A simulation study of irregular respiratory motion and its dosimetric impact on lung tumors.

This study is aimed at providing a dosimetric evaluation of the irregular motion of lung tumors due to variations in patients' respiration. Twenty-three lung cancer patients are retrospectively enrolled in this study. The motion of the patient clinical target volume is simulated and two types of irregularities are defined: characteristic and uncharacteristic motions. Characteristic irregularities are representative of random fluctuations in the observed target motion. Uncharacteristic irregular motion is classified as systematic errors in determination of the target motion during the planning session. Respiratory traces from measurement of patient abdominal motion are also used for the target motion simulations. Characteristic irregular motion was observed to cause minimal changes in target dosimetry with the largest effect of 2.5% ± 0.9% (1σ) reduction in the minimum target dose (D(min)) observed for targets that move 2 cm on average and exhibiting 50% amplitude variations within a session. However, uncharacteristic irregular motion introduced more drastic changes in the clinical target volume (CTV) dose; 4.1% ± 1.7% reduction for 1 cm motion and 9.6% ± 1.7% drop for 2 cm. In simulations with patients' abdominal motion, corresponding changes in target dosimetry were observed to be negligible (<0.1%). Only uncharacteristic irregular motion was identified as a clinically significant source of dosimetric uncertainty.

Conformal prostate treatment planning using a low-energy (6-MV) beam.

A 4-field noncoplanar technique for treatment of prostate cancer developed at the University of Michigan was modified for use with low-energy (6 MV) beams. These modifications include the use of wedges on the 2 anterior inferior-superior oblique fields and adjusting the weights of the oblique and lateral fields appropriately. A margin of 1.5 cm around the physician-defined target region was used to define the blocks on each beam's-eye view. Dose distributions produced using this technique with 6-MV and 24-MV beams were compared visually on several dose planes (transverse and sagittal) and quantitatively by dose volume histograms (target, rectum, and bladder). These comparisons showed insignificant differences between the high-energy and low-energy treatment plans. Much larger differences were observed in comparisons of 2 types of coplanar plans with the noncoplanar setup for the 6-MV photon machine. Rectal doses measured in situ were used to help validate the dose distribution predicted by the treatment planning system for the 6-MV noncoplanar technique.

Calcification in untreated mediastinal lymphoma.

Calcification was demonstrated radiographically within newly diagnosed lymphoma of the mediastinum in two patients. These patients and those in other isolated reports show that radiographic examinations may, rarely, reveal calcification in lymphomatous tissue prior to treatment.