Tumour-Specific Growth Rate as a Potential Predictor of Outcomes in Oligoprogressive Disease Treated With Stereotactic Body Radiotherapy

Aims: Growing data suggest a potential progression-free survival advantage with stereotactic body radiotherapy (SBRT) in oligoprogressive disease (OPD). However, optimal candidates remain uncertain. This study aims to investigate tumour-specific growth rate as a potential predictor of outcomes in OPD. Materials and methods: Patients with <= 5 radiological OPDs were enrolled in a prospective phase II study. SBRT-treated metastases were retrospectively contoured on (1) gross tumour volume (GTV)1-pre-SBRT/baseline computed tomography (CT); (2) GTV2-SBRT planning CT, (3) GTV3-post-SBRT follow-up CT. Specific growth rate for each oligoprogressive lesion (SGR_OP) was calculated according to the literature as (ln(GTVy/GTVx)/t) %/d (t = days). SGR_OP1 was defined as pre-SBRT growth (from SBRT planning CT scan to baseline imaging) and SGR_OP2 as post-SBRT growth (from follow-up CT to planning CT). A high SGR_OP1/2 was defined as one greater than the median SGR_OP1/2 value. The primary endpoint was the impact of SGR1/2 on overall survival (OS) rate, which was estimated using the Kaplan-Meier method and Cox proportional hazards models. Local progression (LP) was progression to the treated lesion, while disease progression (DP) was progression of other nontreated metastases. Cumulative incidence function and Fine-Grey subdistribution hazard models were utilised to estimate progression rates. Results: Thirty-five patients with 55 metastases grouped in gastrointestinal (GI) (40%), genitourinary (GU) (31%), and breast (29%) cancer groups were analysed. The median follow-up was 11.74 months (interquartile range [IQR]: 8.05, 15.65). The median SGR_OP1 and SGR_OP2 value was 0.007 %/d (IQR: 0.004, 0.013) and -0.009 %/d (IQR: -0.014 to 0.002), respectively. Forty-eight percent of patients had high SGR_OP1 (>0.007 %/d). and 50% had high SGR_OP2 (>-0.009 %/d). The 12-month OS rate was 59% (95% confidence interval [CI]: 44.2-78.1), which was significantly lower in the GI group (14% [95% CI: 4-51.5], [P = 0.002]) than in the GU and breast groups. A low SGR_OP1 showed higher rates of OS than high SGR_OP1 (71% vs 47%, P = 0.35). Conclusion: SGR_OP analysis appears to demonstrate a wide range of growth rates within individual cancer group and may allow prediction of patient outcomes independent of histology. Additional validation will be required to confirm if this tool can be used to predict outcomes.