Portal radiography practice in Canada

Abstract

In recent decades most of the effort directed toward ensuring that a treatment prescription is followed has involved the development of accurate dosimetric standards and computational techniques which, under most situations, permit the absolute radiation dose to be known at point with respect to the beam position to within about 5%. More recently, there has been increasing emphasis placed on confirming that the second component of the prescription is followed, viz that the target volume specified by the physician is that irradiated and, in particular, that neighboring radiosensitive tissues are appropriately shielded. Although quantitative clinical evidence of the consequences of inappropriate field placement is severely limited it is a reasonable axiom that the outcome of a course of radiotherapy is influenced by the accuracy with which the prescribed field is actually treated. Verification of treatment field accuracy has benefitted from many technical advances in the methods of portal imaging. These advances have ranged from the development of new screens for the conventional film/screen approach to the construction of computer based systems which can present quality portal images in real time. Achieving these advances has been technically challenging by itself. However, if these efforts are to be translated into a real benefit for the patient the further challenge of developing effective portal verification programs must be met. In preparation for the development of such programs, it is appropriate to review current practice and, in particular, to attempt to identify those areas which may hinder the effective use of portal imaging in the radiotherapy clinic. To this end, a 7-page questionnaire was prepared and sent to 26 radiotherapy facilities in Canada in August 1989. With a little prompting, 25 centers had responded by the end of November 1989. Of these, 25 one did not use portal radiography at all and hence the observations which follow are based on responses from 24 centers. In this brief report we present the main findings of the survey and discuss their significance for the expanding field of portal verification. On average, 67% or more of patients receiving radical treatment at each center have field placement accuracy checked using portal radiography. With the exception of mantle and inverted Y treatments, less than 67%, on average, of palliative treatments are supplemented with portal films. As participating centers were not requested to furnish absolute patient numbers, the survey cannot yield information on the percentage of treatments of a particular type which was accompanied by portal radiography across the country. Rather, the numbers above refer to the average practice over the centers polled. It was also noted, not surprisingly, that there was a very wide variability in practice between centers. To identify the reasons different groups of patients might receive portal radiography, the questionnaire invited respondents to identify, for each site, one or more of eight alternatives. Mantle and inverted Y treatments which are both highly successful and require intricate field shapes are most frequently checked radiographically on all patients (> 90% of centers). The majority of centers (55%) perform routine portal radiography on all their patients treated with radical intent. Cobalt-60 is still widely used in Radiotherapy but, due to its larger penumbra, produces portal radiographs which are difficult to interpret. This survey did not indicate any significant reluctance to perform portal radiography on 6oCo units in spite of the poor resolution films which result. The frequency with which films were taken on those patients who were radiographed on a routine basis was also covered in the study. For those patients who routinely receive portal radiography at the start of treatment (i.e. within the first three days) there is a clear and reasonable trend to repeat the procedure following shielding modification. However, a measurable number of centers do