Beam-specific planning volumes for scattered-proton lung radiotherapy

Hypofractionated Proton Therapy with Concurrent Chemotherapy for Locally Advanced Non-Small Cell Lung Cancer: A Phase 1 Trial from the University of Florida and Proton Collaborative Group

PURPOSE

We report the safety data from the first multicenter phase 1 trial investigating the use of hypofractionated proton therapy with concurrent chemotherapy for patients with stage II or III non-small cell lung cancer.

METHODS AND MATERIALS

From 2013 through 2018, patients with newly diagnosed stage II or III non-small cell lung cancer were enrolled in a multicenter phase 1 clinical trial evaluating concurrent chemotherapy with increasing dose-per-fraction proton therapy. This was a stepwise 5 + 2 dose-intensification protocol with the following dose arms: (1) 2.5 GyRBE per fraction to 60 GyRBE; (2) 3.0 GyRBE per fraction to 60 GyRBE; (3) 3.53 GyRBE per fraction to 60.01 GyRBE; and (4) 4.0 GyRBE per fraction to 60 GyRBE. A dose arm was considered tolerable if no radiation therapy-attributable severe adverse event (SAE) occurred within 90 days of treatment among 5 patients enrolled on the arm or if 1 SAE occurred among 7 patients enrolled. Dose constraints to the heart, brachial plexus, and spinal cord were more conservative at higher doses per fraction.

RESULTS

The study closed early because of slow accrual and competing enrollment in NRG 1308 before accrual was met, with no maximum tolerated dose identified. Eighteen patients were treated, including 5 patients on arms 1 and 2, 7 patients on arm 3, and 1 patient on arm 4. Two SAEs occurred among 7 patients treated at 3.53 GyRBE per fraction; however, per outside expert review, both were attributed to chemotherapy and unrelated to radiation therapy.

CONCLUSIONS

Hypofractionated proton therapy delivered at 2.5 to 3.53 GyRBE per fraction to a dose of 60 GyRBE with concurrent chemotherapy has an acceptable toxicity profile. Further exploration of this regimen is warranted on a phase 2 clinical trial.

Robust Planning for a Patient Treated in Decubitus Position with Proton Pencil Beam Scanning Radiotherapy

A challenging case was reported for a patient treated in decubitus position with proton pencil beam scanning. A regular robust plan with the consideration of the uncertainties of translational alignment and range accuracy cannot ensure the target coverage as revealed in two verification computed tomography (CT) scans during the first week of the treatment. The irreproducibility of daily alignment and anatomical variations in such a position is mainly due to patient’s roll. To mitigate the interfractional effect on the target coverage, a novel robust optimization against the patient’s angular setup uncertainties was implemented to improve the plan quality by introducing two artificial CT image sets by rolling the planning CT three degrees in both clockwise and counter-clockwise directions and adding them into robust optimization scenarios, which was shown to be an effective and simple way to mitigate target dose degradation with respect to interfractional variations. This method can be easily generalized and applied to other situations where angular variations in patient’s setup can introduce large dosimetric effects. It is recommended that angularly robust optimization method should be integrated into the treatment planning system as an option particularly for patient’s treatment subject to large angular variations, such as the one in the decubitus position reported here.

Beam-specific planning target volumes incorporating 4D CT for pencil beam scanning proton therapy of thoracic tumors

The purpose of this study is to determine whether organ sparing and target coverage can be simultaneously maintained for pencil beam scanning (PBS) proton therapy treatment of thoracic tumors in the presence of motion, stopping power uncertainties, and patient setup variations. Ten consecutive patients that were previously treated with proton therapy to 66.6/1.8 Gy (RBE) using double scattering (DS) were replanned with PBS. Minimum and maximum intensity images from 4D CT were used to introduce flexible smearing in the determination of the beam specific PTV (BSPTV). Datasets from eight 4D CT phases, using ±3% uncertainty in stopping power and ±3 mm uncertainty in patient setup in each direction, were used to create 8×12×10=960 PBS plans for the evaluation of 10 patients. Plans were normalized to provide identical coverage between DS and PBS. The average lung V20, V5, and mean doses were reduced from 29.0%, 35.0%, and 16.4 Gy with DS to 24.6%, 30.6%, and 14.1 Gy with PBS, respectively. The average heart V30 and V45 were reduced from 10.4% and 7.5% in DS to 8.1% and 5.4% for PBS, respectively. Furthermore, the maximum spinal cord, esophagus, and heart doses were decreased from 37.1 Gy, 71.7 Gy, and 69.2 Gy with DS to 31.3 Gy, 67.9 Gy, and 64.6 Gy with PBS. The conformity index (CI), homogeneity index (HI), and global maximal dose were improved from 3.2, 0.08, 77.4 Gy with DS to 2.8, 0.04, and 72.1 Gy with PBS. All differences are statistically significant, with p‐values <0.05, with the exception of the heart V45 (p=0.146). PBS with BSPTV achieves better organ sparing and improves target coverage using a repainting method for the treatment of thoracic tumors. Incorporating motion‐related uncertainties is essential.

PACS number: 87.55.D