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Dosimetric uncertainty of low-dose-rate prostate brachytherapy

Surmann, Kathrin (2015) Dosimetric uncertainty of low-dose-rate prostate brachytherapy.

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Abstract:Prostate cancer is the most common type of cancer in males. A well established treatment method is low-dose-rate (LDR) brachytherapy. During LDR brachytherapy, radioactive seeds (e.g. Iodine-125, 125I) are permanently implanted in and around the prostate using transrectal ultrasound (US) guidance. Uncertainties in the procedure include for example target contouring, imaging modalities, treatment planning and source strength. The implementation of LDR brachytherapy differs between institutes and a general list of uncertainties should therefore be tailored to the used procedure. The aim of this project was to (1) investigate and quantify the uncertainties of the LDR procedure as implemented at RISO and (2) suggest improvements to the procedure to possibly reduce these uncertainties. Uncertainties due to target contouring and multi-modality image registration were assessed in a multi-observer study. Six observers contoured the prostate, urethra and rectum on US and computed tomography (CT) and performed registrations of C-arm conebeam CT (CBCT) with US and magnetic resonance imaging (MRI) for eleven patients. US contouring alone led to a prostate D90 variability of 9.3% due to a change in radial distance of 1.1 mm. CT contouring had larger variabilities (D90: 10.3% and 1.9 mm in radial distance) and is not recommended for prostate contouring. Registration variabilities (D90, US{CBCT: 3.1% and MRI{CBCT: 2.1%) were significantly smaller than the contouring variabilities. Manual adjustment of the US{CBCT registrations based on seed and urethra locations was necessary and compensated for the poor fiducial marker visibility on US. The source strengths of five (separately packaged) 125I seeds are measured for each patient as part of the standard internal quality assurance. The source strength of the patients treated in 2014 varied within 3.0% of the value specified by the manufacturer. This was in agreement with the requirements for LDR brachytherapy sources like 125I. MRI scans were incorporated in the LDR procedure for seven patients to visualize suspect lesions, the boosts, within the prostate. At the operating theater, previously contoured MRI scans were registered with the pre-implant US scan to transfer the boost structures to the intraoperative US. MRI scans were registered with the post-implant CBCT for the final boost dosimetry. Satisfactory boost dosimetry was achieved for all patients (average V150: 87%, range: 73 - 100). The large pre-implant US slice spacing of 5 mm caused reconstruction artifacts for small boost volumes. For three patients, the pre-implant US slice spacing was reduced to 2.5 mm, which is equal to the post-implant US scan. Boost volume reconstruction improved. A combined prostate D90 uncertainty of 14% was determined for the LDR procedure as implemented at RISO. The dominant uncertainty of the procedure was target contouring on US (9%). Incorporation of MRI scans for LDR patients with preceding external beam radiotherapy is advised. A smaller pre-implant US slice spacing is recommended for all LDR patients.
Item Type:Essay (Master)
RISO-Radiotherapiegroep, Deventer, The Netherlands
Radboud UMCN, Nijmegen, The Netherlands
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Programme:Electrical Engineering MSc (60353)
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