Proton beams

In proton therapy systems with pencil-beam scanning, output of Halo effect is not necessarily included in Treatment Planning System (TPS). Halo effect (low-intensity tail) can significantly affect a patient’s dose distribution. The output of this dose depends on the field size being irradiated. Although much research has been made to investigate such relation to the field size, the number of reports on dose calculations including the halo effect is small. In this work we have investigated the Halo effect, including field size factor, target depth factor, and air gaps with a range shifter for a Varian ProBeam.
Dose calculations created on the Eclipse Treatment Planning System (vs15.6 TPS) are compared with plane-parallel ionization chambers (PTW Octavius 1500) measurements using PCS and AcurosPT MC model in different isocenters: 5cm, 10cm, and 20cm. We find that in AcurosPT algorithm deviations range between -7.53% (for 2cm field in 25cm air gap with range shifter) up to +7.40% (for 20cm field in 15cm air gap with range shifter). Whereas, in PCS algorithm the deviations are -2.07% (for 20x20cm field in open conditions) to -6.29% (for 20x20cm field in 25cm air gap with range shifter).

Monte Carlo (MC) and Pencil Beam (PB) calculations are compared to their measured planar dose distributions using a 2-D diode array for lung Stereotactic Body Radiation Therapy (SBRT). The planar dose distributions were studied for two different phantom types: an in-house heterogeneous phantom and a homogeneous phantom. The motivation is to mimic the human anatomy during a lung SBRT treatment and incorporate heterogeneities into the pre-treatment Quality Assurance process, where measured and calculated planar dose distributions are compared before the radiation treatment. Individual and combined field dosimetry has been performed for both fixed gantry angle (anterior to posterior) and planned gantry angle delivery. A gamma analysis has been performed for all beam arrangements. The measurements were obtained using the 2-D diode array MapCHECK 2™.

Evaluation of dose optimization using the Pencil Beam (PB) and Monte Carlo (MC) algorithms may allow physicists to apply dosimetric offsets to account for inaccuracies of the PB algorithm for lung cancer treatment with Stereotactic Body Radiotherapy (SBRT). 20 cases of Non-Small Cell Lung Cancer (NSCLC) were selected. Treatment plans were created with Brainlab iPlanDose® 4.1.2. The D97 of the Planning Target Volume (PTV) was normalized to 50 Gy on the Average Intensity Projection (AIP) using the fast PB and compared with MC. This exact plan with the same beam Monitor Units (MUs) was recalculated over each respiratory phase. The results show that the PB algorithm has a 2.3-2.4% less overestimation at the maximum exhalation phase than the maximum inhalation phase when compared to MC. Significantly smaller dose difference between PB and MC is also shown in plans for peripheral lesions (7.7 ± 0.7%) versus central lesions (12.7±0.8%)(p< 0.01).