Megavoltage Energies Research Articles

Radiation therapy alone for stage I non-small cell lung cancer

Purpose : This paper is a retrospective analysis of patients with clinical Stage I non-small cell carcinoma of the lung treated with definitive radiation therapy alone. The results of therapy, patterns of failure and the relationship of technical aspects of the delivery of radiotherapy to outcome are presented. Materials and Methods : From 1980 through 1990, 53 patients with Stage I non-small cell lung cancer were treated with definitive radiation therapy alone at the Radiation Oncology Center of the Mallinckrodt Institute of Radiology and its affiliated hospitals. All patients had a pathologic diagnosis of non-small cell lung cancer and were not candidates for surgical resection because of either patient refusal (10 patients), poor performance status (5 patients), or premorbid medical problems (38 patients). The median age was 73 years. Histologic cell type included squamous (32), adenocarcinoma (11), large cell (4), and unclassified non-small cell (6). Initial tumor size was s 3 cm in 23 patients, between 3 and 5 cm in 13 patients and ≥ 5 cm in 17 patients. Diagnostic staging varied during the study period. All patients had chest X-rays and computed tomography scans of the chest. A majority had liver and bone scans, but only four underwent mediastinoscopy. The radiation therapy was of megavoltage energy in all patients, with a median primary prescription tumor dose of 63.2 Gy. Survival was measured from the date radiation therapy was initiated. Results : The actuarial overall survival rate for the entire group was 19% at 3 years and 6% at 5 years, with a median survival time of 20.9 months. Of the 49 deaths, 35 died of lung cancer; 13 died of intercurrent illness, and one died of pancreatic cancer, which made the actuarial cause-specific survival 33% at 3 years and 13% at 5 years. The actuarial 3-year disease-free survival was 33%. Local primary tumor progression occurred in 22 patients, resulting in a 51% 3-year actuarial freedom from local progression. An additional four patients failed in regional lymph nodes that were included in the original treatment portals. Multivariate analysis found only T stage to be associated with overall survival ( p = .02). However multivariate analysis showed age as a prognostic factor to be approaching statistical significance ( p = .07). Patients under 70 years of age showed an increased survival rate compared to patients over 70 years. Radiation therapy doses ≥ 65 Gy appeared to result in a decreased proportion of patients dying of lung cancer with no apparent increase in either acute or long-term complication rates. Conclusion : Results of definitive radiation therapy for inoperable Stage I non-small cell lung remain inferior to surgical therapy. Potential methods to improve local control with radiotherapy are discussed.

Monte Carlo optimization of metal/phosphor screens at megavoltage energies.

The physics of imaging with metal/phosphor (Gd2O2S:Tb on brass) screens at megavoltage energies has been investigated using Monte Carlo simulation. It has been found that pair production is a significant contributor to energy deposition for Bremsstrahlung beams with energies greater than 6 MV. The effects of different thicknesses of phosphor and metal have been studied, and it is shown that the metal plays a significant role in establishing electronic equilibrium in the phosphor. The transport of optical photons through the phosphor has been modeled, and was found that only 10% to 20% of the light created in the phosphor escapes from the surface, with much of the loss being due to total internal reflection at the surface. Calculated results have been compared with experimental measurements of screen brightness for different phosphor and metal thicknesses. The SNR of a video electronic portal imaging device (VEPID) has been calculated as a function of x-ray and optical photon detection efficiency. The non-Poisson distribution of energy deposition in the phosphor is an important contributor to the SNR. The results of this paper should serve as a useful guide to the engineering design of future electronic portal imaging systems.

The influence of tumor size and pre-treatment staging on outcome following radiation therapy alone for stage I non-small cell lung cancer

From 1970 through 1987, 77 patients with Stage I lung cancer were treated with definitive radiation therapy (RT) alone at the Fox Chase Cancer Center or the Hospital of The University of Pennsylvania. All patients had a pathologic diagnosis of non-small cell lung cancer and were not candidates for surgical resection because of pre-morbid medical problems or patient refusal. The median age was 72 years, although 10 patients were over 80. The histologic cell type was squamous in 44, adenocarcinoma in 15, large cell in 3, adenosquamous in 1, non-small cell in 11, and bronchioli-alveolar in 3. Tumor size was retrievable in 75 patients and 25 were less than or equal to 3 cm, 41 from 3–6 cm, and 9 greater than 6 cm. Diagnostic staging varied during the study period. Twelve patients, evaluated with a CT scan of the chest, including the liver, and a bone scan were classified as having “excellent” staging, 24 patients with conventional tomography, liver-spleen scan and a bone scan had “good” staging, and 41 patients were staged less rigorously. The RT was of megavoltage energy in all patients. The median dose was 60 Gy. The mediastinum was treated in all but eight patients who had poor pulmonary function. Survival was measured from the date of pathologic diagnosis. The actuarial 3-year survival rate of the entire group of patients is 17% with a median survival time of 20 months. Of the 61 deaths, 51 were due to disease and 10 were due to intercurrent disease without evidence of tumor recurrence. The actuarial 3-year disease-specific survival (DSS) was 22%. The 3-year disease-specific survival for patients with tumors less than 3 cm and from 3–6 cm was 30% and 17%, respectively. All nine patients with tumors greater than 6 cm were dead of disease. Local progression occurred in 33 patients, resulting in a 44%, 3-year actuarial freedom from local progression. The median time to local failure was 28 months and there were no local failures after 3 years in the 18 patients eligible for observation beyond this point. Of the patients with “excellent” staging, only 2 of 12 were dead of disease compared with 22 of 24 with “good” staging and 30 of 41 of the remainder. In this large group of Stage I non-small cell lung cancer, thorough pre-treatment staging and smaller tumor size are associated with a more favorable outcome.

Application of a Laplace transform pair model for high‐energy x‐ray spectral reconstruction

A Laplace transform pair model, previously shown to accurately reconstruct x-ray spectra at diagnostic energies, has been applied to megavoltage energy beams. The inverse Laplace transforms of 2-, 6-, and 25-MV attenuation curves were evaluated to determine the energy spectra of these beams. The 2-MV data indicate that the model can reliably reconstruct spectra in the low megavoltage range. Experimental limitations in acquiring the 6-MV transmission data demonstrate the sensitivity of the model to systematic experimental error. The 25-MV data result in a physically realistic approximation of the present spectrum.

Dual energy X-ray beam accelerators in radiation therapy: An overview

The use of high energy megavoltage photon beams alone or in combination with low energy mega voltage X-ray beams and/or electron beams has become a standard practice in the radiation treatment of cancer for selected disease sites. This multi-modality treatment approach has prompted medical accelerator manufacturers to begin producing treatment machines capable of generating both a low energy megavoltage X-ray beam (e.g. 6 MV) and a high energy X-ray beam (18 MV), in addition to a range of electron beam energies. This paper will review the current methods used in accelerators to produce dual energy X-ray beams and multiple energy electron beam modalities. This overview will also review several of the physical characteristics of these dual energy units.

A diagnostic X ray field verification device for a 10 MV linear accelerator

The quality of portal films taken at megavoltage energies is frequently poor, despite the use of various techniques such as lead screens to improve the image. When critical structures, such as the spinal cord, are to be blocked out of the treatment field, it is frequently difficult to determine whether the block is correctly placed. To solve this problem, a diagnostic X ray source has been mounted on the side of a 10 MV linear accelerator to provide accurate verification of patient positioning and location of shielding blocks. The coincidence between the mechanical, 10 MV and diagnostic X ray isocenters is about 1 mm. The system has been designed so that procedures to monitor this coincidence, and adjustment procedures to maintain it, are easily performed.