Rib–tumor distance predicts radiation-induced fracture risk
MedWire News: Japanese scientists say that the distance between rib and tumor is the main predictor for radiation-induced rib fracture (RIRF) in lung cancer patients undergoing hypofractionated stereotactic body radiation therapy (SBRT).
The team, led by Yoshiyuki Shioyama (Kyushu University, Fukuoka), found that ribs less than 2 cm away from the tumor were at significant risk for fracture, and that a high dose volume also correlated with an increased risk for RIRF.
The study followed up 116 patients who underwent SBRT for primary (n=97) or metastatic lung cancer between 2003 and 2007. The tumors were a median of 2.4 cm in diameter and a median of 2.0 cm from a rib.
Overall, 28 patients had 46 rib fractures, occurring a median of 22 months after treatment. This gave an estimated 3-year incidence of 37.7%. Rib fractures were symptomatic in 57.1% of cases, with localized pain and neurologic pain the most common markers.
Analysis found that the distance between rib and tumor was the only clinical risk factor to predict RIRF, with a 3-year incidence of 58.1% in patients whose ribs were less than 2.0 cm from their tumor versus 24.4% for those whose ribs were further away. Age, gender, diagnosis, and gross tumor volume were not significantly associated with risk for RIRF.
Examination of the dose-response curve revealed that the maximum dose (Dmax) was significantly higher in fractured than unfractured ribs, as was the absolute volume receiving 20 Gy or more (V20), V30, and V40.
Using a cutoff value determined by receiver operator characteristics analysis, the team demonstrated that the 3-year risk for RIRF was 45.8% versus 1.4% where the Dmax was 42.4 Gy or more versus below this. For V40, a cutoff of 0.29 cm3 or more versus less than 0.29 cm3 was 51.6% versus 2.0%. For V30 and V20, the cutoffs were 1.35 cm3 (45.8 vs 2.2%), and 3.63 cm3 (42.0 vs 8.5%), respectively.
"RIRF was a not uncommon but relatively tolerable late adverse effect after hypofractionated SBRT," the researchers conclude in the International Journal of Radiation Oncology Biology Physics.
"A high dose volume was more strongly correlated with rib fractures than a low dose volume."
They recommend: "To reduce the risk of RIRF, a restriction of the high dose volume of the rib should be considered, provided that coverage of the tumor will not be compromised."
By Lynda Williams