Monte Carlo-Based Dose Calculation in Postprostatectomy Image-Guided Intensity Modulated Radiotherapy: A Pilot Study

Step-and-shoot (S&S) intensity-modulated radiotherapy (IMRT) using the XiO treatment planning system (TPS) has been routinely used for patients receiving postprostatectomy radiotherapy (PPRT). After installing the Monaco, a pilot study was undertaken with five patients to compare XiO with Monaco (V2.03) TPS for PPRT with respect to plan quality for S&S as well as volumetric-modulated arc therapy (VMAT). Monaco S&S showed higher mean clinical target volume (CTV) coverage (99.85%) than both XiO S&S (97.98%, P = 0.04) and Monaco VMAT (99.44, P = 0.02). Rectal V60Gy volumes were lower for Monaco S&S compared to XiO (46.36% versus 58.06%, P = 0.001) and Monaco VMAT (46.36% versus 54.66%, P = 0.02). Rectal V60Gy volume was lowest for Monaco S&S and superior to XiO (mean 19.89% versus 31.25%, P = 0.02). Rectal V60Gy volumes were lower for Monaco VMAT compared to XiO (21.09% versus 31.25%, P = 0.02). Other organ-at-risk (OAR) parameters were comparable between TPSs. Compared to XiO S&S, Monaco S&S plans had fewer segments (78.6 versus 116.8 segments, P = 0.02), lower total monitor units (MU) (677.6 MU versus 770.7 MU, P = 0.01), and shorter beam-on times (5.7 min versus 7.6 min, P = 0.03). This pilot study suggests that Monaco S&S improves CTV coverage, OAR doses, and planning and treatment times for PPRT.

Stereotactic Hypofractionated Irradiation for Metastatic, Inoperable, and Recurrent Malignancies: A Modern Necessity, rather than a Luxury

Stereotactic-irradiation combines highly conformal delivery of radiation to selected volumes at large doses per fraction, with the treatment completed typically within one to five fractions. The radiobiological equivalence of doses delivered by stereotactic-irradiation (often beyond 80–100 Gy) is much higher in comparison to the doses achievable by conventional fractionation. At the high fraction sizes used in stereotactic-irradiation, evidence suggests the role of various radiobiological mechanisms of actions, which are not traditionally relatable with conventional radiotherapy. In spite of the accumulating evidence in favour of the efficacy of stereotactic irradiation in terms of improving local control and at times attaining increments in survival, the clinical adoption of the technique remains dismal. This review provides a brief description of the available evidence describing the benefits of stereotactic-irradiation for the management of patients with oligometastases, unresectable malignancies and for disease recurrence after prior radiotherapy. Given the growing body of evidence illustrating the efficacy of stereotactic irradiation among patients with conditions which were previously often regarded as untreatable, it is likely that the widespread adoption of stereotactic irradiation may achieve cure in a few patients, while in the remainder providing prospects of long term local control. This could be a step in the direction of converting incurable malignancies into chronic controllable diseases.

Accuracy of Dose Delivery in Multiple Breath-Hold Segmented Volumetric Modulated Arc Therapy: A Static Phantom Study

Purpose. Accuracy of dose delivery in multiple breath-hold segmented volumetric modulated arc therapy (VMAT) was evaluated in comparison to noninterrupted VMAT using a static phantom. Material and Methods. Five VMAT plans were evaluated. A Synergy linear accelerator (Elekta AB, Stockholm, Sweden) was employed. A VMAT delivery sequence was divided into multiple segments according to each of the predefined breath-hold periods (10, 15, 20, 30, and 40 seconds). The segmented VMAT delivery was compared to noninterrupted VMAT delivery in terms of the isocenter dose and pass rates of a dose difference of 1% with a dose threshold of 10% of the maximum dose on a central coronal plane using a two-dimensional dosimeter, MatriXX Evolution (IBA Dosimetry, Schwarzenbruck, Germany). Results. Means of the isocenter dose differences were 0.5%, 0.2%, 0.2%, 0.0%, and 0.0% for the beam-on-times between interrupts of 10, 15, 20, 30, and 40 seconds, respectively. Means of the pass rates were 85%, 99.9%, 100%, 100%, and 100% in the same order as the above. Conclusion. Our static phantom study indicated that the multiple breath-hold segmented VMAT maintains stable and accurate dose delivery when the beam-on-time between interrupts is 15 seconds or greater.

