Public Awareness of the Fencing Response as an Indicator of Traumatic Brain Injury: Quantitative Study of Twitter and Wikipedia Data

BACKGROUND

Traumatic brain injury (TBI) is a disruption in normal brain function caused by an impact of external forces on the head. TBI affects millions of individuals per year, many potentially experiencing chronic symptoms and long-term disability, creating a public health crisis and an economic burden on society. The public discourse around sport-related TBIs has increased in recent decades; however, recognition of a possible TBI remains a challenge. The fencing response is an immediate posturing of the limbs, which can occur in individuals who sustain a TBI and can be used as an overt indicator of TBI. Typically, an individual demonstrating the fencing response exhibits extension in 1 arm and flexion in the contralateral arm immediately upon impact to the head; variations of forearm posturing among each limb have been observed. The tonic posturing is retained for several seconds, sufficient for observation and recognition of a TBI. Since the publication of the original peer-reviewed article on the fencing response, there have been efforts to raise awareness of the fencing response as a visible sign of TBI through publicly available web-based platforms, such as Twitter and Wikipedia.

OBJECTIVE

We aimed to quantify trends that demonstrate levels of public discussion and awareness of the fencing response over time using data from Twitter and Wikipedia.

METHODS

Raw Twitter data from January 1, 2010, to December 31, 2019, were accessed using the RStudio package academictwitteR and queried for the text "fencing response." Data for page views of the Fencing Response Wikipedia article from January 1, 2010, to December 31, 2019, were accessed using the RStudio packages wikipediatrend and pageviews. Data were clustered by weekday, month, half-year (to represent the American football season vs off-season), and year to identify trends over time. Seasonal regression analysis was used to analyze the relationship between the number of fencing response tweets and page views and month of the year.

RESULTS

Twitter mentions of the fencing response and Wikipedia page views increased overall from 2010 to 2019, with hundreds of tweets and hundreds of thousands of Wikipedia page views per year. Twitter mentions peaked during the American football season, especially on and following game days. Wikipedia page views did not demonstrate a clear weekday or seasonal pattern, but instead had multiple peaks across various months and years, with January having more page views than May.

CONCLUSIONS

Here, we demonstrated increased awareness of the fencing response over time using public data from Twitter and Wikipedia. Effective scientific communication through free public platforms can help spread awareness of clinical indicators of TBI, such as the fencing response. Greater awareness of the fencing response as a "red-flag" sign of TBI among coaches, athletic trainers, and sports organizations can help with medical care and return-to-play decisions.

ACR Appropriateness Criteria for external beam radiation therapy treatment planning for clinically localized prostate cancer, part II of II

PURPOSE

To present the most updated American College of Radiology (ACR) Appropriateness Criteria formed by an expert panel on the appropriate delivery of external beam radiation to manage stage T1 and T2 prostate cancer (in the definitive setting and post-prostatectomy) and to provide clinical variants with expert recommendations based on accompanying Appropriateness Criteria for target volumes and treatment planning.

METHODS AND MATERIALS

The ACR Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed annually by a panel of multidisciplinary experts. The guideline development and revision process includes an extensive analysis of current medical literature from peer-reviewed journals and the application of well-established methodologies (RAND/UCLA Appropriateness Method and Grading of Recommendations Assessment, Development, and Evaluation) to rate the appropriateness of imaging and treatment procedures for specific clinical scenarios. In instances in which evidence is lacking or equivocal, expert opinion may supplement available evidence to recommend imaging or treatment.

RESULTS

The panel summarizes the most recent and relevant literature on the topic, including organ motion and localization methods, image guidance, and delivery techniques (eg, 3-dimensional conformal intensity modulation). The panel presents 7 clinical variants, including (1) a standard case and cases with (2) a distended rectum, (3) a large-volume prostate, (4) bilateral hip implants, (5) inflammatory bowel disease, (6) prior prostatectomy, and (7) a pannus extending into the radiation field. Each case outlines the appropriate techniques for simulation, treatment planning, image guidance, dose, and fractionation. Numerical rating and commentary is given for each treatment approach in each variant.

CONCLUSIONS

External beam radiation is a key component of the curative management of T1 and T2 prostate cancer. By combining the most recent medical literature, these Appropriateness Criteria can aid clinicians in determining the appropriate treatment delivery and personalized approaches for individual patients.

