Brachytherapy or interventional radiooncology is a method of radiation therapy. It is a method, where a small encapsulated radioactive source is placed near to / in the tumour and therefore delivers high doses directly to the target volume. Organs at risk (OARs) are spared due to the inverse square dose fall-off.
In the past years there was a slight stagnation in the development of techniques for brachytherapy treatment. While external beam radiotherapy became more and more sophisticated, in brachytherapy traditional methods have been still used. Recently, 3D imaging was considered also as the modality for brachytherapy and more precise brachytherapy could expand.
Nowadays, an image guided brachytherapy is state-of-art in many centres.
Integration of imaging methods lead to the dose distribution individually tailored for each patient. Treatment plan optimization is mostly performed manually as an adaptation of a standard loading pattern. Recently, inverse planning approaches have been introduced into brachytherapy. The aim of this doctoral thesis was to analyze inverse planning and to develop concepts how to integrate inverse planning into cervical cancer brachytherapy.
First part of the thesis analyzes the Hybrid Inverse treatment Planning and Optimization (HIPO) algorithm and proposes a workflow how to safely work with this algorithm. The problem of inverse planning generally is that only the dose and volume parameters are taken into account and spatial dose distribution is neglected. This fact can lead to unwanted high dose regions in a normal tissue. A unique implementation of HIPO into the treatment planning system using additional features enabled to create treatment plans similar to the plans resulting from manual optimization and to shape the high dose regions inside the CTV.
In the second part the HIPO algorithm is compared to the Inverse Planning Simulated Annealing (IPSA) algorithm. IPSA is implemented into the commercial treatment planning system. It has no additional features how to control spatial distribution of high dose regions. It is possible to create a dosimetrically acceptable treatment plans with IPSA.
Nevertheless, the size of high dose regions is not acceptable. In comparison to manual treatment planning as well as to HIPO optimization, IPSA lead to the conflicting results concerning high dose regions. To be able to use IPSA for inverse treatment planning in cervical cancer brachytherapy additional tools have to be developed.
The last part of the thesis is engaged with vaginal wall dosimetry. Dose volume constraints for target volume as well as for organs OARs are an input for inverse planning optimization calculation. A dose tolerance of each OAR has to be known to be able to create appropriate constraints.
Until now only bladder, rectum and sigmoid were considered as OARs. The dose limits for vagina don't exist yet because of several uncertainties during assessing of dose to the vagina and vaginal morbidity. To overcome contouring uncertainties a simplified vagina contour was proposed and tested. The analysis showed that this contour is able to detect differences between different applicators as well as between different treatment plans. A prospective study comparing dosimetric results of this model and side effects in the vagina has to be done to prove whether this contour is working or not.
In conclusion, this thesis proved that inverse planning can be used for cervical cancer brachytherapy. An appropriate implementation of the inverse planning algorithm has to be considered to avoid high dose regions. Prior including of a dose volume constrains of vagina as another input parameter for the inverse planning calculation vaginal dose reporting has to be solved. The feasibility of the proposed simplified vagina contour for dose reporting has to be proven with a prospective study.