Reconstruction of a ring applicator using CT imaging: impact of the reconstruction method and applicator orientation

The purpose of this study is to investigate whether the method of applicator reconstruction and/or the applicator orientation influence the dose calculation to points around the applicator for brachytherapy of cervical cancer with CT-based treatment planning. A phantom, containing a fixed ring applicator set and six lead pellets representing dose points, was used. The phantom was CT scanned with the ring applicator at four different angles related to the image plane. In each scan the applicator was reconstructed by three methods: (1) direct reconstruction in each image (DR), (2) reconstruction in multiplanar reconstructed images (MPR) and (3) library plans, using pre-defined applicator geometry (LIB). The doses to the lead pellets were calculated. The relative standard deviation (SD) for all reconstruction methods was less than 3.7% in the dose points. The relative SD for the LIB method was significantly lower (p < 0.05) than for the DR and MPR methods for all but two points. All applicator orientations had similar dose calculation reproducibility. Using library plans for applicator reconstruction gives the most reproducible dose calculation. However, with restrictive guidelines for applicator reconstruction the uncertainties for all methods are low compared to other factors influencing the accuracy of brachytherapy.

Computed tomography/magnetic resonance based volume changes of the primary tumour in patients with prostate cancer with or without androgen deprivation

BACKGROUND AND PURPOSE

To evaluate changes of the volume of the cancerous prostatic gland during androgen deprivation (AD) started immediately after diagnosis (IAD). Hypothetically, these data would assist the radiotherapist to determine the appropriate duration of pre-radiotherapy downsizing neoadjuvant luteinizing hormone releasing hormone (LHRH) treatment. A second aim was to assess any increase of the prostatic volume during the 1st year of diagnosis in patients who were allocated to a deferred treatment policy (DAD). METHODS AND MATERIALS Thirteen patients in the IAD cohort and 13 patients in the DAD group, all with T1-3pN1-2M0 prostate cancer, had regular computed tomography/magnetic resonance (CT/MR) examinations during the 1st year after randomization within the EORTC-GU trial 30846. Pre-treatment prostate specific antigen (PSA) values were available in only 12 patients.

RESULTS

In the IAD group the prostate gland decreased with significant difference as compared with the DAD patients (P=0.033). As compared with the pre-treatment situation the prostate gland in the IAD group was reduced in size by 18, 35, and 46% at 1, 6, and 12 months, respectively. In four of six evaluable IAD patients the prostatic volume continued to shrink after achievement of the nadir PSA level (at 3 months). In three of the 13 DAD patients the prostate volume increased by >25% during the 1st 3 months after randomization.

CONCLUSION

If neoadjuvant androgen deprivation is applied before local treatment to downsize the volume of the cancerous prostate gland, our limited data suggest that such treatment should last at least 6 months in order to achieve a maximal effect in the majority of patients. In about 1/4 of untreated patients an increase in the prostate volume by >25% may occur within 3 months of diagnosis. If no AD is given, radiotherapy should start within this period.

Determination of 3D dose distribution from intracavitary brachytherapy of cervical cancer by MRI of irradiated ferrous sulphate gel

BACKGROUND AND PURPOSE

MRI ferrous sulphate gel dosimetry has proven to be a valuable method for assessment of dose delivered in teletherapy. The intention of this study was to investigate ferrous sulphate gel as a possible dosimeter for intracavitary brachytherapy applications.

MATERIALS AND METHODS

A plastic duplicate of a cervix ring applicator set was submerged in Fe2(+)-infused gelatin gel. The gel was subsequently irradiated by a stepwise moving 192Ir source, using automatic afterloading equipment (Microselectron, Nucletron-Oldelft International BV, Veenendaal, The Netherlands). A 3D dose distribution was reconstructed from MR images of the gel.

RESULTS

The gel dose measurements were found to be of the same accuracy as TLD measurements. Isodose curves based on gel dosimetry and isodose curves computed by a dose planning system were generally less than 2 mm apart. MR images showing the position of the applicator set in a patient during treatment were used to obtain images describing patient anatomy in the sagittal and ring planes of the applicator set. Isodose curves computed from the gel measurements were then superimposed on these images, illustrating one possible way of linking dosimetrical and anatomical data.

CONCLUSIONS

Our study shows that MRI ferrous sulphate gel dosimetry is a useful tool for studies of dose distributions in brachytherapy and their relation to critical organs. Possible improvements of the gel dosimeter lie in reducing the diffusion of ferric ions and increasing the radiation sensitivity of the gel.