Advantages of high dose rate remote afterloading systems: Physics or biology

Abstract

Brachytherapy, derived from the Greek words brachys and therapia, has come to describe short distance radiation treatment. The earliest forms of implantation used active sources placed directly by the physician into the tumor volume. With the introduction of megavoltage equipment, there was a pronounced decrease in brachytherapy. However, with the introduction of afterloading procedures, brachytherapy experienced a renaissance. It was not long before remote afterloading equipment was developed. These systems allowed both low dose and high dose treatments to be performed. With the ability to use significantly higher activities, a controversy began which focused on whether the biology of high or low dose rate was the superior form of treatment. Unfortunately, there is still a lack of consensus on what is low dose rate. Is 150 to 200 rads a hour low dose or high dose rate? ICRU report number 38 (6) attempts to establish parameters for dose rate and has set the range for low dose rate at 40 to 200 rads per hour, the mid dose rate at 200 to 1200 rads per hour, and high dose rate at greater than 1200 rads per hour or greater than 20 rads per minute. However, the biological effect of radiation, at least experimentally, shows a steep dose response gradient within the region that is described as low dose rate. Inevitably lost in the conflicting biological arguments on the merits of one dose rate versus the other, are the significant advantages of the newer remote afterloading systems when compared to older equipment systems or manual afterloading. All brachytherapy procedures have as their primary advantage a short distance of treatment from source to prescription point which varies from l/R2 to l/R. This leads to better dose localization and improved sparing of surrounding normal tissues. Remote afterloading also provides for unsurpassed radiation protection for personnel. The newer systems, by allowing a single small source to “dwell” at a site for a calculated point and time when combined with dose optimization software programs, provide a significant further improvement in dose distribution, thus taking us up one further rung on the evolutionary ladder of brachytherapy. Further progress is seen with high dose rate units which have the added advantage of a very short treatment time. Roman et al. (14) in this issue of the International Journal of Radiation Oncology, Biology, Physics report on their use of high dose rate remote afterloading intracavitary therapy in conjunction with external irradiation for carcinoma of the cervix. Their stated advantages for the high dose rate system that they used are: a) that treatment time is very short as compared to low dose rate systems, which in turn leads to the next three of their stated advantages, which are b) treatment is now being performed on an outpatient basis, c) with the short treatment time the implant reproducibility is superior to manual systems, and lastly d) there is increased patient comfort. These are all attributes of the physical characteristics of high dose rate, that of delivering a high dose in a greatly reduced time, but do not touch on the question of its radiation biology. They continue to describe the advantages of their protocol which are equipment-related. The tandem and ovoids after insertion are fixed to the treatment couch, thus immobilizing the applicators to a degree not possible with standard vaginal packing, which resists movements, displacement, and allows for exacting reproducibility. The standard vaginal packing is also performed to physically displace the rectum and bladder as far from the applicator as possible and which is standard for conventional implants. However, after computerized dosimetry is performed, they then simulate their high dose rate pellet configuration by using low activity sources to allow comparison of direct dose rate measurements to that calculated by the computer. They have the ability to reconfigure the