2806 - Radiation-Induced Pelvic Insufficiency Fractures and Sacro-Iliac Joint Optimisation in Pelvic External Beam Radiotherapy

E. Herlihy, G. Rangaswamy, J. Nicholson, G. Kennedy, M. Murphy, and O. Boychak; St.Lukes Radiation Oncology Network, Dublin, Ireland

Purpose/Objective(s): Pelvic radiotherapy (RT) is used in the treatment of gynecological and prostate cancers. The incidence of pelvic insufficiency fractures (PIFs) following pelvic RT varies considerably (8.2% - 40%).The majority of these fractures affect the sacroiliac joint (SIJ) and cause significant pain and impairment in quality of life. PIFs are often difficult to diagnose and manage. Studies have shown that reducing the maximum dose (Dmax) from 50Gray (Gy) to 45Gy can reduce the risk of PIFs. This study examined the rationale and potential dosimetric benefits of SIJ avoidance during pelvic RT planning to minimize PIF risk. Materials/

Methods: We retrospectively contoured the SIJ in 30 patients who underwent radical pelvic RT at our institution. Two separate plans were created; one as per standard of care (SOC) and one which was optimized to avoid the SIJ (OP). The right and left SIJ were contoured as separate structures. Coverage of disease (defined as PTV V95%), maximum and mean dose to the SIJ and the volume of the SIJ receiving 40Gy (V40Gy) and 30Gy (V30Gy) were analyzed and compared. The dose to the organs at risk (OARs) was compared.
Results: 20 patients underwent RT for gynecological malignancies and 10 patients underwent prostate RT. The prescribed dose ranged from 45Gy to 78Gy. Tumour coverage was not compromised as result of the optimization of dose to avoid the SIJ in patients who received RT for gynecological tumours (PTV V95% = 93.98% in SOC and 96.61% in OP). However, a reduction in PTV coverage was noted in patients who had prostate RT (PTV V95% = 97.5% vs 92.68%). This may be related to elective nodal irradiation volumes, particularly when extended into the sciatic notch. The dose to the SIJ was significantly reduced in all four endpoints, particularly the V40Gy (SOC SIJ 28.76% vs OP SIJ 5.63%). There was a clinically insignificant increase in the dose to other OARs and any OARs exceeding tolerance did so in both the SOC and SIJ optimized plans. No additional time burden was reported by planners for SIJ optimization.
Conclusion: Risk of PIF is multifactorial including both patient and therapeutic modifiable and non-modifiable risk factors. Literature has shown that a V40 less than 60% can reduce the risk of PIF from 34% to 5%. While the reported incidence varies, the potential consequences necessitate awareness and risk assessment. Contouring the SIJ and optimizing dose to avoid SIJ during RT planning emerges as a promising strategy to reduce PIF risk. However, further research is needed to standardize diagnostic criteria for SIFs, evaluate long-term clinical benefits and safety and investigate the combination of preventative strategies for bone health maintenance after RT. A careful review of compliance with target volume delineation guidelines in both pelvic and prostate nodal radiotherapy will also help in standardizing contouring and reducing the SIJ dose without compromising tumour coverage.