Purpose: Head-First Supine (HFS) patient orientation is widely utilized for certain patients undergoing radiotherapy. The orientation is consistent with what is conventional knowledge on patient anatomy and laterality, therefore our departmental policy is to default to this orientation. Nevertheless, there are sites that may need a different orientation due to limitations on the system. For treatment on a LINAC, we determined the appropriate orientation, HFS versus Feet-First Supine (FFS) to utilize based on simple prior measurable patient and LINAC properties.
Methods: The study was based on 4 Varian TrueBeams and 2 Varian 21Ex LINACs across two Institutions. For each LINAC we measured the maximum extent of the couch towards the gantry from the LINAC isocenter (denoted L). For the patient, the measurement of interest was the distance from the patient’s planning isocenter to the top of the patient’s head (denoted P). Then the simple rule of thumb when deciding between HFS or FFS for certain patients is that if (P + M) is less than or equal to L then scan and treat the patient using HFS otherwise use FFS. Herein M is a margin that accounts for generalized uncertainties.
Results: We used an M of 5cm. The measured L for the TrueBeams(TB) were 91cm(TB1), 92cm(TB2), 93cm(TB3) and 93.8cm(TB4) while those for the 21Ex were 88cm(V21Ex-1) and 92.5cm(V21Ex-2). Therefore when the 5cm margin is taken into account, the simple rule of thumb when deciding between HFS or FFS is that if P is less than or equal to 86cm(TB1), 87cm(TB2), 88cm(TB3), 88.8cm(TB4), 83cm(V21Ex-1) and 87.5cm(V21Ex-2) then scan and treat the patient using the HFS otherwise use the FFS on the indicated LINACs.
Conclusion: A simple rule of thumb has been presented that can be used to decide between an HFS or FFS based on prior measurable patient and LINAC properties.