Top Line: Does compromise of PTV coverage to meet organ-at-risk constraints compromise disease control?
The Study: Stereotactic body radiation therapy (SBRT), or SABR, for oligometastatic cancer has significantly improved progression-free and overall survival in multiple disease sites. An important principle used in many trials is the prioritizing of organ-at-risk constraints over target volume coverage. While this has resulted in relatively safe treatment outcomes, how common are target coverage compromises and do they compromise disease control? In this secondary analysis of the SABR-COMET trial, 29.4% of patients required PTV compromise, which was defined as the ratio of the PTV D99 to prescription dose < 0.9 (coverage compromise index, CCI). Among treatment sites, PTV compromise was required for 36.4% of bone metastases, 0% of liver metastases, 18.7% of lung metastases, and 100% of adrenal metastases. Interestingly, The average CCI for adrenal metastases was 0.68. The most common fractionation schemes in SABR-COMET were 35 Gy in 5 fractions (35.6%) and 60 Gy in 8 fractions (17.4%). Per protocol, adrenal lesions were treated with 60 Gy in either 8 or 12 fractions. But the actual mean adrenal Rx dose was 58.6Gy, average D95 was 49.2Gy, and average D99 was 39.7Gy. Despite these differences in target coverage, neither PTV dose nor CCI were associated with survival, lesion control, or toxicity. In other words, prioritizing OAR constraints over PTV coverage when delivering SBRT for oligometastases does not appear to significantly compromise disease control or survival.
TBL: Even though a third of oligometastatic SBRT targets (particularly adrenal and bone targets) may require compromise of target coverage in order to meet OAR constraints, such compromises are not associated with inferior disease control or survival outcomes. | Van Oirschot, Int J Radiat Oncol Biol Phys 2022