Improving organ at risk sparing in oropharyngeal treatment planning by increasing target dose heterogeneity: A feasibility study.

Target dose homogeneity has historically been a priority in radiotherapy treatment planning. However, in an era of more advanced modulated techniques, there is now greater flexibility in shaping dose distributions suggesting that allowing controlled target dose heterogeneity may consequently improve organ at risk (OAR) sparing. This study sought to determine the feasibility of allowing an increase in target dose heterogeneity in oropharyngeal VMAT plans, and to examine the dosimetric impact this has on target coverage and OARs such as the parotid glands, spinal cord, brainstem and mandible. Nineteen oropharyngeal patients' plans were created with homogeneous dose distributions specified in the London Cancer Head and Neck Radiotherapy Protocol. The upper dose constraint (UDC) objective of the primary planning target volumes (PTV) for each plan were increased in increments of 10% until a maximum of 150% of the prescribed dose was reached. These plans were dosimetrically compared to plans with a uniform dose distribution in terms of OAR sparing and target coverage. Minimal coverage was not compromised, with the largest median changes being a 0.81% decrease [98.6 to 97.8%] to the PTV_70Gy D98% and a 2.86% decrease [99.81 to 96.96%] to the PTV_54Gy D98% at a UDC of 150% of the prescription dose. An OAR sparing effect was observed for the parotid glands, spinal cord and oral cavity sub PTV. Mandible and brainstem Dmax values increased as the PTV UDC increased. Changes in brainstem dose were not statistically significant. All other differences were statistically significant for UDC's above 130%. Target coverage was not compromised as a result of increased target dose heterogeneity. The OAR sparing effect was promising for most organs, however further research with a larger dataset is necessary surrounding the effect on organs that overlap with the PTV.

Adherence to ICRU-83 reporting recommendations is inadequate in prostate dosimetry studies.

PURPOSE: This study aimed to investigate if the International Commission on Radiation Units and Measurements (ICRU) 83 recommendations for reporting dosimetric endpoints are followed in published prostate studies using modulated techniques. METHODS AND MATERIALS: Prostate dosimetry studies using inverse planning techniques were identified through a search of PubMed and EMBASE databases. These studies were analyzed to determine if the endpoints reported followed the recommendations outlined in ICRU-83. A data collection form was completed and any alternative methods of reporting were recorded. Results were analyzed using frequencies, percentages, and Fisher exact tests. RESULTS: The ICRU-83 recommendations were not followed in the majority of studies. For the planning target volume, the dose received by 2% of the volume, the dose received by 98% of the volume, and the dose received by 50% of the volume were reported in 22.9%, 18.8%, and 8.3% of studies, respectively. The adherence to reporting for the clinical target volume was below 5% for all specifications. The mean dose, the dose received by a specified volume, and dose received by 2% of the volume for organs at risk were reported in 47.1%, 83.3%, and 16.7%, respectively. The homogeneity index was used in 14.6% of studies. Conformity was discussed in 45.8% of studies. Confidence intervals were included in 37.5% of studies. CONCLUSIONS: The reporting recommendations of ICRU-83 were not adhered to in the majority of the dosimetry studies reviewed, highlighting the need for greater diligence for authors and reviewers when publishing planning outcomes for modulated techniques.

The impact of intensity-modulated radiation therapy plan normalization in the postprostatectomy setting-does it matter?

The International Commission on Radiation Units & Measurements -83 recommends prescribing intensity-modulated radiation therapy (IMRT) in a dose-volume manner. Despite this, clinical variation still exists in how prostate IMRT plans are prescribed. This study aims to investigate the impact of different plan normalization methods for postprostatectomy IMRT. IMRT treatment plans were created retrospectively for 20 postprostatectomy patients. These were normalized such that the dose received by 98% (D98) of the planning target volume (PTV) was equal to 100% of the prescribed dose. All plans were individually optimized to achieve target coverage and organ at risk (OAR) dose constraints. Each patient's plan was then copied and normalized such that the mean dose (Dmean) received by the PTV was equal to 100% of the prescribed dose. The International Commission on Radiation Units & Measurements -83 recommended dosimetric end points were extracted for targets and Quantitative Analyses of Normal Tissue Effects in the Clinic or Radiation Therapy Oncology Group 0534 end points extracted for OARs. Statistical analysis using the Wilcoxon signed-rank test to measure the difference between data from plans normalized to D98 and Dmean was conducted. Extracted dosimetric end points of the targets and OARs were significantly higher in plans normalized to D98 than Dmean (p < 0.05) with the exceptions of D2 of the rectum and right femoral head. Normalization impacts on dosimetric end points of a plan. Hence, reporting the normalization method used is necessary to allow for meaningful interpretation of IMRT dosimetric studies.

