A pre-optimised dosimetry system using a rigid applicator for intracavitary treatment of cervical carcinoma.

AIMS: Tumour control and complication risk have been major concerns in the treatment of cervical carcinoma. A review of dose distribution for intracavitary treatment of cervical carcinoma revealed that modification of the Manchester dosimetry system is necessary for cases of narrow-sized vagina. A revised dosimetry system was introduced in the present study, with the objective of optimising the dose coverage for the parametrium while minimising the bladder and rectum dosage by restricting the rectal dose so as not to exceed 75% of the brachytherapy prescription dose. MATERIALS AND METHODS: A suitable-sized applicator was selected according to the patient's anatomy. The revised system is optimised based on the fixed geometry of the applicator. The system was therefore predefined and the distribution of the treatment dose already determined before application. The revised system was applied to 135 cases, involving 540 applications. The clinical outcome in terms of local tumour control and complication rates is reported. The differences between the revised system and the Manchester system in terms of dose coverage for the parametrium and the rectum dose were compared. RESULTS: The results showed that higher rectal and parametrial dosages were obtained with the Manchester system as compared with the revised system. Our study showed that over 50% of our patients would have received a rectal dose close to 100% of the point A dose if the Manchester system was applied, whereas it was restricted to below 75% using the revised system. Using the revised system, the significance of the parametrial dosage coverage in relation to local control was assessed: the mean dose to the rectum and the bladder as a percentage of point A was 65.7 +/- 5% (range 50-85%) and 66.4 +/- 14% (range 29-116%), respectively. The 5-year actuarial local failure-free survival rates were 90, 92.9, 86.8, 100, 69.7 and 0% for stages IB, IIA, IIB, IIIA, IIIB and IV (P < 0.0001), respectively. The 3-year actuarial complication rates (grade 3/4) for proctitis and cystitis were 1.4 and 0.5%, respectively. The dosage coverage for the parametrium was found to be significant (P = 0.029) in relation to local control for early-stage disease. CONCLUSIONS: The favourable local tumour control and low complication rates shown by our results indicate that the revised system presents an optimal dose distribution, particularly for the application of small ovoids, whereas morbidity was reduced to a lower level without compromising local control.

CT verification of isocentre relocatability using stereotactic mask fixation system.

AIMS: This study introduces a non-invasive method based on computed tomography (CT) verification to ensure patients are accurately positioned before fractionated stereotactic radiotherapy. It enables quality control of mask positioning with reference to the CT images of the treatment plan. MATERIALS AND METHODS: A mask system, together with a dental impression moulded mouth bite, was used for patient immobilisation. In order to facilitate relevant image comparison, special alignment during CT localisation was discussed in the study. The accuracy of patient set-up was studied by assessing the isocentre position in relation to the patient's anatomical structure. The planning CT images were applied as a reference and the study was applied to 261 cranial applications. RESULTS: The results show that the mean and the maximum overall displacements at the isocentre were 0.7 and 2.5 mm, respectively. The mean and the maximum rotational displacement in the axial plane were 0.56 degrees and 2 degrees, respectively. The mean translational displacement and rotational displacement were close to zero when considering the direction of movement. CONCLUSIONS: The results indicate that the systematic error of the mask system and the verification method are minimal. Advantages of this technique include the simple set-up, three-dimensional quantification and short study time (10-15 min). It is therefore practical to implement on a routine basis. Investigation of the ability to relocate the mask is also recommended to justify the required safety margin between the clinical and planning target volumes.