What is the annual PDD or TMR and OAF constancy tolerance?

• A. 0% (PDD/TMR)
• B. 2% (PDD/TMR) or 2 mm PDD shift (TSET)
• C. 1% (TBI) or 1 mm PDD shift (TSET)
• D. 1.5% (TBI) or 1.5 mm PDD shift (TSET)

Answer: (C)

Annual constancy tolerances for PDD/TMR and OAF follow the treatment modality, because different setups stress the beam data in different ways. For Total Body Irradiation (TBI), the PDD/TMR profile must stay within about 1% to keep dose uniform over the large, shallow fields used. For Total Skin Electron Therapy (TSET), measurements are taken at very shallow depths, so a small shift in the PDD curve translates to a larger dose change; the tolerance is expressed as a 1 mm PDD shift. So the best answer reflects these modality-specific tolerances: 1% for TBI or 1 mm PDD shift for TSET.