The dilemma of parotid gland and pharyngeal constrictor muscles preservation-Is daily online image guidance required? A dosimetric analysis.

With margin reduction common in head and neck radiotherapy, it is critical that the dosimetric effects of setup deviations are quantified. With past studies focusing on the quantification of positional and volumetric changes of organs at risk (OARs), this study aimed to measure the dose delivered to these the parotid gland (PG) and pharyngeal constrictor muscles (PCMs) using cone beam computed tomography (CBCT). Furthermore, this investigation sought to establish a potential time trend of change in dose delivered to target volumes secondary to ascertaining the need for daily image guidance (IG) to reduce the dose burden to these important OARs. Intensity modulated radiotherapy (IMRT) plans for 5 locally advanced head and neck patients׳ plans were created and mapped to weekly CBCTs. Each plan was recalculated without heterogeneity correction allowing for dosimetric comparison. Dosimetric endpoints recorded to assess the effect of positional variation were as per ICRU 83 and included D95 and D98 for the target volumes, mean dose (MD) and V30Gy for the PGs, and V50Gy and MD for the PCMs. Results were deemed statistically significant if p < 0.05. No significant time trends were established for these OARs. A significant decrease in V50Gy was observed for all PCMs (p < 0.001) on all CBCTs relative to the original plan. Regarding target volumes, a highly significant decrease in MD (MD = 20Gy, CI: -20.310 to -19.820) in D98 of the high-dose planning target volume (PTV [70Gy]; PTVD98% = 70Gy) for case 3 was found (p ≤ 0.001). A nonpredictable, yet significant dosimetric effect was found. A clinically acceptable balance must be achieved between OAR dosimetry and target coverage as can be achieved by frequent IG.

A Dosimetric Evaluation of Threshold Bladder Volumes for Prostate Cancer Radiotherapy.

BACKGROUND: An interfraction variation in bladder filling results in uncertainties of dose received and also has workflow implications for busy departments. This study aims to examine the dosimetric impact of a reduced bladder volume while determining a suitable threshold for treatment. MATERIALS AND METHODS: A total of 15 definitive prostate patients were included for this retrospective dosimetry study. Each patient was planned to receive 80 Gy in 40 fractions using intensity-modulated radiation therapy. For each patient, a series of shrunken bladder volumes were created in 50-mL increments. The volume of bladder receiving 65 Gy (V65), 70 Gy, 75 Gy, and 80 Gy for each "shrunken" bladder volume were analyzed with paired samples t-tests. The effect of the shrunken volume relative to the established dose-volume constraint (DVC) was then assessed using single sample t-tests. RESULTS: The mean planning bladder volume was 345.01 ± 138.51 mL. Under maximum bladder shrinkage, mean difference between the percentage dose received and each DVC was seen to be statistically significant (P < .05). However, for the majority of patients, DVCs were only violated once the bladder volume shrunk to less than 150 mL. On average, the DVCs were violated once the bladder volume fell below 150 mL for the V75 and V80 constraints, with no violations noted for V65 and V70. CONCLUSION: Even under exacerbated bladder shrinkage, bladder DVC violations were found to be rare. A bladder threshold of 150 mL would prove sufficient to meet bladder DVCs in over 90% of patients; however, case-by-case assessment is required to ensure patient suitability.

Daily Rectal Dose-volume Histogram Variation in Prostate Intensity-modulated Radiation Therapy: Is It Clinically Significant in the Era of Image Guidance?

AIM: Because of the steep dose gradient associated with dose-escalated intensity-modulated radiation therapy, interfraction motion and variation in rectal volume may result in the rectum receiving a larger dose than predicted at treatment planning. This study aims to quantify the variation in daily rectal dose-volume histograms (DVHs) from the treatment plan and to discuss the potential clinical significance of this variation. MATERIALS AND METHODS: Daily cone beam computed tomography scans of nine patients treated with definitive prostate intensity-modulated radiation therapy were collected. The daily dose distribution to the rectum was calculated retrospectively. The variation between the planned and on-treatment rectal DVHs was determined using Friedman tests with post hoc analysis and Wilcoxon matched-pairs tests. The on-treatment DVHs were compared with dose-volume constraints (DVCs) to assess the potential clinical significance of this variation using Wilcoxon signed-rank tests. RESULTS: Significant variation (P < .05) was observed between the planned and on-treatment DVHs. The DVCs for the volume receiving 50 Gy (V50), V60, and V65 were adhered to. The mean V70 and V75 values were above the DVC; however, this variation was not statistically significant. CONCLUSION: The initial treatment plan does not accurately represent the dose received by the rectum on treatment. Investigation into the most effective rectal protocol is recommended to reduce the likelihood of these variations occurring on a daily basis.

