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

Abstract

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

Prospective study of proton‐beam radiation therapy for limited‐stage small cell lung cancer

“…For double‐scattering and uniform‐scanning plans, beam range compensators were designed to account for the properties of the proton beam and range uncertainties by providing proximal and distal margins with respect to each PTV. Blocking was designed to create a lateral margin with respect to each PTV, with margins individualized for each patient according to formulas by Moyers et al For IMPT, single‐field uniform‐dose optimization was used, and beam‐specific PTVs were created similarly to PTVs in the literature . Patients were treated with 2 or 3 beams for all PBT plans.…”

“…Blocking was designed to create a lateral margin with respect to each PTV, with margins individualized for each patient according to formulas by Moyers et al 10 For IMPT, single-field uniform-dose optimization was used, and beam-specific PTVs were created similarly to PTVs in the literature. 11 Patients were treated with 2 or 3 beams for all PBT plans.…”

Evaluation of motion mitigation using abdominal compression in the clinical implementation of pencil beam scanning proton therapy of liver tumors

“…A setup margin of 3 mm was used from iCTV to BSPTV, perpendicular to the beam direction. The setup shifts (3 mm in one direction at once) and calibration errors (±3%) were quadratically summed with motion‐induced treatment margins in water‐equivalent thickness (WET) along the beam direction . Treatment margins to proximal and distal CTV was translated into geometric margins in the BSPTV on the average CT. BSPTVs were created for coplanar beam angles every 10° and reviewed for their volumes and intersection with OARs.…”

“…Beam angle selection is crucial to minimize dose degradation of static PBSPT plans in 4DDD. To ensure target coverage against uncertainty of respiratory motion, beam‐specific planning target volume (BSPTV) have been suggested for Single Field Uniform Dose (SFUD) PBSPT. Such a methodology can also ensure coverage against uncertainties in patient setup and CT calibration and assist in the selection of the beam angles .…”

Technical Note: Monte Carlo methods to comprehensively evaluate the robustness of 4D treatments in proton therapy

“…However, this approximation is violated in proton therapy due to the finite range of charged particles. Adaptation of the PTV has been suggested by integrating range uncertainties along the beam direction within the margin leading to the so‐called beam‐specific PTV . However, the latter gets overall inadequate in the context of multifield intensity modulated proton therapy (IMPT), due to possibly high in‐field dose gradients.…”