Room: ePoster Forums
Purpose: For interstitial brachytherapy, the American Brachytherapy Society (ABS) recommends that treatment applicators be checked before each treatment to ensure that the implant has not moved. However, they do not specify tolerance levels for external implant measurements. Here, we investigated tolerance levels for external measurements and planning margins necessary for maintaining adequate target coverage.
Methods: Guided by preliminary evaluations of previous treatments, a planning study was designed. Simple clinical target volumes (CTVs) of identical diameter (2 cm) but varying lengths in the needle-wise direction (1, 2, 3, 4, 5 cm) were created. From these CTVs, planning target volumes (PTVs) were created by adding varying planning margins in the needle-wise direction. For each PTV (20 total), plans were created to represent HDR inpatient interstitial treatments with 9 parallel needle applicators placed symmetrically through all PTVs. Plans were manually optimized for high PTV coverage and PTV conformity. Then, varying shifts (0, 0.1, 0.2, 0.5, 1.0 cm) were imposed on planned dwell positions in the outward direction to represent inter-fraction motion of the applicators. Maintenance of target coverage was evaluated for the shifted dose distributions based on the ABS recommendation of CTV D90 > 100%.
Results: For plans without margins, tolerances for external implant measurements varied depending on target length, with shorter targets requiring stricter tolerances to maintain coverage (1.4-4.7 mm tolerances for all 5 CTVs). Alternatively, assuming a set 0.5 cm tolerance on external measurements, a recipe for margins could be used, with shorter targets requiring larger margins (0.4-4.3 mm margins for all 5 CTVs).
Conclusion: Planning margins and tolerance levels for inter-fraction motion are intrinsically tied to one another, and both relate to target length in the needle-wise direction. Inter-fraction motion has a detrimental impact on target coverage, which should be considered. Future work will evaluate these relationships using patient CTVs.