Patient-specifc quality assurance for helical tomotherapy An evaluation of two different detector systems

Intensity-modulated radiation therapy (IMRT) has during the recent years become a frequently used technique in radiotherapy. Lately IMRT has also been combined with rotational beam delivery, with a simultaneously moving couch, a technique called tomotherapy. The great benefit with this treatment method is the ability to give highly conformal treatments, with high dose to the tumour, while sparing organs at risk. Patient-specic quality assurance (QA) of thegenerated tomotherapy treatment plan is essential to ensure that the patient will receive the planned dose. During the last years the use of electronic array dosimeters in the QA process has increased substantially, since they provide almost instantaneous readout in terms of absolute dose distribution. In the present work one such array dosimeter has been implemented, namely the Delta4® (ScandiDos AB, Uppsala, Sweden). The Delta4® has subsequently been compared with a two-dimensional array dosimeter called MapCHECK 2 (Sun Nuclear Corporation, Melbourne,USA), that is already in clinical use at Skåne University Hospital in Lund. The gamma evaluation method has been used for comparison of dose distributions. 25 patient plans were recalculated on CT-scans of the phantoms that constituted the two dosimetry systems. Delivery QA plans for pretreatment evaluation were then created and measured with the two dosimetry systems respectively. The pass rate i.e. the percentage of diodes passing the gamma evaluation was calculated for each plan for dierent gamma criteria and thresholds. The pass rate was then used to compare the dosimetry systems with each other. The results from the measurements showed a much poorer agreement between planned- and measured dose distribution than expected for the MapCHECK 2 system whereas the Delta 4 system showed a more decent agreement. Even though the Delta4 system proved to be superior the MapCHEK 2 it showed on a noteworthy weakness in dose difference agreement, often below 85 % for a 3 % dose difference criterion. Though on the other hand the Delta4 showed an almost perfect agreement in distance-to-agreement, which keep the pass rate of the gamma evaluation on a still acceptable level. For the MapCHECK 2 an alternative way of calculating the gamma evaluation (using Van Dyk percent difference) was also evaluated which gave a gamma pass rate comparable with that for the Delta4. The Van Dyk percent difference is however highly controversial, since it does not reect local dose differences which might be more relevant for organs at risk. The principle conclusion of this thesis is that the Delta4 system is superior the MapCHECK 2 system in validating treatment plans for tomotherapy. It should however be stressed that the Delta4 system, in its current state, is not the optimal system for patient-specic QA since the agreement in dose difference is somewhat low