Protons offer reduced bone marrow, small bowel, and urinary bladder exposure for patients receiving neoadjuvant radiotherapy for resectable rectal cancer.
BACKGROUND
To assess the potential benefit of proton therapy (PT) over photon therapy, we compared 3-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiotherapy (IMRT), and PT plans in patients undergoing neoadjuvant chemoradiation for resectable rectal cancer at our institution.


METHODS
Eight consecutive patients with resectable (T2-T3) rectal cancers underwent 3DCRT, IMRT, and 3-dimensional conformal PT treatment planning. Initial target volumes (PTV1) were contoured using the Radiation Therapy Oncology Group anorectal atlas guidelines. Boost target volumes (PTV2) consisted of the gross rectal tumor plus a uniform 2-cm expansion. Plans delivered 45 Gray (Gy) or Cobalt Gray Equivalent (CGE) to the PTV1 and a 5.4-Gy (CGE) boost to the PTV2. Ninety-five percent of the PTVs received 100% of the target dose and 100% of the PTVs received 95% of the target dose. Standard normal-tissue constraints were utilized. Wilcoxon paired t-tests were performed to compare various dosimetric points between the 3 plans for each patient.


RESULTS
All plans met all normal-tissue constraints and were isoeffective in terms of PTV coverage. The proton plans offered significantly reduced median normal-tissue exposure over the 3DCRT and IMRT plans with respect to pelvic bone marrow at the V5Gy, V10Gy, V15Gy, and V20Gy levels and the small bowel space at the V10Gy and V20Gy levels. The proton plans also offered significantly reduced median normal-tissue exposure over the 3DCRT plans with respect to the small bowel at the V30Gy and V40Gy levels and the urinary bladder at the V40Gy level.


CONCLUSIONS
By reducing bone marrow exposure, PT may reduce the acute hematologic toxicity of neoadjuvant chemoradiation and increase the likelihood of uninterrupted chemotherapy delivery. Bone marrow sparing may also facilitate the delivery of salvage chemotherapy for patients who subsequently develop hematogenous metastasis. Reduced small bowel exposure using PT may also reduce toxicity and possibly facilitate the use of more-aggressive chemotherapy with radiotherapy.
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Rovel J
RJ
Colaco
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Romaine Charles
RC
Nichols
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Soon
S
Huh
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Nataliya
N
Getman
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Meng Wei
MW
Ho
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Zuofeng
Z
Li
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Christopher G
CG
Morris
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
William M
WM
Mendenhall
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Nancy P
NP
Mendenhall
University of Florida Proton Therapy Institute, Jacksonville, Florida, USA.
Bradford S
BS
Hoppe
Proton therapy; dosimetry; gastrointestinal cancer; particle therapy; rectal cancer