A High-Precision Method for In Vitro Proton Irradiation

Linear energy transfer optimized proton therapy for rectal cancer

PURPOSE

To evaluate the feasibility and utility of an LET-optimized proton treatment planning algorithm in locally advanced rectal cancer and to assess whether the degree of LET-optimization achieved in clinical plans improves efficacy and toxicity in preclinical models.

MATERIALS AND METHODS

A series of five rectal cancer patients treated with standard 25 fraction clinical proton plans were re-planned using an LET-optimization treatment planning algorithm and evaluated for dosimetric endpoints. LET-optimized plans were generated using an algorithm which iteratively increases the weights of higher LET spots in GTV and lower LET in OARs. Murine and in vitro preclinical models of tumor efficacy and normal tissue toxicity were evaluated using comparable LETd range to that achieved in clinical LET-optimized plans.

RESULTS

LET-optimized proton plans increased dose-averaged LET (LETd) in the GTV and LET-weighted dose in the GTV, and CTV5625cGy V100% coverage. At the same time, LET-optimization also decreased mean LET-weighted dose to bladder and small bowel, as well as small bowel V30Gy(cc) compared to standard proton plans. Optimizing the LETd to a volume of GTV-3 mm further increased LETd compared to total GTV. LET-optimization in preclinical models increased tumor efficacy in colorectal cancer cell lines in vitro and decreased small bowel radiation enteropathy in murine models of normal tissue toxicity.

CONCLUSIONS

LET-optimized proton plans increased LETd in gross tumor while maintaining or improving target coverage and OAR sparing, with acceptable plan robustness. Preclinical models demonstrated that comparable LET-optimization may increase tumor efficacy and decrease normal tissue toxicity in rectal cancer.

Dosimetric Assessment of a High Precision System for Mouse Proton Irradiation to Assess Spinal Cord Toxicity

The uncertainty associated with the relative biological effectiveness (RBE) in proton therapy, particularly near the Bragg peak (BP), has led to the shift towards biological-based treatment planning. Proton RBE uncertainty has recently been reported as a possible cause for brainstem necrosis in pediatric patients treated with proton therapy. Despite this, in vivo studies have been limited due to the complexity of accurate delivery and absolute dosimetry. The purpose of this investigation was to create a precise and efficient method of treating the mouse spinal cord with various portions of the proton Bragg curve and to quantify associated uncertainties for the characterization of proton RBE. Mice were restrained in 3D printed acrylic boxes, shaped to their external contour, with a silicone insert extending down to mold around the mouse. Brass collimators were designed for parallel opposed beams to treat the spinal cord while shielding the brain and upper extremities of the animal. Up to six animals may be accommodated for simultaneous treatment within the restraint system. Two plans were generated targeting the cervical spinal cord, with either the entrance (ENT) or the BP portion of the beam. Dosimetric uncertainty was measured using EBT3 radiochromic film with a dose-averaged linear energy transfer (LETd) correction. Positional uncertainty was assessed by collecting a library of live mouse scans (n = 6 mice, two independent scans per mouse) and comparing the following dosimetric statistics from the mouse cervical spinal cord: Volume receiving 90% of the prescription dose (V90); mean dose to the spinal cord; and LETd. Film analysis results showed the dosimetric uncertainty to be ±1.2% and ±5.4% for the ENT and BP plans, respectively. Preliminary results from the mouse library showed the V90 to be 96.3 ± 4.8% for the BP plan. Positional uncertainty of the ENT plan was not measured due to the inherent robustness of that treatment plan. The proposed high-throughput mouse proton irradiation setup resulted in accurate dose delivery to mouse spinal cords positioned along the ENT and BP. Future directions include adapting the setup to account for weight fluctuations in mice undergoing fractionated irradiation.