Practical methods for improving dose distributions in Monte Carlo‐based IMRT planning of lung wall‐seated tumors treated with SBRT

Abstract

Current commercially available planning systems with Monte Carlo (MC)‐based final dose calculation in IMRT planning employ pencil‐beam (PB) algorithms in the optimization process. Consequently, dose coverage for SBRT lung plans can feature cold‐spots at the interface between lung and tumor tissue. For lung wall (LW)‐seated tumors, there can also be hot spots within nearby normal organs (example: ribs). This study evaluated two different practical approaches to limiting cold spots within the target and reducing high doses to surrounding normal organs in MC‐based IMRT planning of LW‐seated tumors. First, “iterative reoptimization”, where the MC calculation (with PB‐based optimization) is initially performed. The resultant cold spot is then contoured and used as a simultaneous boost volume. The MC‐based dose is then recomputed. The second technique uses noncoplanar beam angles with limited path through lung tissue. Both techniques were evaluated against a conventional coplanar beam approach with a single MC calculation. In all techniques the prescription dose was normalized to cover 95% of the PTV. Fifteen SBRT lung cases with LW‐seated tumors were planned. The results from iterative reoptimization showed that conformity index (CI) and/or PTV dose uniformity (UPTV) improved in 12/15 plans. Average improvement was 13%, and 24%, respectively. Nonimproved plans had PTVs near the skin, trachea, and/or very small lung involvement. The maximum dose to 1cc volume (D1cc) of surrounding OARs decreased in 14/15 plans (average 10%). Using noncoplanar beams showed an average improvement of 7% in 10/15 cases and 11% in 5/15 cases for CI and UPTV, respectively. The D1cc was reduced by an average of 6% in 10/15 cases to surrounding OARs. Choice of treatment planning technique did not statistically significantly change lung V5. The results showed that the proposed practical approaches enhance dose conformity in MC‐based IMRT planning of lung tumors treated with SBRT, improving target dose coverage and potentially reducing toxicities to surrounding normal organs.PACS numbers: 87.55.de, 87.55.kh

To gate or not to gate - dosimetric evaluation comparing Gated vs. ITV-based methodologies in stereotactic ablative body radiotherapy (SABR) treatment of lung cancer

“…Thirty-six tumors were located centrally, defined as being within 2 cm of the proximal bronchial tree [ 24 ] or mediastinal structures, and 114 lesions were located peripherally. Peripheral lesions were subdivided into “island” ( n = 57) or lung wall-seated tumors ( n = 57) using definitions given by Altman et al [ 25 ]. Island tumors are enclosed by lung parenchyma, and lung wall-seated tumors abut the lung wall.…”

Characterisation of small photon field outputs in a heterogeneous medium using X-ray voxel Monte Carlo dose calculation algorithm

“…The point dose measurements were carried out with two different detecting systems, CC01 micro chamber with active volume of 0•01 cc and the SRS diode detector (Type 60018) (PTW Freiburg GmbH, Freiburg, Germany) with active volume of 0•3 mm 3 . The effective point of measurements for CC01 and the SRS diode were 1 and 1•3 mm, respectively, from the tip of the detector.…”

“…In addition, the dosimetric uncertainties in a heterogeneous medium attracts a lot of research. [1][2][3][4][5] The majority of the stereotactic treatment planning systems (TPSs) ignore the presence of heterogeneities such as the cranial bone and the air cavities at the skull base and do not include heterogeneity corrections, as the typical beam arrangement that are used compensate the perturbation. Also, there are computation time limitations since the irradiation technique includes multiple arcs and multiple isocenters.…”

A Monte Carlo approach to lung dose calculation in small fields used in intensity modulated radiation therapy and stereotactic body radiation therapy

“…[1][2][3][4][5][6] However, the application of these new modalities with small beams for lung cancers has made the dose calculations more intricate and sophisticated. In general, the main goal of small field irradiation is to provide a desired dose distribution to a fine described small target with the slightest dose to the neighboring normal tissue of the lung.…”

“…In general, the main goal of small field irradiation is to provide a desired dose distribution to a fine described small target with the slightest dose to the neighboring normal tissue of the lung. [1,7] However, in these techniques such as IMRT and SBRT of lung cancers, small fields are used routinely as a part of the series of small beamlets to provide a non-homogenous dose distribution inside planning target volume. The use of small photon fields, normally in the presence of low density heterogeneous substances, may become a complicated scenario to be studied by the treatment planning system (TPS) in order to resolve dose portions.…”