16 Cervix Carcinoma

Cervix Cancer 319

8

Dose Calculation and Treatment Planning

8.1 General introduction Because of the very high dose gradient around the sources used in brachytherapy (about 10% per mm), there were many difficulties in past decades in expressing the dose in intracavitary brachytherapy. Historically, the classical methods of Paris and Stockholm used mg.h to prescribe the dose in a radium implant. With the Manchester system fixed distances related to anatomy and applicator, respectly were used: reference points: A and B. Later on, considering that these different parameters were insufficient to express the dose, the volume concept was developed. Around the world different schools, according to their own experience, tried to express the dose in intracavitary brachytherapy to adapt the dose distribution to the different clinical situations and to be able to compare their experience with each other. The first basis of a “common language” was established at the end of the seventies by the GEC (15). Based on this, some years later recommendations on dose and volume specification for reporting intracavitary therapy in gynecology were published by the ICRU (63). Dose calculation and treatment planning in intracavitary brachytherapy is performed according to different levels. However, independent of the level of dosimetry which is applied, the aim is to achieve the optimal therapeutic effect possible: from mg.h to TRAK; from standard dose distributions (atlas/computer library) and standard dose specification at points to individualisation; from dose adaptation at points to adaptation in volumes; from 2 D to 3 D dose calculation; from radiography based treatment planning to sectional imaging based 3D treatment planning with individual assessment of target volume and critical organs. All over the world, a large amount of intracavitary gynaecological brachytherapy is performed based on standard application techniques and standard dosimetry programs. Standard dosimetry for a given standard geometry includes a defined loading pattern of the vaginal and the intrauterine sources and their relation to each other. This is usually derived from a classical method with a long tradition of defined loading patterns for radium. This loading pattern leads to a pear-shaped distribution when viewed from anterioposterior and a banana-shaped distribution when viewed laterally (Fig 14.10 - 14). The aim of this differential loading has always been to achieve a significant dose in the pear/banana- shaped target, whereas the critical organs nearby (anterior/posterior) have to be spared as much as possible. Therefore, the minimum requirements for dose calculation - independent of the method applied - are the calculation of the dose at one reference point (e.g. at point A, see Fig 14.16) or for more than one reference point (Fig 14.15C,D) and for certain points at critical organs like the ICRU rectum and bladder point (Fig 14.15B). In addition, the height, width and thickness of the brachytherapy volume is indicated which is characterised by the isodose going through point A (Fig 14.16) and related to the dimensions of the GTV and PTV, respectively (height, width and thickness). The lateral dimensions of the pear-shaped dose distribution can be adapted to the lateral dimensions of the GTV and the PTV, respectively. In order to meet these minimum requirements for dosimetry a treatment planning approach based on projection images (radiographs) is necessary, correlating the radiography based data from the applicator and from reference points to isodose dostributions for the given geometry of sources. For this approach, there are precalculated isodose distributions available either in a hard copy atlas or in a “library” of a computerized treatment planning system, which are usually based on the experience of one of the traditional schools.