FLUKA Monte Carlo for Basic Dosimetric Studies of Dual Energy Medical Linear Accelerator

General purpose Monte Carlo code for simulation of particle transport is used to study the basic dosimetric parameters like percentage depth dose and dose profiles and compared with the experimental measurements from commercial dual energy medical linear accelerator. Varian Clinac iX medical linear accelerator with dual energy photon beams (6 and 15 MV) is simulated using FLUKA. FLAIR is used to visualize and edit the geometry. Experimental measurements are taken for 100 cm source-to-surface (SSD) in 50 × 50 × 50 cm3 PTW water phantom using 0.12 cc cylindrical ionization chamber. Percentage depth dose for standard square field sizes and dose profiles for various depths are studied in detail. The analysis was carried out using ROOT (a DATA analysis frame work developed at CERN) system. Simulation result shows good agreement in percentage depth dose and beam profiles with the experimental measurements for Varian Clinac iX dual energy medical linear accelerator.

Variations in Rectal Volumes and Dosimetry Values Including NTCP due to Interfractional Variability When Administering 2D-Based IG-IMRT for Prostate Cancer

We estimated variations in rectal volumes and dosimetry values including NTCP with interfractional motion during prostate IG-IMRT. Rectal volumes, DVH parameters, and NTCPs of 20 patients were analyzed. For this patient population, the median (range) volume on the initial plan for the rectum was 45.6 cc (31.3–82.0), showing on-treatment spread around the initial prediction based on the initial plan. DVH parameters of on-treatment CBCT analyses showed systematic regularity shift from the prediction based on the initial plan. Using the Lyman-Kutcher-Burman model, NTCPs of predicted late rectal bleeding toxicity of rectal grade ≥ 2 (RTOG) and the QUANTEC update rectal toxicity for the prediction based on the initial plan were 0.09% (0.02–0.24) and 0.02% (0.00–0.07), respectively, with NTCPs from on-treatment CBCT analyses being 0.35% (0.01–6.16) and 0.12% (0.00–4.11), respectively. Using the relative seriality model, for grade ≥ 2 bleeding rectal toxicity, NTCP of the prediction based on the initial plan was 0.64% (0.15–1.22) versus 1.48% (0.18–7.66) for on-treatment CBCT analysis. Interfraction variations in rectal volumes occur in all patients due to physiological changes. Thus, rectal assessment during 2D-based IG-IMRT using NTCP models has the potential to provide useful and practical dosimetric verification.

Decreasing the Dose to the Rectal Wall by Using a Rectal Retractor during Radiotherapy of Prostate Cancer: A Comparative Treatment Planning Study

Aim. The aim of the study was to examine the dosimetric effect of rectal retraction, using a rectal retractor, by performing a comparative treatment planning study. Material and Methods. Treatment plans using volumetric arc therapy (VMAT) were produced for ten patients both with and without rectal retraction. A hypofractionation scheme of 42.7 Gy in seven fractions was used. The dose to the rectal wall was evaluated for both methods (with and without retraction) using four dose-volume criteria: V40.1 Gy, V38.3 Gy, V36.5 Gy, and V32.6 Gy. Results. The retraction of the rectal wall increased the distance between the rectal wall and the prostate. The rectal wall volume was reduced to zero for all dose-volume values except for V32.6 Gy, which was 0.2 cm3 in average when the rectal retractor was used. Conclusion. There was a significant decrease of V40.1 Gy, V38.3 Gy, V36.5 Gy, and V32.6 Gy when the rectal retractor was used without compromising the dose coverage of planning target volume (PTV).

Repeat Whole Brain Radiation Therapy with a Simultaneous Infield Boost: A Novel Technique for Reirradiation

The treatment of patients who experience intracranial progression after whole brain radiation therapy (WBRT) is a clinical challenge. Novel radiation therapy delivery technologies are being applied with the objective of improving tumor and symptom control in these patients. The purpose of this study is to describe the clinical outcomes of the application of a novel technology to deliver repeat WBRT with volume modulated arc therapy (VMAT) and a simultaneous infield boost (WB-SIB) to gross disease. A total of 16 patients were initially treated with WBRT between 2000 and 2008 and then experienced intracranial progression, were treated using repeat WB-SIB, and were analyzed. The median dose for the first course of WBRT was 35 Gy (range: 30–50.4 Gy). Median time between the initial course of WBRT and repeat WB-SIB was 11.3 months. The median dose at reirradiation was 20 Gy to the whole brain with a median boost dose of 30 Gy to gross disease. A total of 2 patients demonstrated radiographic disease progression after treatment. The median overall survival (OS) time from initial diagnosis of brain metastases was 18.9 months (range: 7.1–66.6 (95% CI: 0.8–36.9)). The median OS time after initiation of reirradiation for all patients was 2.7 months (range: 0.46–14.46 (95% CI: 1.3–8.7)). Only 3 patients experienced CTCAE grade 3 fatigue. No other patients experienced any ≥ CTCAE grade 3 toxicity. This analysis reports the result of a novel RT delivery technique for the treatment of patients with recurrent brain metastases. Side effects were manageable and comparable to other conventional repeat WBRT series. Repeat WB-SIB using the VMAT RT delivery technology is feasible and appears to have acceptable short-term acute toxicity. These results may provide a foundation for further exploration of the WB-SIB technique for repeat WBRT in future prospective clinical trials.