The spatial accuracy of two frameless, linear accelerator-based systems for single-isocenter, multitarget cranial radiosurgery

Single‐isocenter, multitarget cranial stereotactic radiosurgery (SRS) is more efficient than using an isocenter for each target, but spatial positioning uncertainties can be magnified at locations away from the isocenter. This study reports on the spatial accuracy of two frameless, linac‐based SRS systems for multitarget, single‐isocenter SRS as a function of distance from the isocenter. One system uses the ExacTrac platform for image guidance and the other localizes with cone beam computed tomography (CBCT). For each platform, a phantom with 12 target BBs distributed up to 13.8 cm from the isocenter was aligned starting from five different initial offsets and then imaged with the treatment beam at seven different gantry and couch angles. The distribution of the resulting positioning errors demonstrated the value of adding a 1‐mm PTV margin for targets up to about 7–8 cm from the isocenter. For distances 10 cm or more, the CBCT‐based alignment remained within 1.1 mm while the ExacTrac‐based alignment differed by up to 2.2 mm.

Robustness quantification methods comparison in volumetric modulated arc therapy to treat head and neck cancer

BACKGROUND

To compare plan robustness of volumetric modulated arc therapy (VMAT) with intensity modulated radiation therapy (IMRT) and to compare the effectiveness of 3 plan robustness quantification methods.

METHODS AND MATERIALS

The VMAT and IMRT plans were created for 9 head and neck cancer patients. For each plan, 6 new perturbed dose distributions were computed using ±3 mm setup deviations along each of the 3 orientations. Worst-case analysis (WCA), dose-volume histogram (DVH) band (DVHB), and root-mean-square dose-volume histogram (RVH) were used to quantify plan robustness. In WCA, a shaded area in the DVH plot bounded by the DVHs from the lowest and highest dose per voxel was displayed. In DVHB, we displayed the envelope of all DVHs in band graphs of all the 7 dose distributions. The RVH represents the relative volume on the vertical axis and the root-mean-square-dose on the horizontal axis. The width from the first 2 methods at different target DVH indices (such as D_95% and D_5%) and the area under the RVH curve for the target were used to indicate plan robustness. Results were compared using Wilcoxon signed-rank test.

RESULTS

The DVHB showed that the width at D_95% of IMRT was larger than that of VMAT (unit Gy) (1.59 vs 1.18) and the width at D_5% of IMRT was comparable to that of VMAT (0.59 vs 0.54). The WCA showed similar results between IMRT and VMAT plans (D_95%: 3.28 vs 3.00; D_5%: 1.68 vs 1.95). The RVH showed the area under the RVH curve of IMRT was comparable to that of VMAT (1.13 vs 1.15). No statistical significance was found in plan robustness between IMRT and VMAT.

CONCLUSIONS

The VMAT is comparable to IMRT in terms of plan robustness. For the 3 quantification methods, WCA and DVHB are DVH parameter-dependent, whereas RVH captures the overall effect of uncertainties.

RO-ILS: Radiation Oncology Incident Learning System: A report from the first year of experience

PURPOSE

Incident learning is a critical tool to improve patient safety. The Patient Safety and Quality Improvement Act of 2005 established essential legal protections to allow for the collection and analysis of medical incidents nationwide.

METHODS AND MATERIALS

Working with a federally listed patient safety organization (PSO), the American Society for Radiation Oncology and the American Association of Physicists in Medicine established RO-ILS: Radiation Oncology Incident Learning System (RO-ILS). This paper provides an overview of the RO-ILS background, development, structure, and workflow, as well as examples of preliminary data and lessons learned. RO-ILS is actively collecting, analyzing, and reporting patient safety events.

RESULTS

As of February 24, 2015, 46 institutions have signed contracts with Clarity PSO, with 33 contracts pending. Of these, 27 sites have entered 739 patient safety events into local database space, with 358 events (48%) pushed to the national database.

CONCLUSIONS

To establish an optimal safety culture, radiation oncology departments should establish formal systems for incident learning that include participation in a nationwide incident learning program such as RO-ILS.

