Purpose: To analyze the dose-volume histograms (DVHs) of the tumor volume and organs at risk by CT-based treatment planning compared with conventional radiography-based treatment planning for intracavitary brachytherapy in cancer of the cervix.
Methods and materials: Fifteen consecutive patients with cancer of the cervix (1 IB1, 3 IB2, 7 IIB, 4 IIIB) were treated with plastic CT-compatible HDR intracavitary applicators and underwent postimplant pelvic CT scans with applicators in place. CT-images were transferred to the PLATO treatment planning system. The gross tumor volume (GTV) and organs at risk were digitized. Dwell positions in the uterine tandem and colpostats were identified and registered for each patient. All patients were treated with 6 Gy per fraction to Point A using radiography-based planning. For the CT-based planning, DVHs were performed for the GTV, bladder, rectum, sigmoid colon, and small bowel in the pelvis. The dose delivered to 3% volume of the organs at risk (D3%) was compared with the respective ICRU reference doses.
Results: For stage IB(I), IB2, IIB, and IIIB disease the mean GTV was 20.5 cc, 56.6 cc (54.2-57.2), 63.7 cc (55.4-118.9), and 77.6 cc (49.4-102.9), respectively. The 6 Gy pear-shaped volume (PSV) encompassed an average GTV of 98.5%, 89.5%, 79.5%, and 59.5% for stages IBI, IB2, IIB, and IIIB, respectively. The mean dose for the ICRU bladder point and D3% was 3.72 Gy (1.51-5.53) and 4.74 Gy (1.70-10.10), respectively. The mean dose for the ICRU rectal point and D3% was 3.97 Gy (2.09-5.37) and 3.52 Gy (2.05-4.08), respectively. The D3% for the sigmoid colon was highest (3.88 Gy), followed by the rectum (3.52 Gy), and the small bowel (3.36 Gy).
Conclusion: Radiography-based conventional treatment planning overestimates tumor dose, especially those with more advanced tumors. To correlate DVHs for tumor control, improved tumor imaging is necessary.