Parametrial Boost Using Midline Shielding Results in an Unpredictable Dose to Tumor and Organs at Risk in Combined External Beam Radiotherapy and Brachytherapy for Locally Advanced Cervical Cancer

Abstract

Midline-blocked boost (MBB) fields are frequently used in the treatment of locally advanced cervical cancer. The purpose of this study was to evaluate the dose contribution from MBBs to tumor and organs at risk. Six patients with locally advanced cervical cancer (IIB-IIIB) treated with definitive chemoradiotherapy and magnetic resonance imaging (MRI)-guided brachytherapy were analyzed. A three-phase plan was modeled: 45 Gy (1.8 Gy per fraction) four-field box, 9 Gy (1.8 Gy per fraction) MBB (midline-shielded anteroposterior/posteroanterior fields), and intracavitary MRI-guided brachytherapy boost of 28 Gy (7 Gy per fraction). Midline shields 3, 4, and 5 cm wide were simulated for each patient. Brachytherapy and MBB plans were volumetrically summed. The rectum, sigmoid, and bladder minimum dose in the most exposed 2 cm(3) of an organ at risk (D(2 cc)) and high-risk clinical target volume (HR-CTV) and intermediate-risk clinical target volume (IR-CTV) D90 and D100 were evaluated. The intended HR-CTV D90 was 85 Gy or greater, and the intended IR-CTV D90 was greater than 60 Gy. After a 4-cm MBB, HR-CTV D90 remained lower than 85 Gy in all cases (mean, 74 Gy; range, 64-82 Gy). High-risk clinical target volume (85 Gy) coverage increased slightly from 73% (range, 64-82%) to 78% (range, 69-88%). Mean IR-CTV D90 increased from 56 Gy (range, 53-64 Gy) to 62 Gy (range, 59-67 Gy). Intermediate-risk clinical target volume 60-Gy dose coverage increased from 81% (range, 72-96%) to 96% (range, 90-100%). The mean volume irradiated to 85 Gy increased by 14 cm(3) (range, 10-22 cm(3)), whereas the volume irradiated to 60 Gy increased from 276 cm(3) (range, 185-417 cm(3)) to 592 cm(3) (range, 385-807 cm(3)). Bladder, rectum, or sigmoid D(2 cc) increased by more than 50% of the boost dose in 4 of 6 patients. Midline-blocked boosts contribute substantial dose to rectum, sigmoid, and bladder D(2 cc). HR-CTV dose and 85-Gy coverage remain compromised in large tumors despite MBB. IR-CTV 60-Gy coverage improved at the expense of a considerable increase in volume of normal tissue irradiated to 60 Gy.