Lung cancer -- Treatment

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™.

Empirical methods of beam angle optimization (BAO) are tested against the BAO
that is currently employed in Eclipse treatment planning software. Creating an improved
BAO can decrease the amount of time a dosimetrist spends on making a treatment plan,
improve the treatment quality and enhance the tools an inexperienced dosimetrist can use
to develop planning techniques. Using empirical data created by experienced dosimetrists
from 69 patients treated for lung cancer, the most frequently used gantry angles were
applied to four different regions in each lung to gather an optimal set of fields that could
be used to treat future lung cancer patients. This method, given the moniker FAU BAO,
is compared in 7 plans created with the Eclipse BAO choosing 5 fields and 9 fields. The
results show that the conformality index improved by 30% or 3% when using the 5 and 9
fields. The conformation number was better by 12% from the 5 fields and 9% from the 9
fields. The organs at risk (OAR) were overall more protected to produce fewer
nonstochastic effects from the radiation treatment with the FAU BAO.

The purpose of this research is to determine the feasibility of introducing the Monte Carlo (MC) dose calculation algorithm into the clinical practice. Unlike the Ray Tracing (RT) algorithm, the MC algorithm is not affected by the tissue inhomogeneities, which are significant inside the chest cavity. A retrospective study was completed for 102 plans calculated using both the RT and MC algorithms. The D95 of the PTV was 26% lower for the MC calculation. The first parameter of conformality, as defined as the ratio of the Prescription Isodose Volume to the PTV Volume was on average 1.27 for RT and 0.67 for MC. The results confirm that the RT algorithm significantly overestimates the dosages delivered confirming previous analyses. Correlations indicate that these overestimates are largest for small PTV and/or when the ratio of the volume of lung tissue to the PTV approaches 1.

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).

An improved method is introduced for prediction of local tumor control following lung
stereotactic body radiation therapy (SBRT) for early stage non-small cell lung cancer
(NSCLC) patients using 18F-fluorodeoxyglucose positron emission tomography (18F-FDG
PET). A normalized background-corrected tumor maximum Standard Uptake Value
(SUVcmax) is introduced using the mean uptake of adjacent aorta (SUVref), instead of
the maximum uptake of lung tumor (SUVmax). This method minimizes the variations
associated with SUVmax and objectively demonstrates a strong correlation between the
low SUVcmax (< 2.5-3.0) and local control of post lung SBRT. The false positive rates
of both SUVmax and SUVcmax increase with inclusion of early (<6 months) PET scans,
therefore such inclusion is not recommended for assessing local tumor control of post
lung SBRT.