An improvement in IMRT QA results and beam matching in linacs using statistical process control

The purpose of this study is to apply the principles of statistical process control (SPC) in the context of patient specific intensity‐modulated radiation therapy (IMRT) QA to set clinic‐specific action limits and evaluate the impact of changes to the multileaf collimator (MLC) calibrations on IMRT QA results. Ten months of IMRT QA data with 247 patient QAs collected on three beam‐matched linacs were retrospectively analyzed with a focus on the gamma pass rate (GPR) and the average ratio between the measured and planned doses. Initial control charts and action limits were calculated. Based on this data, changes were made to the leaf gap parameter for the MLCs to improve the consistency between linacs. This leaf gap parameter is tested monthly using a MLC sweep test. A follow‐up dataset with 424 unique QAs were used to evaluate the impact of the leaf gap parameter change. The initial data average GPR was 98.6% with an SPC action limit of 93.7%. The average ratio of doses was 1.003, with an upper action limit of 1.017 and a lower action limit of 0.989. The sweep test results for the linacs were ‐1.8%,0%, and +1.2% from nominal. After the adjustment of the leaf gap parameter, all sweep test results were within 0.4% of nominal. Subsequently, the average GPR was 99.4% with an SPC action limit of 97.3%. The average ratio of doses was 0.997 with an upper action limit of 1.011 and a lower action limit of 0.981. Applying the principles of SPC to IMRT QA allowed small differences between closely matched linacs to be identified and reduced. Ongoing analysis will monitor the process and be used to refine the clinical action limits for IMRT QA.

PACS number: 87.55.Qr

Outcome and toxicity for patients treated with intensity modulated radiation therapy for localized prostate cancer

PURPOSE

We evaluate long-term disease control and chronic toxicities observed in patients treated with intensity modulated radiation therapy for clinically localized prostate cancer.

MATERIALS AND METHODS

A total of 302 patients with localized prostate cancer treated with image guided intensity modulated radiation therapy between July 2000 and May 2005 were retrospectively analyzed. Risk groups (low, intermediate and high) were designated based on National Comprehensive Cancer Network guidelines. Biochemical control was based on the American Society for Therapeutic Radiology and Oncology (Phoenix) consensus definition. Chronic toxicity was measured at peak symptoms and at last visit. Toxicity was scored based on Common Terminology Criteria for Adverse Events v4.

RESULTS

The median radiation dose delivered was 75.6 Gy (range 70.2 to 77.4) and 35.4% of patients received androgen deprivation therapy. Patients were followed until death or from 6 to 138 months (median 91) for those alive at last evaluation. Local and distant recurrence rates were 5% and 8.6%, respectively. At 9 years biochemical control rates were 77.4% for low risk, 69.6% for intermediate risk and 53.3% for high risk cases (log rank p = 0.05). On multivariate analysis T stage and prostate specific antigen group were prognostic for biochemical control. At last followup only 0% and 0.7% of patients had persistent grade 3 or greater gastrointestinal and genitourinary toxicity, respectively. High risk group was associated with higher distant metastasis rate (p = 0.02) and death from prostate cancer (p = 0.0012).

CONCLUSIONS

This study represents one of the longest experiences with intensity modulated radiation therapy for prostate cancer. With a median followup of 91 months, intensity modulated radiation therapy resulted in durable biochemical control rates with low chronic toxicity.

The clinical case for proton beam therapy

Over the past 20 years, several proton beam treatment programs have been implemented throughout the United States. Increasingly, the number of new programs under development is growing. Proton beam therapy has the potential for improving tumor control and survival through dose escalation. It also has potential for reducing harm to normal organs through dose reduction. However, proton beam therapy is more costly than conventional x-ray therapy. This increased cost may be offset by improved function, improved quality of life, and reduced costs related to treating the late effects of therapy. Clinical research opportunities are abundant to determine which patients will gain the most benefit from proton beam therapy. We review the clinical case for proton beam therapy. SUMMARY SENTENCE: Proton beam therapy is a technically advanced and promising form of radiation therapy.

Radiation dose escalation for localized prostate cancer: intensity-modulated radiotherapy versus permanent transperineal brachytherapy

BACKGROUND

In the current study, the effects of dose escalation for localized prostate cancer treatment with intensity-modulated radiotherapy (IMRT) or permanent transperineal brachytherapy (BRT) in comparison with conventional dose 3-dimensional conformal radiotherapy (3D-CRT) were evaluated.

METHODS

This study included 853 patients; 270 received conventional dose 3D-CRT, 314 received high-dose IMRT, 225 received BRT, and 44 received external beam radiotherapy (EBRT) + BRT boost. The median radiation doses were 68.4 grays (Gy) for 3D-CRT and 75.6 Gy for IMRT. BRT patients received a prescribed dose of 144 Gy with iodine-125 (I-125) or 120 Gy with palladium-103 (Pd-103), respectively. Patients treated with EBRT + BRT received 45 Gy of EBRT plus a boost of 110 Gy with I-125 or 90 Gy with Pd-103. Risk group categories were low risk (T1-T2 disease, prostate-specific antigen level <or=10 ng/mL, and a Gleason score <or=6), intermediate risk (increase in value of 1 of the factors), and high risk (increase in value of >or=2 factors).

