A comparison of two systems of patient immobilization for prostate radiotherapy.
BACKGROUND
Reproducibility of different immobilization systems, which may affect set-up errors, remains uncertain. Immobilization systems and their corresponding set-up errors influence the clinical target volume to planning target volume (CTV-PTV) margins and thus may result in undesirable treatment outcomes. This study compared the reproducibility of patient positioning with Hipfix system and whole body alpha cradle with respect to localized prostate cancer and investigated the existing CTV-PTV margins in the clinical oncology departments of two hospitals.


METHODS
Forty sets of data of patients with localized T1-T3 prostate cancer were randomly selected from two regional hospitals, with 20 patients immobilized by a whole-body alpha cradle system and 20 by a thermoplastic Hipfix system. Seven sets of the anterior-posterior (AP), cranial-caudal (CC) and medial-lateral (ML) deviations were collected from each patient. The reproducibility of patient positioning within the two hospitals was compared using a total vector error (TVE) parameter. In addition, CTV-PTV margins were computed using van Herk's formula. The resulting values were compared to the current CTV-PTV margins in both hospitals.


RESULTS
The TVE values were 5.1 and 2.8 mm for the Hipfix and the whole-body alpha cradle systems respectively. TVE associated with the whole-body alpha cradle system was found to be significantly less than the Hipfix system (p < 0.05). The CC axis in the Hipfix system attained the highest frequency of large (23.6%) and serious (7.9%) set-up errors. The calculated CTV to PTV margin was 8.3, 1.9 and 2.3 mm for the Hipfix system, and 2.1, 3.4 and 1.8 mm for the whole body alpha cradle in CC, ML and AP axes respectively. All but one (CC axis using Hipfix) margin calculated did not exceed the corresponding hospital protocol. The whole body alpha cradle system was found to be significantly better than the Hipfix system in terms of reproducibility (p < 0.05), especially in the CC axis.


CONCLUSIONS
The whole body alpha cradle system was more reproducible than the Hipfix system. In particular, the difference in CC axis contributed most to the results and the current CC margin for the Hipfix system might be considered as inadequate.
The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. htpwhite@polyu.edu.hk.
Peter
P
White
Chui Ka
CK
Yee
Lee Chi
LC
Shan
Lee Wai
LW
Chung
Ng Ho
NH
Man
Yik Shing
YS
Cheung
D006801:Humans; D007103:Immobilization / Q000295:instrumentation / Q000379:methods*; D015994:Incidence; D008297:Male; D056888:Patient Positioning / Q000295:instrumentation / Q000379:methods*; D011471:Prostatic Neoplasms / Q000453:epidemiology / Q000473:pathology / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods; D061106:Radiotherapy Setup Errors / Q000706:statistics & numerical data; D020266:Radiotherapy, Conformal / Q000009:adverse effects / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods; D015203:Reproducibility of Results; D012189:Retrospective Studies; D047368:Tumor Burden