Risk of Malignant Transformation of Giant Cell Tumors of Bone Is 8 Times Lower with Megavoltage vs. Orthovoltage Radiation Therapy

Background. The first-line treatment for most giant cell tumors (GCTs) of bone is surgical; radiotherapy (RT) is reserved for inoperable or refractory cases. While RT techniques have undergone a dramatic change over the past few decades, with the higher energy megavoltage RT replacing orthovoltage RT, concerns for high rates of malignant transformation following RT have limited its use. Evidence suggests a lower incidence of secondary malignancy after treatment with megavoltage compared with orthovoltage RT, but this has not been studied in GCTs. Our main purpose was to compare the incidence of malignant transformation of GCTB between patients treated with orthovoltage vs. megavoltage RT. Methods. A literature review was performed to identify studies reporting GCTBs treated with RT from 01/1900 through 12/2019. Studies that did not report RT modality or separate orthovoltage and megavoltage results were excluded. Included in the analysis were 6 patients from our institution. Primary outcome was the incidence of malignant transformation; secondary outcomes were time to transformation and incidence of local recurrence. Fisher’s exact tests and independent sample t-tests were used, and significance was set at p < 0.05 . Results. Twenty-two studies were included, which reported on 168 GCTBs treated with orthovoltage and 393 treated with megavoltage RT. Transformation incidence was 14% (n = 24) for orthovoltage and 1.8% (n = 7) for megavoltage RT, an 8-fold difference (odds ratio (OR) 9.1, 95% confidence interval (CI) 3.9–22, p < 0.001 ). Mean time to transformation was 8.7 years for orthovoltage and 11.2 years for megavoltage RT ( p = 0.28 ). Incidence of local recurrence was 38% (63/167) for orthovoltage and 17% (66/393) for megavoltage RT (OR 3.3, 95% CI 2.0–4.6, p < 0.001 ). Conclusions. The risk of developing a malignancy after RT of GCTB is 8 times lower with megavoltage than with orthovoltage. Malignant transformation with megavoltage, while not zero, is lower than that in historical series. Use of modern RT techniques in inoperable or refractory GCTB may be appropriate.