RESULTS

With a median follow-up of 58 months, the 5-year biochemical control (bNED) rates were 74% for 3D-CRT, 87% for IMRT, 94% for BRT, and 94% for EBRT + BRT (P <.0001). For the intermediate-risk group, high-dose IMRT, BRT, or EBRT + BRT achieved significantly better bNED rates than 3D-CRT (P <.0001), whereas no improvement was noted for the low-risk group (P = .22). There was no increase in gastrointestinal (GI) toxicity from high-dose IMRT compared with conventional dose 3D-CRT, although there was more grade 2 genitourinary (GU) toxicity (toxicities were graded at the time of each follow-up visit using a modified Radiation Therapy Oncology Group [RTOG] scale). BRT caused more GU but less GI toxicity, whereas EBRT + BRT caused more late GU and GI toxicity than IMRT or 3D-CRT.

CONCLUSIONS

The data from the current study indicate that radiation dose escalation improved the bNED rate for the intermediate-risk group. IMRT caused less acute and late GU toxicity than BRT or EBRT + BRT.

Analysis of biochemical control and prognostic factors in patients treated with either low-dose three-dimensional conformal radiation therapy or high-dose intensity-modulated radiotherapy for localized prostate cancer

PURPOSE

To identify prognostic factors and evaluate biochemical control rates for patients with localized prostate cancer treated with either high-dose intensity-modulated radiotherapy (IMRT) or conventional-dose three-dimensional conformal radiotherapy 3D-CRT.

METHODS

Four hundred sixteen patients with a minimum follow-up of 3 years (median, 5 years) were included. Two hundred seventy-one patients received 3D-CRT with a median dose of 68.4 Gy (range, 66-71 Gy). The next 145 patients received IMRT with a median dose of 75.6 Gy (range, 70.2-77.4 Gy). Biochemical control rates were calculated according to both American Society for Therapeutic Radiology and Oncology (ASTRO) consensus definitions. Prognostic factors were identified using both univariate and multivariate analyses.

RESULTS

The 5-year biochemical control rate was 60.4% for 3D-CRT and 74.1% for IMRT (p < 0.0001, first ASTRO Consensus definition). Using the ASTRO Phoenix definition, the 5-year biochemical control rate was 74.4% and 84.6% with 3D-RT and IMRT, respectively (p = 0.0326). Univariate analyses determined that PSA level, T stage, Gleason score, perineural invasion, and radiation dose were predictive of biochemical control. On multivariate analysis, dose, Gleason score, and perineural invasion remained significant.

CONCLUSION

On the basis of both ASTRO definitions, dose, Gleason score, and perineural invasion were predictive of biochemical control. Intensity-modulated radiotherapy allowed delivery of higher doses of radiation with very low toxicity, resulting in improved biochemical control.

Intraoperative radiation therapy using mobile electron linear accelerators: report of AAPM Radiation Therapy Committee Task Group No. 72

Intraoperative radiation therapy (IORT) has been customarily performed either in a shielded operating suite located in the operating room (OR) or in a shielded treatment room located within the Department of Radiation Oncology. In both cases, this cancer treatment modality uses stationary linear accelerators. With the development of new technology, mobile linear accelerators have recently become available for IORT. Mobility offers flexibility in treatment location and is leading to a renewed interest in IORT. These mobile accelerator units, which can be transported any day of use to almost any location within a hospital setting, are assembled in a nondedicated environment and used to deliver IORT. Numerous aspects of the design of these new units differ from that of conventional linear accelerators. The scope of this Task Group (TG-72) will focus on items that particularly apply to mobile IORT electron systems. More specifically, the charges to this Task Group are to (i) identify the key differences between stationary and mobile electron linear accelerators used for IORT, (ii) describe and recommend the implementation of an IORT program within the OR environment, (iii) present and discuss radiation protection issues and consequences of working within a nondedicated radiotherapy environment, (iv) describe and recommend the acceptance and machine commissioning of items that are specific to mobile electron linear accelerators, and (v) design and recommend an efficient quality assurance program for mobile systems.

