ANALYSIS OF DIFFERENT MIDLINE BLOCKING TECHNIQUES IN COMBINED RADIOTHERAPY FOR CERVIX CANCER USING CT IMAGES

Purpose: In this- study, the combined dose distribution of intracavitary brchytherapy (ICBT) and external beam radiotherapy (EBRT)' was evaulated regarding bladder and rectum dose volume histograms (DVH) and reference point doses with external photon beam for four different midline blocking tecniques were analysed. Material and Method: The treatment plans of three patients who received 60 Gy by LDR ICBT and 50 Gy with 18 MVX photon EBRT were studied. Four midline blocking tecniques were used for dosimetrical analysis and comparison as;1) standart blocks, 2) customized blocks (CB) with %50 of reference isodose blocked, 3)CB with 75% of reference isodose blocked; and 4) step wedge CB according to reference isodose (100% with 5HVL, 75% with 1 HVL). Results: The lowest value of the maximum ARW (anterior rectum wall)) dose was obtained with step wedge blocking while the highest value with standard blocking. Critical organs received lower doses with all three CBTs than standard blocks. When the combination of the external and intracavitary dose distribution was considered CBTs achieved more homogenous dose distribution in paracentral tissues than standard blocking. Conclusion: CBT should be preferred since it gives much more homogenous dose distribution in paracentral tissues and dose decrease in critical organs.

Keywords: intracavitary brachytherapy, external radioterapy, midline block, rectum and bladder dosimetry