Adaptive radiotherapy for bladder cancer – a systematic review

Radiotherapy has been offered as a multi-modality treatment for bladder cancer patients. Due to the significant variation of bladder volume observed throughout the course of treatment, large margins in the range of 20 – 30 mm have been used, unnecessarily irradiating a large volume of normal tissue. With the capability of visualizing soft tissue in Cone Beam Computerized Tomography, there is opportunity to modify or to adapt the plan based on the variation observed during the course of treatment for quality improvement. A literature search was conducted in May 2016, with the aim of examining the adaptive strategies that have been developed for bladder cancer and assessing the efficacy in improving treatment quality. Among the 18 identified publications, three adaptive strategies were reported: Plan of the Day, patient-specific planning target volume and daily reoptimization. Overall, any of the adaptive strategies achieved a significant improvement in reducing the irradiated volume compared to the non-adaptive approach, outweighing the additional resource required for its execution. The amount and the type of resource required vary from strategy to strategy, suggesting the need for the individual institution to assess feasibility based on the existing infrastructure in order to identify the most appropriate strategy for implementation

Comparison of 3 image-guided adaptive strategies for bladder locoregional radiotherapy

The objective of this study was to compare the dosimetric differences of a population-based planning target volume (PTV) approach and 3 proposed adaptive strategies: plan of the day (POD), patient-specific PTV (PS-PTV), and daily reoptimization (ReOpt). Bladder patients (n = 10) were planned and treated to 46 Gy in 23 fractions with a full bladder in supine position by the standard strategy using a population-based PTV. For each patient, the adaptive strategy was executed retrospectively as follows: (1) POD—multiple distributions of various PTV sizes were generated, and the appropriate distribution based on the bladder of the day was selected for each fraction; (2) PS-PTV—population-based PTV was used for the first 5 fractions and a new PTV derived using information from these fractions was used to deliver the remaining 18 fractions; and (3) ReOpt—distribution was reoptimized for each fraction based on the bladder of the day. Daily dose was computed on all cone beam computed tomographies (CBCTs) and deformed back to the planning computed tomography (CT) for dose summation afterward. V95_Accu, the volume receiving an accumulated delivered dose of 43.7 Gy (95% prescription dose), was measured for comparison. Mean V95_Accu (cm3) values were 1410 (standard deviation [SD]: 227), 1212 (SD: 186), 1236 (SD: 199), and 1101 (SD: 180) for standard, POD, PS-PTV, and ReOpt, respectively. All adaptive strategies significantly reduced the irradiated volume, with ReOpt demonstrating the greatest reduction compared with the standard (− 25%), followed by PS-PTV (− 16%) and POD (− 12%). The difference in the magnitude of reduction between ReOpt and the other 2 strategies reached statistical significance (p = 0.0006). ReOpt is the best adaptive strategy at reducing the irradiated volume because of its frequent adaptation based on the daily geometry of the bladder. The need to adapt only once renders PS-PTV to be the best alternative adaptive strategy

The Effect of dose grid resolution on dose volume histograms for slender organs at risk during pelvic intensity-modulated radiotherapy

Purpose: There are enduring uncertainties regarding the optimal dose grid resolution for use with pelvic intensity-modulated radiotherapy (IMRT) plans in which the adjacent organs at risk are slender and transect the field edge. Therefore, this study evaluated the effect of dose grid resolution on bladder wall dose-volume histogram (DVH) calculations for prostate IMRT plans. Materials and Methods: The planning computed tomography scans and clinical plans from 15 prostate cancer patients were included in this analysis. For each study computed tomography, the entire inner and outer bladder surfaces were delineated. Nine versions of the clinical plan were created, varying interval between the dose grid calculation points uniformly in three dimensions, whereas all other plan parameters were kept constant. The dose grid increments tested were 1-10 mm. The plans were recalculated and the bladder wall DVH compared against the study benchmark (1 mm grid). Results: All the dose grid increments evaluated resulted in a systematic overestimation of the bladder wall volume receiving low doses and an underestimation of the volume receiving high doses. Grid increments 5.0 mm resulted in mean volume differences greater than 2 cm³. Individual patient analysis revealed that only the 1.5 mm increment resulted in maximum volume differences ≤1 cm³ for every patient across the full length of the DVH curve. Bladder wall thickness ranged from 1.7 to 4.4mm and displayed no correlation with the magnitude of the dose grid effect. Conclusions: For an accurate DVH calculation for bladder wall during IMRT planning for prostate cancer, a 1.5 mm dose grid increment is recommended. This finding was unaffected by a normal range in bladder wall thickness. It is suggested that the application of any new treatment planning technique or organ delineation method be accompanied with an evaluation of optimal dose grid resolution.6 page(s

The Effects of external beam radiotherapy on the normal urinary bladder - a histopathological review

Introduction: This literature review presents information relevant to medical radiation technologists with respect to new knowledge on the function of the urinary bladder. These new insights are also explored in relation to radiation-induced histopathological effects and the symptoms of bladder dysfunction reported after external beam radiotherapy. Methods and Materials: The peer-reviewed scientific literature was examined using various electronic medical search engines with appropriate keywords and MeSH headings. Inclusion criteria comprised English language articles published between 1999 and January 2010, with full manuscript available. A critical review was then performed, synthesizing the information contained in those multiple sources into the following subject categories: normal urinary bladder function (basic review and new knowledge), and effect of fractionated radiotherapy on normal bladder (histopathological changes and symptoms of dysfunction). Findings: Previously considered an inert vessel for urine storage, the urinary bladder is actually a complex system of morphologically different tissues, which play an interconnected role in its physiological functions. Injury or abnormal repair in any of the bladder cell layers results in a multifaceted display of interrelated manifestations of dysfunction. In this complex environment, not only can a single symptom of dysfunction have multiple histopathological causes, but the presence of one symptom may exacerbate the presentation of another. To date, this new knowledge has had little impact on radiotherapy clinical practice because subjective methods of collecting toxicity data prevent the identification of a link between radiotherapy dose and urinary dysfunction. The new understanding of the histopathological cause of radiation-induced symptoms, however, has led to the preclinical investigation of many promising methods to prevent or reduce radiotherapy toxicity.9 page(s