Shielding evaluation and acceptance testing of a prefabricated, modular, temporary radiation therapy treatment facility

We have recently commissioned a temporary radiation therapy facility that is novel in two aspects: it was constructed using modular components, and the LINAC was installed in one of the modular sections before it was lifted into position. Additional steel and granular fill was added to the modular sections on‐site during construction. The building will be disassembled and removed when no longer needed. This paper describes the radiation shielding specifications and survey of the facility, as well as the ramifications for acceptance testing occasioned by the novel installation procedure. The LINAC is a Varian 21EX operating at 6 MV and 18 MV. The radiation levels outside the vault satisfied the design criteria, and no anomalous leakage was detected along the joints of the modular structure. At 18 MV and 600 monitor units (MU) per minute, the radiation level outside the primary barrier walls was 8.5μSv/h of photons; there were no detectable neutrons. Outside the direct‐shielded door, the levels were 0.4μSv/h of photons and 3.0μSv/h of neutrons. The isocentricity of the accelerator met the acceptance criteria and was not affected by its preinstallation into an integrated baseframe and subsequent transport to the building site.

PACS numbers: 87.52.Df, 87.52.Ga

Treatment planning for dose escalation in non-small cell lung cancer (NSCLC)

This study was performed to examine potential field arrangements for irradiating non-small cell lung cancer (NSCLC) on a dose escalation study. An example patient was chosen and 7 coplanar treatment plans were created to treat a NSCLC. Two plans included prophylactic nodal irradiation (PNRT) and 5 did not. Four plans used 4 fields, 2 plans used 5 fields, and 1 plan included dynamic conformal 360 degrees rotational therapy. All plans delivered 80 Gy to the isocenter with 10-MV x-rays. Each plan was initially created without dose inhomogeneity corrections and then was recalculated with these corrections, maintaining the same weighting and number of monitor units. Avoiding PNRT spared a considerable volume of normal tissue from radiation. Plans with 5 fields generally spared normal tissues better than 4-field plans. There was no benefit to the dynamic conformal 360 degrees rotational plan. Inhomogeneity corrections revealed that higher doses were delivered to both the tumor and normal structures. Seven beam arrangements for the treatment of NSCLC were compared to develop potential beam arrangements that would be applicable to treating NSCLC on a multi-institutional dose escalation study. We favor the use of at least 5 beams in most situations. It is possible that the use of more fields would further improve plans up to a point of diminishing returns, as exemplified by the lack of benefit seen with the dynamic conformal 360 degrees rotational plan. It is possible that the use of noncoplanar fields or intensity-modulated radiation therapy (IMRT) may further improve the therapeutic ratio.

Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee

Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add-on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional (3-D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf-collimator-based IMRT delivery systems, goal-based inverse treatment planning, and clinical implementation of IMRT with patient-specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.

Initial experience with ultrasound localization for positioning prostate cancer patients for external beam radiotherapy

PURPOSE

Transabdominal ultrasound localization of the prostate gland and its immediate surrounding anatomy has been used to guide the positioning of patients for the treatment of prostate cancer. This process was evaluated in terms of (1) the reproducibility of the ultrasound measurement; (2) a comparison of patient position between ultrasound localization and skin marks determined from a CT treatment planning scan; (3) the predictive indicators of patient anatomy not well suited for ultrasound localization; (4) the measurement of prostate organ displacement resulting from ultrasound probe pressure; and (5) quality assurance measures.

METHODS AND MATERIALS

The reproducibility of the ultrasound positioning process was evaluated for same-day repeat positioning by the same ultrasound operator (22 patients) and for measurements made by 2 different operators (38 patients). Differences between conventional patient positioning (CT localization with skin markings) and ultrasound-based positioning were determined for 38 patients. The pelvic anatomy was evaluated for 34 patients with pretreatment CT scans to identify predictors of poor ultrasound image quality. The displacement of the prostate resulting from pressure of the ultrasound probe was measured for 16 patients with duplicate CT scans with and without a simulated probe. Finally, daily, monthly, and semiannual quality assurance tests were evaluated.

