Long-term stability of the Leksell Gamma Knife® Perfexion™ patient positioning system (PPS).

PURPOSE: To assess the long-term mechanical stability and accuracy of the patient positioning system (PPS) of the Leksell Gamma Knife(®) Perfexion™ (LGK PFX). METHODS: The mechanical stability of the PPS of the LGK PFX was evaluated using measurements obtained between September 2007 and June 2011. Three methods were employed to measure the deviation of the coincidence of the radiological focus point (RFP) and the PPS calibration center point (CCP). In the first method, the onsite diode test tool with single diode detector was used together with the 4 mm collimator on a daily basis. In the second method, a service diode test tool with three diode detectors was used biannually at the time of the routine preventive maintenance. The test performed with the service diode test tool measured the deviations for all three collimators 4, 8, and 16 mm and also for three different positions of the PPS. The third method employed the conventional film pin-prick method. This test was performed annually for the 4 mm collimator at the time of the routine annual QA. To estimate the effect of the patient weight on the performance of the PPS, the focus precision tests were also conducted with varying weights on the PPS using a set of lead bricks. RESULTS: The average deviations measured from the 641 daily focus precision tests were 0.1 ± 0.1, 0.0 ± 0.0, and 0.0 ± 0.0 mm, respectively, for the 4 mm collimator in the X (left/right of the patient), Y (anterior/posterior of the patient), and Z (superior/inferior of the patient) directions. The average of the total radial deviations as measured during ten semiannual measurements with the service diode test tool were 0.070 ± 0.029, 0.060 ± 0.022, and 0.103 ± 0.028 mm, respectively for the central, long, and short diodes for the 4 mm collimator. Similarly, the average total radial deviations measured during the semiannual measurements for the 4, 8, and 16 mm collimators and using the central diode were 0.070 ± 0.029, 0.097 ± 0.025, 0.159 ± 0.028 mm, respectively. The average values of the deviations as obtained from the five annual film pin-prick tests for the 4 mm collimator were 0.10 ± 0.06, 0.06 ± 0.09, and 0.03 ± 0.03 mm for the X, Y, Z stereotactic directions, respectively. Only a minor change was observed in the total radial deviations of the PPS as a function of the simulated patient weight up to 202 kg on the PPS. CONCLUSIONS: Excellent long-term mechanical stability and high accuracy was observed for the PPS of the LGK PFX. No PPS recalibration or any adjustment in the PPS was needed during the monitored period of time. Similarly, the weight on the PPS did not cause any significant disturbance in the performance of the PPS for up to 202 kg simulated patient weight.

X-ray doses to patients undergoing full-spine radiographic examination.

Patients undergoing full-spine radiographic examinations receive x-ray doses of 115 mrad (1,150 microGy) to 85% of the body's total active bone marrow, 235 to 465 mrad (2,350 to 4,650 microGy) to the breasts, 165 mrad (1,650 microGy) to the embryo and ovaries, and 700 mrad (7,000 microGy) to the thyroid. These doses can be doubled for patients exposed to the x rays before and after treatment.

Single port technique of mantle field treatment in Hodgkin's disease using very high energy x-ray beams.

Treatment pd irradiation of Hodgkin's disease is described indicating the advantage of this energy range and the appropriate dose build-up modifications necessary to achieve a single port irradiation technique.

Variation of percentage depth dose with beam area of 43 MV roentgen ray beam from a betatron.

The central axis percentage depth dose of a 43 MV roentgen ray beam from a betatron, is found to decrease with increasing beam area at depths more than the depth of maximum dose build-up. At depths less than the depth of maximum dose build-up, a reverse trend is seen. This type of variation of central axis percentage depth dose with beam area is found due to the presence of extraneous radiation originating in the field flattening filter (compensator) and the collimator of the betatron.

Biological effects evaluated as a function of patient thickness, beam quality, SSD,and treatment schedule.

Continuing efforts are being made by clinical radiotherapists to evaluate radiationcomplications to normal tissue and organs by specific time-dose parameters. Currently,the NSD concept of Ellis is receiving wide application in the literature in the reporting of radiation complications and normal tissue tolerances. To afford an easy and broad application of the NSD concept to the evaluation of physiological, functional, or structural changes, the authors have evolved mathematical expressions for the calculations of NSD as a function of patient thickness, beam energy, SSD, and treatment schedule involving coplanar field arrangements whether the fields are treated alternately or simultaneously. Several interesting aspects evolving form the concepts of treatment planning interms of the NSD or biological effects indicate that 1) for beam energies above 22 MeV, treatment is more ideally performed by treating only one field per day, since the depth of electronic equiliberium provides more effective sparing of superficial organs andtissues; 2) large-field therapy, such as the total nodal irradiation of Hodgkin'sdisease, can be more effectively treated in terms of tissue sparing by higher energy beamsthan cobalt-60 or 4-MeV for practically all patient dimensions; 3) a new concept ofintegral biological dose,the "gram-ret", is proposed, which represents the quantitation of total biological effect; 4) a series of tables with multiplication factors programmed on a digital computeris presented, which very quickly make available the NSD in any fractionated radiation treatment cycle to any plane of the body as a fuction of the beam energy, SSD, patient thickness, and continuous or split-course therapy schedule.