O8. Validation of margins from setup errors in head and neck radiotherapy: Preliminary results

• Published in: Physica medica 2016-09, Vol.32, p.143-143
• Main Authors: van der Merwe, L, van der Merwe, D.G
• Format: Article
• Language: English
• Subjects: Radiology
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2016.07.016

Aim The aim of this study was to quantify random and systematic setup errors in a population of head and neck cancer patients for the purpose of evaluating departmental positioning and immobilization techniques, verification and treatment protocols, as well as validating the treatment margins used. Methods and materials Random and systematic setup errors in all three principal directions were calculated for two groups of head and neck cancer patients, using record and verify system couch position data. For one group containing 20 patients the positioning and immobilization device system was localized to the treatment couch, and for the other group containing 38 patients it was loose on the treatment couch. Within both groups of patients, some treatment course patient positions were verified online with portal imaging whereas others were verified offline on a conventional radiotherapy simulator. Some courses were also treated with electron fields in addition to the photon fields. Results For the patient group treated with the base plate loose on the treatment table the population Random and Systematic setup errors, all in centimeters, for photon field treatments with online verification are ± 0.15, ± 1.67, ± 4.00 and ± 0.14, ± 1.43, ± 2.85 in the anterior-posterior (AP), mediolateral (ML) and superior-inferior (SI) directions respectively. For photon field treatments verified offline corresponding results are ± 0.10, ± 1.10, ± 3.13 and ± 0.11, ± 1.06, ± 2.55. For electron field treatments corresponding results are ± 0.93, ± 1.32, ± 3.36 and ± 1.54, ± 2.72, ± 2.87. Conclusion Preliminary results indicate that only anterior-posterior (AP) random and systematic setup errors are useful for photon field treatments where the base plate is not localized to the treatment couch. No electron field setup errors appear to be useful. Reasons for this are both the mobility of the base plate on the treatment couch, as well as the mobility of lead stuck onto the electron applicator. Offline and Online systematic setup errors are comparable.