Image-guided radiotherapy for cervix cancer: high-tech external beam therapy versus high-tech brachytherapy

Many studies comparing external-beam therapy (EBT) and brachytherapy (BT) are biased because advanced EBT is compared with conventional BT. This study compares high-tech EBT against high-tech BT.

Nine patients were selected with locally advanced cervix cancer, representing typical clinical situations according to initial tumor extension and response after EBT. Patients were treated either with intracavitary, combined interstitial/intracavitary, or complex interstitial BT. Gross tumor volume, high-risk clinical target volume (CTV), intermediate-risk CTV, bladder, rectum, and sigmoid were delineated. Magnetic resonance-guided BT planning was manually optimized with respect to organ dose limits. Margins (3 and 5 mm) were added to BT CTVs to construct planning target volumes (PTVs) for EBT. Inversely planned EBT with photons (IMRT) and protons (IMPT) was challenged to deliver the highest possible doses to PTVs while respecting D(1cc) and D(2cc) limits from BT, assuming the same fractionation (4 x 7 Gy). The D90 for target structures and normal tissue volumes receiving fractionated doses between 3 and 7 Gy were compared.

High-risk CTV doses depended on the clinical situation and radiation quality. If IMRT was limited to D(2cc) and D(1cc) from BT, the D90 for high-risk PTV and intermediate-risk PTV was mostly lower. Volumes receiving 60 Gy (in equivalent dose in 20 Gy fractions) were approximately twice as large for IMRT compared with BT. For IMPT, this volume ratio was lower. Planning target volume doses of IMPT plans with 3-mm margins were comparable to those with BT. Gross tumor volume doses were mostly lower for both IMRT and IMPT.

For benchmarking high-tech EBT, high-tech BT techniques have to be used. For cervix cancer boost treatments, both IMRT and IMPT seem to be inferior to advanced BT.