A new strategy for craniospinal axis localization and adaptive dosimetric evaluation using cone beam CT.

BACKGROUND AND AIM: Computational complexities encountered in craniospinal irradiation (CSI) have been widely investigated with different planning strategies. However, localization of the entire craniospinal axis (CSA) and evaluation of adaptive treatment plans have traditionally been ignored in CSI treatment. In this study, a new strategy for CSI with comprehensive CSA localization and adaptive plan evaluation has been demonstrated using cone beam CT with extended longitudinal field-of-view (CBCTeLFOV). MATERIALS AND METHODS: Multi-scan CBCT images were acquired with fixed longitudinal table translations (with 1 cm cone-beam overlap) and then fused into a single DICOM-set using the custom software coded in MatLab™. A novel approach for validation of CBCTeLFOV was demonstrated by combined geometry of Catphan-504 and Catphan-604 phantoms. To simulate actual treatment scenarios, at first, the end-to-end workflow of CSI with VMAT was investigated using an anthropomorphic phantom and then applied for two patients (based on random selection). RESULTS: The fused CBCTeLFOV images were in excellent agreement with planning CT (pCT). The custom developed software effectively manages spatial misalignments arising out of the uncertainties in treatment/setup geometry. Although the structures mapped from pCT to CBCTeLFOV showed minimal variations, a maximum spatial displacement of up to 1.2 cm (and the mean of 0.8 ± 0.3 cm) was recorded in phantom study. Adaptive plan evaluation of patient paradigms showed the likelihood of under-dosing the craniospinal target. CONCLUSION: Our protocol serves as a guide for precise localization of entire CSA and to ensure adequate dose to the large and complex targets. It can also be adapted for other complex treatment techniques such as total-marrow-irradiation and total-lymphoid-irradiation.

A Hybrid Conformal Planning Technique with Solitary Dynamic Portal for Postmastectomy Radiotherapy with Regional Nodes.

PURPOSE: This study focuses on incorporation of a solitary dynamic portal (SDP) in conformal planning for postmastectomy radiotherapy (PMRT) with nodal regions with an intention to overcome the treatment planning limitations imposed by conventional techniques. MATERIALS AND METHODS: Twenty-four patients who underwent surgical mastectomy followed by PMRT were included in this study. Initially, a treatment plan comprising tangential beams fitted to beam's-eye-view (BEV) of chest wall (CW) and a direct anterior field fitted to BEV of nodal region, both sharing a single isocenter was generated using Eclipse treatment planning system. Multiple field-in-fields with optimum beam weights (5% per field) were added primarily from the medial tangent, fitted to BEV of entire target volume, and finally converted into a dynamic portal. Dosimetric analysis for the treatment plans and fluence verification for the dynamic portals were performed. RESULTS AND DISCUSSION: Conformal plans with SDP showed excellent dose coverage (V95%>95%), higher degree of tumor dose conformity (≤1.25) and homogeneity (≤0.12) without compromising the organ at risk sparing for PMRT with nodal region. Treatment plans with SDP considerably reduced the lower isodose spread to the ipsilateral lung, heart, and healthy tissue without affecting the dose homogeneity. Further, gamma evaluation showed more than 96% pixel pass rate for standard 3%/3 mm dose difference and distance-to-agreement criteria. Moreover, this plan offers less probability of "geometrical miss" at the highly irregular CW with regional nodal radiotherapy. CONCLUSION: Hybrid conformal plans with SDP would facilitate improved dose distribution and reduced uncertainty in delivery and promises to be a suitable treatment option for complex postmastectomy CW with regional nodal irradiation.

Enhancing the longevity of three-dimensional dose in a diffusion-controlled Fricke gel dosimeter.

INTRODUCTION: The principle of Fricke gel dosimeter is the oxidation of ferric ions on exposure to radiation. The major limitation in this dosimeter is the post-irradiation diffusion of ferric ions leading to degradation of spatial dose information. AIMS AND OBJECTIVES: The primary objective of this study is to reduce diffusion of ferric ions post-irradiation and enhance the spatial stability of the dose for an acceptable period, within which it can be read out. MATERIALS AND METHODS: A novel method has been proposed to achieve this aim by incorporation of an anti-oxidant in the present Fricke gel dosimeter. The modified gel prepared in this study consisted of 50 mM sulfuric acid, 0.05 mM xylenol orange, 0.5 mM ferrous ammonium sulfate, and an optimal concentration of anti-oxidant. Different concentrations of the anti-oxidant (ascorbic acid and glycine) based gel dosimeters were prepared. The performance evaluations of the same were characterized dosimetrically with high energy photons (x- and gamma rays). Spectrophotometric measurements of gel dosimeters were performed at a wavelength of 585 nm and the post-irradiation diffusion was studied by observing the dose response over time. The spatial dose information from the large volume cylindrical gel phantoms was acquired using an in-house optical computed tomography scanner. RESULTS: Auto-oxidation and diffusion were controlled in the enhanced Fricke gel dosimeter by the incorporation of glycine as anti-oxidant. The post-irradiation dose in the gel dosimeter was stable up to 6 hours, thereby enhancing the longevity of three-dimensional (3D) dose. CONCLUSION: The widely established limitations of Fricke gel dosimeter viz., auto-oxidation and diffusion were overcome using a novel method that incorporated optimal quantity of glycine as a suitable anti-oxidant. This modified Fricke gel dosimeter could be used as an effective 3D dosimeter for practical applications in radiotherapy.

Determination of dosimetric leaf gap using amorphous silicon electronic portal imaging device and its influence on intensity modulated radiotherapy dose delivery.

As complex treatment techniques such as intensity modulated radiotherapy (IMRT) entail the modeling of rounded leaf-end transmission in the treatment planning system, it is important to accurately determine the dosimetric leaf gap (DLG) value for a precise calculation of dose. The advancements in the application of the electronic portal imaging device (EPID) in quality assurance (QA) and dosimetry have facilitated the determination of DLG in this study. The DLG measurements were performed using both the ionization chamber (DLGion) and EPID (DLGEPID) for sweeping gap fields of different widths. The DLGion values were found to be 1.133 mm and 1.120 mm for perpendicular and parallel orientations of the 0.125 cm(3) ionization chamber, while the corresponding DLGEPID values were 0.843 mm and 0.819 mm, respectively. It was found that the DLG was independent of volume and orientation of the ionization chamber, depth, source to surface distance (SSD), and the rate of dose delivery. Since the patient-specific QA tests showed comparable results between the IMRT plans based on the DLGEPID and DLGion, it is concluded that the EPID can be a suitable alternative in the determination of DLG.