Impact of residual setup error on parotid gland dose in intensity-modulated radiation therapy with or without planning organ-at-risk margin.
OBJECTIVE
To estimate the dosimetric impact of residual setup errors on parotid sparing in head-and-neck (H&N) intensity-modulated treatments and to evaluate the effect of employing an PRV (planning organ-at-risk volume) margin for the parotid gland.


METHODS
Ten patients treated for H&N cancer were considered. A nine-beam intensity-modulated radiotherapy (IMRT) was planned for each patient. A second optimization was performed prescribing dose constraint to the PRV of the parotid gland. Systematic setup errors of 2 mm, 3 mm, and 5 mm were simulated. The dose-volume histograms of the shifted and reference plans were compared with regard to mean parotid gland dose (MPD), normal-tissue complication probability (NTCP), and coverage of the clinical target volume (V95% and equivalent uniform dose [EUD]); the sensitivity of parotid sparing on setup error was evaluated with a probability-based approach.


RESULTS
MPD increased by 3.4%/mm and 3.0%/mm for displacements in the craniocaudal and lateral direction and by 0.7%/ mm for displacements in the anterior-posterior direction. The probability to irradiate the parotid with a mean dose > 30 Gy was > 50%, for setup errors in cranial and lateral direction and < 10% in the anterior-posterior direction. The addition of a PRV margin improved parotid sparing, with a relative reduction in NTCP of 14%. The PRV margin compensates for setup errors of 3 mm and 5 mm (MPD < or = 30 Gy in 87% and 60% of cases), without affecting clinical target volume coverage (V95% and EUD variations < 1% and < 1 Gy).


CONCLUSIONS
The parotid gland is more sensitive to craniocaudal and lateral displacements. A setup error of 2 mm guarantees an MPD < or = 30 Gy in most cases, without adding a PRV margin. If greater displacements are expected/accepted, an adequate PRV margin could be used to meet the clinical parotid gland constraint of 30 Gy, without affecting target volume coverage.
Delana
Anna
A
Department of Medical Physics, S. Chiara Hospital, APSS Trento, Trento, Italy. anna_delana@yahoo.it
Menegotti
Loris
L
Bolner
Andrea
A
Tomio
Luigi
L
Valentini
Aldo
A
Lohr
Frank
F
Vanoni
Valentina
V
D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008207:Lymphatic Metastasis / Q000532:radiotherapy; D010039:Otorhinolaryngologic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D010306:Parotid Gland / Q000528:radiation effects*; D011336:Probability; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000209:etiology* / Q000517:prevention & control; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*