Comparing Setup Errors Using Portal Imaging in Patients With Gynecologic Cancers by Two Methods of Alignment

Abstract

Aims: Alignment tattoos on a lax abdomen contribute to misalignment of patients undergoing abdomino-pelvic radiotherapy (RT). The present study was undertaken to assess setup reproducibility in gynecologic cancer patients positioned identically but aligned for treatment to machine isocenter by two different ways.

Materials and methods: A prospective study in 35 women treated with radical RT for gynecologic malignancy was undertaken. A RT planning contrast-enhanced computed tomography scan in the supine position using an foot and ankle positioning device was done, and three reference points tattooed on the reference plane, anteriorly at the mons pubis and one on each side laterally at a fixed table top-to-vertical height of 10 cm, whereas a fourth point was tattooed at the xiphoid in the anterior midline. Patients were aligned using either a field center, that is, conventional method (Arm I, n = 18) or by a new setup isocenter (Arm II, n = 17) defined by a cranial offset of 4 cm to the reference plane for daily treatment. Anterior and right lateral digitally reconstructed radiograph setup fields were created at the treatment isocenters and compared with orthogonal megavoltage portal images (PI) taken during initial 3 days of RT and subsequently twice weekly. Setup deviations-rotations and translations were analysed in mediolateral (ML), craniocaudal, and anteroposterior direction. No online and offline corrections were performed. Population systematic error and random error were calculated and planning target volume margins required were estimated using van Herk's formula.

Results: Arm I had 209 PI while Arm II had 188 PI. Patients in arm II had a lesser systematic error in the ML direction. Patients with large pelvic girth (>95 cm) were susceptible for greater movements during treatment, more so in Arm I, major shifts (>5 mm) with respect to Arm II in the ML direction (37% vs. 22%, P = .001). A larger planning target volume expansion was required in Arm I (1.6 cm) compared with Arm II (0.9 cm). The margin expansion required from clinical target volume in anteroposterior direction was about 0.6 cm and about a cm in the craniocaudal direction in both the arm.

Conclusions: Alignment of patient with anterior tattoo at the relatively immobile portion of lower abdomen (mons pubis) Arm II (setup) is superior to a more cranial location over the flabby abdomen during radiation treatment.

Keywords: Radiotherapy technique; carcinoma cervix; portal imaging; positioning devices; setup errors.