A review of the use of fiducial markers for image-guided bladder radiotherapy.

BACKGROUND: Enhancing target visualization and reducing set-up errors in image-guided radiotherapy (IGRT) are issues faced when trying to implement more conformal and partial bladder techniques. This review examines the evidence available pertaining to the clinical use of Lipiodol and gold fiducials for IGRT for bladder cancer. MATERIAL AND METHODS: Nine published articles relating to the feasibility of using Lipiodol injections or gold fiducial markers in IGRT for bladder patients were recruited from a database search strategy. Set-up errors were evaluated in addition to the stability and visibility of each on verification imaging. Adverse reactions from the insertion of each method were also assessed. RESULTS: Both Lipiodol and gold fiducials have the potential to remain stable and visible in the bladder, however, fading, washout and seed loss was also reported. Set-up errors can be reduced by using Lipiodol or fiducial registration when compared to other registration techniques. Adverse reactions reported were minimal for each. CONCLUSION: Current evidence suggests that Lipiodol injections and gold fiducial markers present as promising and highly accurate methods of overcoming interfraction bladder motion in IGRT.

Effect of stereotactic dosimetric end points on overall survival for Stage I non-small cell lung cancer: a critical review.

Stereotactic body radiation therapy (SBRT) delivers a high biologically effective dose while minimizing toxicities to surrounding tissues. Within the scope of clinical trials and local practice, there are inconsistencies in dosimetrics used to evaluate plan quality. The purpose of this critical review was to determine if dosimetric parameters used in SBRT plans have an effect on local control (LC), overall survival (OS), and toxicities. A database of relevant trials investigating SBRT for patients with early-stage non-small cell lung cancer was compiled, and a table of dosimetric variables used was created. These parameters were compared and contrasted for LC, OS, and toxicities. Dosimetric end points appear to have no effect on OS or LC. Incidences of rib fractures correlate with a lack of dose-volume constraints (DVCs) reported. This review highlights the great disparity present in clinical trials reporting dosimetrics, DVCs, and toxicities for lung SBRT. Further evidence is required before standard DVCs guidelines can be introduced. Dosimetric end points specific to stereotactic treatment planning have been proposed but require further investigation before clinical implementation.

A class solution for volumetric-modulated arc therapy planning in postprostatectomy radiotherapy.

This study is aimed to test a postprostatectomy volumetric-modulated arc therapy (VMAT) planning class solution. The solution applies to both the progressive resolution optimizer algorithm version 2 (PRO 2) and the algorithm version 3 (PRO 3), addressing the effect of an upgraded algorithm. A total of 10 radical postprostatectomy patients received 68 Gy to 95% of the planning target volume (PTV), which was planned using VMAT. Each case followed a set of planning instructions; including contouring, field setup, and predetermined optimization parameters. Each case was run through both algorithms only once, with no user interaction. Results were averaged and compared against Radiation Therapy Oncology Group (RTOG) 0534 end points. In addition, the clinical target volume (CTV) D100, PTV D99, and PTV mean doses were recorded, along with conformity indices (CIs) (95% and 98%) and the homogeneity index. All cases satisfied PTV D95 of 68 Gy and a maximum dose < 74.8 Gy. The average result for the PTV D99 was 64.1 Gy for PRO 2 and 62.1 Gy for PRO 3. The average PTV mean dose for PRO 2 was 71.4 Gy and 71.5 Gy for PRO 3. The CTV D100 average dose was 67.7 and 68.0 Gy for PRO 2 and PRO 3, respectively. The mean homogeneity index for both algorithms was 0.08. The average 95% CI was 1.17 for PRO 2 and 1.19 for PRO 3. For 98%, the average results were 1.08 and 1.12 for PRO 2 and PRO 3, respectively. All cases for each algorithm met the RTOG organs at risk dose constraints. A successful class solution has been established for prostate bed VMAT radiotherapy regardless of the algorithm used.

A volumetric approach to path-length measurements is essential when treating radiotherapy with modulated beams.

