a urethral intraprostatic metallic stent on the dose absorbed by the surrounding tissue. The study was designed to mimic the conditions under which the prostatic stent is placed in the body during pelvic irradiation. A urethral stent composed of a 50% nickel-50% titanium alloy (Uracoil-InStent) was imbedded in material mimicking normal tissue (bolus) at a simulated body depth of 10 cm. The distribution of the absorbed dose of irradiation was determined by film dosimetry using Kodak X-Omat V film. Irradiation was done in a single field at the isocenter of a 6 MV linear accelerator with a field size of 7 x 7 cm. The degree of film blackening was in direct proportion to the absorbed dose. The measurements showed an increase in dose of up to 20% immediately before the stent and a decrease of up to 18% immediately after the stent. These changes occurred within a range of 1-3 mm from both sides of the stent. In practice, irradiation in prostate cancer is given by two pairs of opposed co-axial fields; a total of four fields (Box Technique). The dose perturbations are partly cancelled in a pair of opposed beams resulting in a net variation of +/- 4%; therefore, the presence of the intraprostatic stent should not influence radiotherapy planning for prostate cancer.

Reduction of the rectal dose in gynecological brachytherapy: modification to the Fletcher-Suit applicator.

The dose to the anterior rectal wall is a known limiting factor for the delivery of radical doses of radiation to the uterine cervix with brachytherapy. We developed a modification to the Fletcher-Suit afterloading applicator, consisting of two small inflatable balloons attached to the posterior end of each colpostat. The balloons are connected to catheters that emerge from the vagina attached to the colpostat's handles. The balloons were affixed to the colpostats with a plastic adaptor and are inserted empty. After an anterior radiograph is taken, the balloons are filled with radiological contrast material and a lateral orthogonal film is made. This lateral film taken with the balloons filled with contrast typically shows a significant posterior displacement of the anterior rectal wall away from the vaginal sources. The International Commission on Radiation Units (ICRU) rectal point is then determined 5 mm beyond the posterior boundary of the opacified balloons. We have performed 90 applications using this device, including brachytherapy applications for cervical cancer, as well as vaginal applications for endometrial carcinoma following TAH-BSO. On average, the ICRU rectal point was displaced 14 mm away from the colpostats, thus reducing the dose rate by 60% and resulting in an average dose sparing of about 1000 cGy to the anterior rectal wall.

The time-dependent LQ model applied to the matching line of adjacent fields in radiotherapy.

The matching volume of adjacent therapeutic fields that are not given concomitantly is subject to a different fractionation schedule than that within the portals. The time-dependent LQ formalism is applied to this problem in general and to the case of Hodgkin's disease in particular. The fields should overlap but this is clinically infeasible.