Spot scanning proton beam therapy for prostate cancer: treatment planning technique and analysis of consequences of rotational and translational alignment errors.
OBJECTIVE
Conventional proton therapy with passively scattered beams is used to treat a number of tumor sites, including prostate cancer. Spot scanning proton therapy is a treatment delivery means that improves conformal coverage of the clinical target volume (CTV). Placement of individual spots within a target is dependent on traversed tissue density. Errors in patient alignment perturb dose distributions. Moreover, there is a need for a rational planning approach that can mitigate the dosimetric effect of random alignment errors. We propose a treatment planning approach and then analyze the consequences of various simulated alignment errors on prostate treatments.


METHODS
Ten control patients with localized prostate cancer underwent treatment planning for spot scanning proton therapy. After delineation of the clinical target volume, a scanning target volume (STV) was created to guide dose coverage. Errors in patient alignment in two axes (rotational and yaw) as well as translational errors in the anteroposterior direction were then simulated, and dose to the CTV and normal tissues were reanalyzed.


RESULTS
Coverage of the CTV remained high even in the setting of extreme rotational and yaw misalignments. Changes in the rectum and bladder V45 and V70 were similarly minimal, except in the case of translational errors, where, as a result of opposed lateral beam arrangements, much larger dosimetric perturbations were observed.


CONCLUSIONS
The concept of the STV as applied to spot scanning radiation therapy and as presented in this report leads to robust coverage of the CTV even in the setting of extreme patient misalignments.
Meyer
Jeff
J
University of Texas-M.D. Anderson Cancer Center, Houston, USA. jmeye3@utsouthwestern.edu
Bluett
Jaques
J
Amos
Richard
R
Levy
Larry
L
Choi
Seungtaek
S
Nguyen
Quynh-Nhu
QN
Zhu
X Ron
XR
Gillin
Michael
M
Lee
Andrew
A
D016022:Case-Control Studies; D005270:Femur Head / Q000528:radiation effects; D006801:Humans; D008297:Male; D056888:Patient Positioning / Q000379:methods*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D011522:Protons; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012007:Rectum / Q000528:radiation effects; D012399:Rotation; D014057:Tomography, X-Ray Computed; D047368:Tumor Burden; D001743:Urinary Bladder / Q000528:radiation effects
D011522:Protons