Dynamics of High Risk Clinical Target Volume reduction during Brachytherapy and impact on its coverage in patients with inoperable cervical cancer

 Margita Pobijakova,  Danijela Scepanovic, Marek Paluga, Marta Fekete, Jozef Mardiak

Abstract:

Cervical cancer is the third most common cancer in women worldwide. Standard of care for patients with node positive or locally advanced tumors >4 cm is definitive radiotherapy and concurrent chemotherapy. Brachytherapy is an integral part of definitive radiotherapy for cervical cancer. The aim of the study was to show a dynamics of High Risk Clinical Target Volume (HR-CTV) reduction during Brachytherapy (BT) as a part of definitive treatment (External Beam Radiotherapy /EBRT/ +/– Chemotherapy /ChT/) depending on the initial Gross Tumor Volume (GTV) and its impact on HR-CTV coverage in patients with inoperable cervical cancer. We analyzed the dosimetric data for BT of 54 patients who have had Three Dimensional Planning of BT (3D BT). The Gross Tumor Volume, HR-CTV and organs at risk (OARs) were contoured on the magnetic resonance imaging (MRI), subsequently on the co-registered MRI images with computed tomography (CT). Point A and ICRU 38 rectal and bladder points were defined on reconstructed CT images. Patients were categorized on the basis of whether the 100% isodose line of the point-A prescription dose encompassed the HR-CTV (1st group) or not (2nd group). The 30cc volume has been determined as a cut-off value, which represented the most acceptable value of intermediate size of volumes. The initial mean value of GTV was 42cc. After completion of EBRT/ChT, the mean GTV was 3.24cc what was 91% reduction rate in relation to the initial value. We followed the dynamics of HR-CTV reduction during BT and have noted its minimal reduction from 24.3cc (mean value) at the time of the first fraction to the 24.1cc before fourth fraction. The mean V100 was 98% and increased with decreasing of the volume size (p=0.0063, Fisher’s exact test). D90 (mean value was 96.3 Gy10) has been correlated with V100 and also, it increased with decreasing of the volume size (p=0.0003). The mean D0.1cc and D2cc of rectum doses were 80 Gy3 and 65.6 Gy3, respectively. The mean ICRU rectal dose for all patients was 67.2 Gy3. The mean D0.1cc (99.5 Gy3), D2cc (79.5 Gy3) and ICRU (75.2 Gy3) of bladder doses were acceptable. Dynamics of HR-CTV reduction during BT was minimal, although, significant reduction of the GTV was achieved after EBRT/ChT. This study revealed that the dose prescription of 7 Gy × 4 fractions to point A was not sufficient indicator for dose coverage of the HR-CTV. However, dosimetric parameters as V100 and D90 were strong indicators for coverage of HR-CTV which was inversely related to the volume of the target and the extension of tumor. However, dosimetric parameters for rectum and bladder (D0.1cc, D2cc and ICRU) did not show dependence on the target volumes.