The established dosimetric benefits of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy have lead to their increased use in prostate radiotherapy. Complimenting these techniques, volumetric image guidance has supported increased positional accuracy. In addition, 3-dimensional image guidance has also allowed for assessment of potential dosimetric variation that can be attributed to a deformation of either internal or external structures, such as rectal distension or body contour. Compounding these issues is the variation of tissue density through which the new field position passes and also the variation of dose across a modulated beam. Despite the growing level of interest in this area, there are only a limited number of articles that examine the effect of a variation in beam path length, particularly across a modulated field. IMRT and volumetric-modulated radiation therapy (VMAT) fields are dynamic in nature, and the dose gradient within these fields is variable. Assessment of variation of path length away from the beam׳s central axis and across the entire field is vital where there is considerable variation of dose within the field, such as IMRT and VMAT. In these cases, reliance on the traditional central axis to focus skin distances is no longer appropriate. This article discusses these more subtle challenges that may have a significant clinical effect if left unrecognized and undervalued.

Volumetric-modulated arc therapy in postprostatectomy radiotherapy patients: a planning comparison study.

The purpose of this study was to compare postprostatectomy planning for volumetric-modulated arc therapy (VMAT) with both single arc (SA) and double arcs (DA) against dynamic sliding window intensity-modulated radiotherapy (IMRT). Ten cases were planned with IMRT, SA VMAT, and DA VMAT. All cases were planned to achieve a minimum dose of 68Gy to 95% of the planning target volume (PTV) and goals to limit rectal volume >40Gy to 35% and >65Gy to 17%, and bladder volumes >40Gy to 50% and >65Gy to 25%. Plans were averaged across the 10 patients and compared for mean dose, conformity, homogeneity, rectal and bladder doses, and monitor units. The mean dose to the clinical target volume and PTV was significantly higher (p<0.05) for SA compared with DA or IMRT. The homogeneity index was not significantly different: SA = 0.09; DA = 0.08; and IMRT = 0.07. The rectal V40 was lowest for the DA plan. The rectal V20 was significantly lower (p<0.05) for both the VMAT plans compared with IMRT. There were no significant differences for bladder V40 or rectal and bladder V65. The IMRT plans required 1400MU compared with 745 for DA and 708 for SA. This study shows that for equivalent dose coverage, SA and DA VMAT plans result in higher mean doses to the clinical target volume and PTV. This greater dose heterogeneity is balanced by improved low-range rectal doses and halving of the monitor units.

Image-guided dose-escalated intensity-modulated radiation therapy for prostate cancer: treating to doses beyond 78 Gy.

UNLABELLED: Study Type - Therapy (case series) Level of Evidence 4 What's known on the subject? and What does the study add? High doses of radiotherapy correspond to higher cure rates. Most radiation centers in the UK and Australia limit prescription doses to 74-78 Gy and do not use IGRT and IMRT for all patients. The combination of IGRT and IMRT can limit the rectum and bladder to low doses allowing doses above 78 Gy to be delivered, including pelvic lymph node treatment, with low side effects. This paper provides a detailed guide of planning technique and expected toxicity to enable implementation of similar dose escalated prostate programmes in other radiotherapy centres. OBJECTIVE: To determine the short-term side-effects of dose-escalated image-guided intensity-modulated radiation therapy (IG-IMRT) for the definitive treatment of prostate cancer. PATIENTS AND METHODS: Eligible patients were treated at doses >78 Gy and with definitive intent. Patients from a prospective database with International Prostate Symptom Score (IPSS) and Common Terminology Criteria for Adverse Events, version 3.0 (CTCAE) toxicity were recorded weekly during treatment and at follow-up. RESULTS: In all, 101 patients were included, with doses ranging from 78.3 to 84 Gy (median = 79.3). The median (range) age was 71 (46-83) years; 60% received concurrent androgen deprivation and 35% had pelvic lymph node treatment. The median (range) IPSS at baseline, on treatment and at first follow-up were 7 (0-35), 15 (1-34) and 6 (0-30), respectively. The percentages of patients with grade 2 or higher toxicity during treatment, at 3 months and at 2 years were, respectively, 6.9%, 1% and 2% for gastrointestinal symptoms and 39%, 6.9% and 3% for genitourinary symptoms. There was no significant difference in gastrointestinal toxicity during treatment (P= 0.860) or at 3 months (P= 0.334) between patients with and without lymph node treatment. CONCLUSION: Doses >78 Gy delivered using daily image guidance and IMRT are well tolerated, and by 3 months short-term side-effects have normalized for the majority of patients.