Ivan Rosenberg

Optimization of radical radiotherapy with beam's eye view techniques for non-small cell lung cancer

The presence of vital and sensitive organs such as the spinal cord, heart, and lungs makes curative radiotherapy of non-small cell lung cancer difficult to implement and necessitates use of oblique portals. Defining the target volumes in oblique portals is very difficult. We now show, for non-small cell lung cancer, how beams eye view-based radiotherapy can be used for accurate delineation of treatment volumes and for avoidance of real or dosimetric geographic misses. Furthermore, the beams eye view-based method enables one to project accurately a 2-dimensional image of 3-dimensional disease extension, especially in oblique fields, thus facilitating the design of accurate customized blocking and avoiding inadvertent blocking of the tumor or unnecessary irradiation of normal tissues. Beams eye view volumetric analysis is helpful for devising a customized treatment plan for each patient. Such customization may minimize local failure, which is one cause of poor results of radiotherapy in this site. Beams eye view-based radiotherapy has the potential of improving local control and hence may improve the survival of patients with non-small-cell lung cancer.

Beams eye view-based photon radiotherapy I

Geographic miss, dosimetric miss (underdosing), and proximity of the tumor to sensitive normal tissues are some of the causes of inadequate radiation dose delivery; this is one of many causes of failure after radiotherapy. In the past decade, computerized tomography (CT)-based treatment planning has helped to overcome some of these problems. Beams eye view (BEV)-based radiotherapy planning is an improvement over CT-based treatment planning that may further increase the therapeutic ratio. Since January 1988, we have treated 198 patients with BEV-based photon radiotherapy. About 40% of our patients treated with radical radiotherapy undergo BEV-based treatment, and about 70% of patients who undergo planning CT in the treatment position receive BEV-based radiotherapy. Our findings are as follows: (a) routine use of BEV-based RT (BEVRT) is possible in a busy radiation oncology department; (b) BEVRT improves geometric coverage of tumors; (c) BEVRT is extremely useful in the design of oblique portals; (d) time commitments for various members of the RT treatment-planning team are reasonable; (e) BEVRT helps individualize RT technique; (f) preliminary data suggest decreased acute toxicity with the use of BEVRT for prostate cancer patients. Whether these advantages will help to improve the outcome (i.e., improve local control and survival) and/or decrease the long-term toxicity is not yet known.

Beam's eye view based prostate treatment planning: Is it useful?

Prostate cancer is a common malignancy often treated with radiation therapy. Treatment optimization may improve local control while reducing acute and long-term complications. We routinely obtained CT scans on prostate cancer patients in treatment position after simulation. We analyzed the impact and implications of using our 3-D Beams Eye View (BEV) capability on field definition and blocking for 12 consecutive patients. Conclusions include: (a) it is necessary to use multiple bony landmarks to align BEV images with simulator films; (b) it is difficult to enter volumes precisely, that is, the exact inferior extent of prostate; (c) Beams Eye View-based plans show more individual variability in field size and position than are allowed for by recommendations in the literature; and (d) in this small series we found no significant correlation between prostate volume and clinical staging. In addition, computerized Beams Eye View capability enables us to do normal tissue dosimetry. We have used Dose Volume Histograms (DVH) to study the impact of Beams Eye View on optimization of dose to the bladder and rectum while adequately treating the prostate, with or without the seminal vesicles. Dose Volume Histograms using Beams Eye View are compared with Dose Volume Histograms using target volumes from the literature. The results will be discussed, as well as the relative advantages of using Beams Eye View for prostate cancer on a routine basis.

Dosimetry of Sr-90 ophthalmic applicators.

Sr-90 ophthalmic applicators are commonly used for the treatment of superficial eye disorders. Although a variety of dosimetric devices such as film, thermoluminescent dosimeters (TLDs), ion chambers, and radiochromic foils have been used to measure the peak dose at the applicator surface, there is no internationally agreed upon calibration procedure. Recently, large discrepancies among calibrations of the same applicator at three institutions have been reported. Here we describe a technique to obtain the peak dose rate at the applicator surface using LiF TLDs. The technique can be used for the calibration of flat as well as curved surface applicators. Results for two flat and three concave applicators are presented. Our measurement of the surface dose rate for one of the flat applicators is compared with those obtained by four other institutions, each using different dosimetric devices.

An anterior appositional electron field technique with a hanging lens block in orbital radiotherapy: A dosimetric study

A technique for orbital radiotherapy is presented consisting of an anterior, appositional electron beam with a hanging lens block. The beam was modified by introducing two 1.6 mm thick plastic spoilers, at about 3 cm and 15 cm from the lens, to boost in-scattering of electrons under the block. The 9 mm diameter, 2 cm long stainless steel cylindrical block was suspended 0.5-1.0 cm above the eye. We performed film, TLD (Thermo Luminescent Dosimetry), and diode dosimetry to determine the dose fill-in behind the lens. The introduction of the spoilers dramatically changed the dose distribution. The maximum dose under the block increased from 66% to 85% of the open field dose. Moreover, the dose to the posterior surface of the globe directly underneath the block, at a depth of 3 cm, increased from 48% to 76% of maximum dose, while the dose to the lens was still below 20%. This is a simple and easily reproducible treatment and is an improvement on a previously described technique. The dose distribution is adequate for cases where the target volume surrounds and is posterior to the globe.

The Use of a Negative Beam to Simulate a Midline Block

The physics behind the use of a negatively weighted beam to calculate the dose distribution under a midline block in computerized treatment planning systems is reviewed. To correctly reproduce the dose under the block, it is necessary and sufficient to know the relative dose at one reference depth on the central axis under the block. If the relative weight of the negative beam is then adjusted to produce agreement at that depth, good agreement can be obtained throughout. Comparisons between calculated and measured dose distributions under a midline block are presented for two therapy beams with 4MV and 6MV nominal energies.

Conceptual design of beryllium targets for the generation of neutron beams for radiation therapy by the (p,n) reaction

A conceptual design is presented showing that, by judiciously choosing the beryllium target thickness and the backstop material, improvements can be achieved in dose-rate, skin sparing, and penetration of neutron beams generated by the same proton accelerator.

Absolute neutron dosimetry: Effects of ionization chamber wall thickness

To assess the effect of ionization chamber wall thickness on absolute neutron absorbed dose determinations, measurements were made of the charge collected by an A-150 tissue-equivalent plastic ionization chamber irradiated by a p(66)Be(49) neutron therapy beam as a function of chamber wall thickness both in air and in four different media: tissue-equivalent solution, water, motor oil, and glycerin. Wall thicknesses ranged from 1 to 31 mm, where isolation of the chamber gas volume from protons originating outside the chamber wall was assured. The in-air measurements compare favorably with earlier buildup measurements performed with an A-150 extrapolation chamber in an A-150 phantom. The in-phantom results may be explained if the effect of charged particles reaching the gas volume from the medium and the wall as well as the differences in neutron attenuation by the wall and the medium displaced by the wall are taken into account. The errors in absolute absorbed dose determination caused by ignoring the above processes are assessed.