Volumetric Modulated Arc Radiotherapy for Early Stage Non-Small-Cell Lung Carcinoma: Is It Better Than the Conventional Static Beam Intensity Modulated Radiotherapy?

This study compared the performance of volumetric modulated arc therapy (VMAT) techniques: single arc volumetric modulated arc therapy (SA-VMAT) and double arc volumetric modulated arc therapy (DA-VMAT) with the static beam conventional intensity modulated radiotherapy (C-IMRT) for non-small-cell lung carcinoma (NSCLC). Twelve stage I and II NSCLC patients were recruited and their planning CT with contoured planning target volume (PTV) and organs at risk (OARs) was used for planning. Using the same dose constraints and planning objectives, the C-IMRT, SA-VMAT, and DA-VMAT plans were optimized. C-IMRT consisted of 7 static beams, while SA-VMAT and DA-VMAT plans consisted of one and two full gantry rotations, respectively. No significant difference was found among the three techniques in target homogeneity and conformity. Mean lung dose in C-IMRT plan was significantly lower than that in DA-VMAT plan P=0.04. The ability of OAR sparing was similar among the three techniques, with no significant difference in V20, V10, or V5 of normal lungs, spinal cord, and heart. Less MUs were required in SA-VMAT and DA-VMAT. Besides, SA-VMAT required the shortest beam on time among the three techniques. In treatment of early stage NSCLC, no significant dosimetric superiority was shown by the VMAT techniques over C-IMRT and DA-VMAT over SA-VMAT.

Assessing Response to Radiation Therapy Treatment of Bone Metastases: Short-Term Followup of Radiation Therapy Treatment of Bone Metastases with Diffusion-Weighted Magnetic Resonance Imaging

This study examined the usefulness of diffusion-weighted (DW) Magnetic Resonance Imaging (MRI) in monitoring bone metastases response to radiation therapy in 15 oligometastatic patients. For each metastasis, both mean apparent diffusion coefficient (ADC) changes and high b-value DW metastasis/muscle signal intensity ratio (SIR) variations were evaluated at 30 ± 5 days and 60 ± 7 days after the end of treatment. On baseline DW-MRI, all bone metastases were hyperintense and had signal intensities higher than normal bone marrow on calculated ADC maps. At follow-up evaluations, 4 patterns of response were identified: (I) decreased high b-value DW SIR associated with increased mean ADC (83.3% of cases); (II) increased mean ADC with no change of high b-value DW SIR (10% of cases); (III) decreased both high b-value DW SIR and mean ADC (3.3% of cases); (IV) a reduction in mean ADC associated with an increase in high b-value DW SIR compared to pretreatment values (3.3% of cases). Patterns (I) and (II) suggested a good response to therapy; pattern (III) was classified as indeterminate, while pattern (IV) was suggestive of disease progression. This pattern approach may represent a useful tool in the differentiation between treatment-induced necrosis and highly cellular residual tumor.

Dosimetric Studies of Mixed Energy Intensity Modulated Radiation Therapy for Prostate Cancer Treatments

Dosimetric studies of mixed field photon beam intensity modulated radiation therapy (IMRT) for prostate cancer using pencil beam (PB) and collapsed cone convolution (CCC) algorithms using Oncentra MasterPlan treatment planning system (v. 4.3) are investigated in this study. Three different plans were generated using 6 MV, 15 MV, and mixed beam (both 6 and 15 MV). Fifteen patients with two sets of plans were generated: one by using PB and the other by using CCC for the same planning parameters and constraints except the beam energy. For each patient’s plan of high energy photons, one set of photoneutron measurements using solid state neutron track detector (SSNTD) was taken for this study. Mean percentage of V66 Gy in the rectum is 18.55±2.8, 14.58±2.1, and 16.77±4.7 for 6 MV, 15 MV, and mixed-energy plans, respectively. Mean percentage of V66 Gy in bladder is 16.54±2.1, 17.42±2.1, and 16.94±41.9 for 6 MV, 15 MV, and mixed-energy plans, respectively. Mixed fields neutron contribution at the beam entrance surface is 45.62% less than at 15 MV photon beam. Our result shows that, with negligible neutron contributions, mixed field IMRT has considerable dosimetric advantage.