The Manchester system of cervical brachytherapy was one of the most commonly used methods of cervical brachytherapy, till the arrival of computer-based dosimetry. Before a look into the reason why this system was so popular we must look into the history of cervical brachytherapy and dosimetric systems as a whole.
Dosimetric Systems
Any dosimetric system is a set of rules for arrangement of a specific set of radio isotopes in a specialised applicator to deliver a designated dose to a designated point.
Each system that specifies the following:
- Type of radioisotope to be used.
- The geometrical arrangement of radioisotope.
- Specification of the treatment in terms of the dose, time and administration.
- A specified set of tables to allow, reproducible and easy calculation in most of the encountered clinical scenarios.
Older Dosimetric Systems
Before the advent of the Manchester system, to other systems were commonly used: the Paris system and the Stockholm system.a brief description of these systems is given below:
Paris System
In the Paris system of cervical brachytherapy, a single application of radium was specified.the system incorporated two cork colpostats in the form of a cylinder and an intrauterine tube.
The system was designed to deliver a dose of 7000- 8000 mg-hrs of radium over a period of five days.in this system, almost an equal amount of radium was used in the uterus and the vagina.
The intrauterine sources contained three radioactive sources, with source strengths in the ratio of 1:1:0.5. the colpostats harbored sources with the same strength as the topmost uterine source
Stockholm System:
The Stockholm system of cervical brachytherapy was the predecessor of modern-day cervical brachytherapy dosimetric systems. In this system of fractionated course of radiotherapy was delivered and the total course of radiation therapy was delivered over a period of one month. Usually 2-3 applications were used, with each application lasting for a period of 20 to 30 hours.
In this system, intravaginal boxes were used, made up of lead or gold. The intrauterine tube was made up of flexible rubber. Unlike the Paris system, the Stockholm system advocated an unequal loading or of the uterine and the vaginal radium. 30 to 90 mg of radium was placed inside the uterus, while 60 - 80 mg were placed inside the vagina. A total dose of 6500-7100 mg Ra was prescribed and 4500 mg Ra was contributed by the vaginal box.
Common Features
Both the Paris and the Stockholm Systems used intrauterine tubes, which were separate from the vaginal colpostats. Thus these systems had a loose geometry. in addition, the uterine sources in both the systems were arranged from the top of the uterine cavity, down below to the external os. In addition, the longest possible uterine tubes were preferred to allow the highest dose to the paracervical tissue and the pelvic lymph nodes.
Problems with older dosimetric systems
Two basic problems with the dose specification methods of the older dosimetric systems were as follows:
With the use of external-beam radiotherapy with the dose is specified in terms of the absorbed dose, the use of Milligram-hours Radium as a unit in brachytherapy was no longer acceptable.
In addition, dose prescription in this unit ignored the importance of tolerance of different critical organs to radiation. This was because the dose to important anatomical targets could not be quantified adequately with the use of this dose prescription method.
Principles of the Manchester system of cervical brachytherapy
When Todd and Meredith designed the Manchester system of cervical brachytherapy, they had three points in mind:
To define the treatment in terms of dose to a point. According to the authors, this point should have met the following criteria to be acceptable:
- It should have been anatomically comparable from patient to patient. Should have been in a region where the dosage is not highly sensitive to small alteration in applicator position. Should have been in position, so that it allowed correlation of the dose levels with the clinical effects
- To design a set of applicators and their loading (with a given amount of radium), which would give the same dose rate irrespective of the combination of applicators used.
- To formulate a set of rules regarding the activity, relationship and positioning of the radium sources in the uterine tumors and the vaginal ovoids, which will give rise to the desired dose rate.
Actual design of the system
Designation of point A:
When Todd and Meredith, started planning this system they wanted a point, which would satisfy the criteria as stated above. External loss and the mucosa of the vagina vault although being attractive options clinically, where not considered because they were not comparable from patient to patient and because the dose in these regions was highly sensitive to small changes in applicator positions. In addition, in 1938, Todd F showed that the initial lesion of radiation necrosis was not due to the direct effects of radiation on the rectum or the bladder, but due to the high dose effects in the area in the medial edge of the broad ligament, where the uterine vessels cross the ureter. This area is also known as the paracervical triangle. Keeping this triangle in mind, the authors defined a point, which was 2 cm lateral to the center of the uterine canal and 2 cm superior to the mucosa of the lateral fornix, in the plane of the uterus.