RESULTS

Self-verification tests of ultrasound positioning indicated a shift of <3 mm in approximately 95% of cases. Interoperator tests indicated shifts of <3 mm in approximately 80-90% of cases. The mean difference in patient positioning between conventional and ultrasound localization for lateral shifts was 0.3 mm (SD 2.5): vertical, 1.3 mm (SD 4.7 mm) and longitudinal, 1.0 mm (SD 5.1). However, on a single day, the differences were >10 mm in 1.5% of lateral shifts, 7% of longitudinal shifts, and 7% of vertical shifts. The depth to the isocenter, thickness of tissue overlying the bladder, and position of the prostate relative to the pubic symphysis, but not the bladder volume, were significant predictive indicators of poor ultrasound imaging. The pressure of the ultrasound probe displaced the prostate in 7 of the 16 patients by an average distance of 3.1 mm; 9 patients (56%) showed no displacement. Finally, the quality assurance tests detected ultrasound equipment defects.

CONCLUSION

The ultrasound positioning system is reproducible and may indicate the need for significant positioning moves. Factors that predict poor image quality are the depth to the isocenter, thickness of tissue overlying the bladder, and position of the prostate relative to the pubic symphysis. The prostate gland may be displaced a small amount by the pressure of the ultrasound probe. A quality assurance program is necessary to detect ultrasound equipment defects that could result in patient alignment errors.

The overshoot phenomenon in step-and-shoot IMRT delivery

The control loop in the Varian DMLC system (V4.8) requires approximately 65 msec to monitor and halt the irradiation of a segment, causing an "overshoot" effect: the segment ends on a fractional monitor unit larger than that planned. As a result, the actual MU delivered may differ from that planned. In general, for step-and-shoot treatments, the first segment receives more, the last receives less, and intermediate segments vary. The overshoot for each segment (DeltaMU) is small, approximately 0.6 MU at 600 MU/min. Our IMRT planning system (Corvus) produces plans often having more than 20% of the segments with less than 1 MU/segment. Such segments may be skipped if the DeltaMU exceeds the segments' planned MU. Furthermore, QA filming often requires reducing the total MU by a factor of 4-6, increasing the potential for dosimetric error. This study measured DeltaMU over a range of MU/min and MU/segment. At >5 MU/segment, the DeltaMU was stable, corresponding to a delay of 62 msec. DeltaMU became larger and more variable at <1 MU/segment. The behavior was modeled in a computer program that predicted the change in delivered MU/segment and total change in delivered MU to each beamlet. Beams were analyzed for patients receiving 5 field prostate or 9 field head and neck treatments. At 400 MU/min, 28% and 16%, respectively, of the planned segments were skipped. For QA filming, up to 75% of the segments were skipped. The cumulative error averaged <0.1 MU/beamlet, but individual beamlets had errors exceeding 200%. The effect is most significant for low dose regions. Recommendations are given for deciding when to treat or do QA studies with lower MU/min. In general, treatments are not significantly affected, but QA films taken at reduced MU may be improved if irradiated at lowered MU/min.

The overshoot phenomenon in step-and-shoot IMRT delivery

The control loop in the Varian DMLC system (V4.8) requires approximately 65 msec to monitor and halt the irradiation of a segment, causing an "overshoot" effect: the segment ends on a fractional monitor unit larger than that planned. As a result, the actual MU delivered may differ from that planned. In general, for step-and-shoot treatments, the first segment receives more, the last receives less, and intermediate segments vary. The overshoot for each segment (DeltaMU) is small, approximately 0.6 MU at 600 MU/min. Our IMRT planning system (Corvus) produces plans often having more than 20% of the segments with less than 1 MU/segment. Such segments may be skipped if the DeltaMU exceeds the segments' planned MU. Furthermore, QA filming often requires reducing the total MU by a factor of 4-6, increasing the potential for dosimetric error. This study measured DeltaMU over a range of MU/min and MU/segment. At >5 MU/segment, the DeltaMU was stable, corresponding to a delay of 62 msec. DeltaMU became larger and more variable at <1 MU/segment. The behavior was modeled in a computer program that predicted the change in delivered MU/segment and total change in delivered MU to each beamlet. Beams were analyzed for patients receiving 5 field prostate or 9 field head and neck treatments. At 400 MU/min, 28% and 16%, respectively, of the planned segments were skipped. For QA filming, up to 75% of the segments were skipped. The cumulative error averaged <0.1 MU/beamlet, but individual beamlets had errors exceeding 200%. The effect is most significant for low dose regions. Recommendations are given for deciding when to treat or do QA studies with lower MU/min. In general, treatments are not significantly affected, but QA films taken at reduced MU may be improved if irradiated at lowered MU/min.

