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Patient-specific dosimetric verification methods for IMRT treatments are variable, time-consuming and frequently qualitative, preventing evidence-based reduction in the amount of verification performed. This paper addresses some of these issues by applying a quantitative analysis parameter to the dosimetric verification procedure. Film measurements in different planes were acquired for a series of ten IMRT prostate patients, analysed using the quantitative parameter, and compared to determine the most suitable verification plane. Film and ion chamber verification results for 61 patients were analysed to determine long-term accuracy, reproducibility and stability of the planning and delivery system. The reproducibility of the measurement and analysis system was also studied. The results show that verification results are strongly dependent on the plane chosen, with the coronal plane particularly insensitive to delivery error. Unexpectedly, no correlation could be found between the levels of error in different verification planes. Longer term verification results showed consistent patterns which suggest that the amount of patient-specific verification can be safely reduced, provided proper caution is exercised: an evidence-based model for such reduction is proposed. It is concluded that dose/distance to agreement (e.g., 3%/3 mm) should be used as a criterion of acceptability. Quantitative parameters calculated for a given criterion of acceptability should be adopted in conjunction with displays that show where discrepancies occur. Planning and delivery systems which cannot meet the required standards of accuracy, reproducibility and stability to reduce verification will not be accepted by the radiotherapy community.

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Objectives-To study longitudinal biological monitoring data on urinary and blood cadmium collected in a small cohort of nine workers who had been brazing for several years with solders containing cadmium. Methods-Cadmium was measured by neutron activation analysis in livers and kidneys, and estimates of renal function were carried out in 1983 and 1995. During the intervening period exposure to cadmium was dramatically reduced by local exhaust ventilation control and substitution of the solder containing cadmium. Results-From urinary protein measurements there was evidence within the group of increasing renal tubular damage over the 12 year period, even though exposure to cadmium was dramatically reduced over this period and almost eliminated by 1995. There was no evidence from serum creatinine of decreasing glomerular filtration rate, and the renal tubular handling of calcium, phosphate, or urate had not worsened significantly. Blood and urinary cadmium concentrations reduced significantly over the 12 year period but were still substantial in 1995. Blood cadmium concentrations tended to reflect cadmium body burden in 1995 when exposure had been low for several years, and decreased most significantly during 1983-90. By contrast urinary cadmium concentrations only decreased significantly from about 1990 onwards. Urinary cadmium was not significantly correlated with liver or kidney cadmium concentration in either 1983 or 1995. This may be due to the level of tubular dysfunction in the cohort. Calculated cumulative excretion of cadmium over the 12 year period was substantially greater than the loss of cadmium measured in livers and kidneys and the derived loss in body burden. Reasons for this are discussed. It is possible that in cohorts, where renal damage is apparent, urinary concentrations reflect a substantial component of current exposure rather than stored body losses. Conclusions-The data reinforce the concept that blood cadmium concentrations may not always reflect recent exposure, but may reflect body burden derived from historical exposure depending on the degree of current exposure; and that the decline in urinary and blood cadmium measurements after removal from, or reduction in, exposure will be slow and depend on the historical body burden. (Occup Environ Med 1999;56:553-558)