Use of an electron reflector to improve dose uniformity at the vertex during total skin electron therapy.

PURPOSE: The vertex of the scalp is always tangentially irradiated during total skin electron therapy (TSET). This study was conducted to determine the dose distribution at the vertex for a commonly used irradiation technique and to evaluate the use of an electron reflector, positioned above the head, as a means of improving the dose uniformity. METHODS AND MATERIALS: Phantoms, simulating the head of a patient, were irradiated using our standard procedure for TSET. The technique is a six-field irradiation using dual angled electron beams at a treatment distance of 3.6 meters. Vertex dosimetry was performed using ionization methods and film. Measurements were made for an unmodified 6 MeV electron beam and for a 4 MeV beam obtained by placing an acrylic scattering plate in the beam line. Studies were performed to examine the effect of electron scattering on vertex dose when a lead reflector, 50 x 50 cm in area, was positioned above the phantom. RESULTS: The surface dose at the vertex, in the absence of the reflector, was found to be less than 40% of the prescribed skin dose. Use of the lead reflector increased this value to 73% for the 6 MeV beam and 99% for the degraded 4 MeV beam. Significant improvements in depth dose were also observed. The dose enhancement is not strongly dependent on reflector distance or angulation since the reflector acts as a large source of broadly scattered electrons. CONCLUSION: The vertex may be significantly underdosed using standard techniques for total skin electron therapy. Use of an electron reflector improves the dose uniformity at the vertex and may reduce or eliminate the need for supplemental irradiation.

Implementation of total skin electron therapy using an optional high dose rate mode on a conventional linear accelerator.

A technique for total skin electron therapy (TSET) has been implemented using a standard accelerator that has been equipped with an optional special procedures mode to permit high dose-rate therapy with a 6-MeV electron beam. Patients are treated in a standing position using dual angled fields at a source to skin distance of 3.6 m. Dosimetric characteristics of the dual field technique were investigated for the 6-MeV beam as well as for a lower energy beam produced by the introduction of an acrylic beam degrader. A treatment stand, which incorporates the degrader in addition to devices used for patient support and shielding, is described. Acceptable beam uniformity and depth dose have been achieved while maintaining a low level of x-ray contamination. Treatment times are reasonably short since the output of the machine in the high-dose-rate mode is 25 Gy/min at the isocenter. Beam uniformity, dose rate, and x-ray contamination are relatively unaffected by the presence of the beam degrader if it is positioned near the treatment plane. The high dose-rate electron option is a useful treatment mode that provides the advantage of reduced treatment times while retaining proper functioning of all accelerator dosimetry systems and interlocks. Use of a dual field technique permits TSET in a treatment room of standard dimensions. The machine is easily set up for treatment, and patient setup is simplified through use of a customized support system.

Optical properties of normal and diseased human breast tissues in the visible and near infrared.

The optical absorption and scattering coefficients have been determined for specimens of normal and diseased human breast tissues over the range of wavelengths from 500 to 1100 nm. Total attenuation coefficients were measured for thin slices of tissue cut on a microtome. The diffuse reflectance and transmittance were measured for 1.0 mm thick samples of these tissues, using standard integrating sphere techniques. Monte Carlo simulations were performed to derive the scattering and absorption coefficients, as well as the mean cosine of the scattering angle. The results indicate that scatter exceeds absorption by at least two orders of magnitude. Absorption is most significant at wavelengths below 600 nm. The scattering coefficients lie in the range 30-90 mm-1 at 500 nm, and fall smoothly with increasing wavelength to between 10 and 50 mm-1 at 1100 nm. The scattering coefficient for adipose tissue differs, in that it is invariant with wavelength over this spectral range. For all tissues examined, the scattered light is highly forward peaked, with the mean cosine of the scattering angle in the range 0.945-0.985. Systematic differences between the optical properties of some tissue types are demonstrated.

Modification of a standard cobalt-60 unit for total body irradiation at 150 cm SSD.

A cobalt-60 teletherapy unit has been modified to permit total body irradiation (TBI) with a vertical beam in a conventional treatment room. This technique has been implemented at low cost using a few easily made accessories. Removal of the adjustable collimator assembly provides a field 2.3 meters in diameter at 150 cm SSD. A copper flattening filter has been constructed to improve beam uniformity and remove electron contamination. Machine set up time for TBI requires less than 15 minutes and does not affect the routine clinical use of the unit. A dose rate of 32 cGy per minute (midplane) is attainable in a 20 cm thick patient. The dosimetry and technical aspects are presented in this paper.

Total skin electron irradiation for mycosis fungoides: failure analysis and prognostic factors.

From 1970 to 1980, 106 patients with mycosis fungoides received total skin electron irradiation to full tolerance. The majority received 30 Gy of 3 MeV electrons in 12 treatments over three weeks. Eighty-eight patients had received prior therapy. Fifty patients had cutaneous plaques only (T1-2N0), and 56 had more advanced disease. At five years, actuarial survival is 66.7% and disease-free survival 21.4%. The median time to relapse is 12 months; prolonged survival is seen only with complete response. Compared with more advanced stages, T1-2N0 patients have more frequent complete response (96% vs 71%) and better relapse-free survival at five years (32 vs 7%). Of 14 patients with T2 disease in continuous complete remission for from 45-113 months, only one has relapsed. This suggests that cure is possible in up to 26% of patients with T2 disease who achieve complete response. In advanced stages, complete response is more likely with doses over 25 Gy (80 vs 50%). First recurrences were predominently in sites of previous involvement. Death resulted mainly from extracutaneous dissemination or failure to induce remission.

Laser backpointer for use with a radiotherapy simulator.

A radiotherapy simulator with image intensification has been equipped with a laser backpointer which does not need to be removed during fluoroscopic examinations, provides negligible interference with the fluoroscopic image, and results in significant reductions in the time required for mark-up of patients. Accuracy of set-ups may be improved since rechecking of patient positioning has been simplified.