Development of a treatment planning protocol for prostate treatments using intensity modulated radiotherapy

We have developed a treatment planning protocol for intensity-modulated radiation therapy of the prostate using commercially available inverse planning software. Treatment plans were developed for ten patients using the Corvus version 3.8 planning system, testing various prescription options, including tissue types, dose volume histogram values for the target and normal structures, beam arrangements, and number of intensity levels. All plans were scaled so that 95% of the clinical target volume received 75.6 Gy; mean doses to the prostate were typically 79 Gy. The reproducibility of the inverse planning algorithm was tested by repeating a set of the plans five times. Plans were deemed acceptable if they satisfied predefined dose constraints for the targets and critical organs. Figures of merit for target coverage, target dose uniformity, and organ sparing were used to rank acceptable plans. Certain systematic behaviors of the optimizer were noted: the high dose regions for both targets and critical organs were 5-10 Gy more than prescribed; reducing bladder and rectum tolerance increased the range of doses within the target; increasing the number of fields incrementally improved plan quality. A set of planning parameters was found that usually satisfied the minimum requirements. Repeating the optimization with different beam order produced similar but slightly different dose distributions, which was sometimes useful for finding acceptable solutions for difficult cases. The standard set of parameters serves as a useful starting point for individualized planning.

Application of a genetic algorithm to optimizing radiation therapy treatment plans for pancreatic carcinoma

The performance of an automated treatment planning algorithm was tested using cases of patients with pancreatic carcinoma; the system implements optimization tools that suggest high-quality plans for consideration by the planner and physician, making best use of the capabilities of a conventional linear accelerator: isocentric setup, shaped fields, and wedges. Ten consecutive patients presenting with pancreatic cancer were first planned using a conventional 3-field protocol to provide a basis for comparison. Each was then planned using an automated optimization technique using a genetic algorithm and a dose-based score function subject to volume-dose constraints. Two sets of optimized plans were created, 1 using only axial beams and the other permitting non-axial beams. The improvement afforded by the optimization was assessed by comparing the score function results and by computing the combined normal tissue complication probability (NTCP) for a constant isocenter dose. In all 10 cases, optimization improved the dose-based score function. In 9 cases, the non-axial plan scored higher than the axial plan. Optimization driven by the dose-based score function improved or equaled the predicted NTCP in 8 axial and 9 nonaxial plans. This study demonstrates progress toward the goal of developing an automated planning tool that can robustly suggest high-quality plans.

A manual algorithm for computing dwell times for two-catheter endobronchial treatments using HDR brachytherapy

A method has been developed to permit the calculation of dwell times for endobronchial high dose rate (HDR) brachytherapy treatment using two catheters, without using a dedicated treatment planning system. Worksheets were developed to guide a user through manual calculations, and a computer program was written to automate the process. This empirical algorithm produces dose distributions that are clinically safe and reasonable. The total dwell times match those predicted by individually optimized distributions to within a few percent. The method has been used most frequently as a quality assurance check on optimized plans produced by a commercial treatment planning system, but it can serve as a back-up method should the commercial system fail. [Worksheets with example calculations may be retrieved using anonymous ftp from the American Institute of Physics, Electronic Physics Auxillary Publication Service (EPAPS).]

Limitations of the straight-line assumption for endobronchial HDR brachytherapy treatments

It is useful to expedite high dose rate brachytherapy endobronchial treatments by using standardized dwell weights and worksheets, avoiding computerized planning where possible. Such methods assume the treatment catheter is straight. This study uses inverse-square considerations to quantify the curvature that invalidates the straight-line approximation. The ratio of the distance between end dwell positions and the active length can be used as a measure of the curvature. The variation in mean dose and maximum dose at the prescription points is presented as a function of active length and curvature, allowing the planner to determine quickly when a pre-treatment computer plan is necessary.

Genetic and geometric optimization of three-dimensional radiation therapy treatment planning

The thesis of this report is that potentially useful treatment beams can be chosen based on geometric heuristics and that a genetic algorithm (GA) can be constructed to find an optimal combination of beams based on a formal objective function. The paper describes the basic principles of a GA and the particular implementation developed. The code represents each plan in the population as two paired lists comprised of beam identifiers and relative weights. Reproduction operators, which mimic sexual reproduction with crossover, mutation, cloning, spontaneous generation, and death, manipulate the lists to grow optimal plans. The necessary gene pool is created by software modules which generate beams, distribute calculation points, obtain clinical constraints, add wedges, and calculate doses. The code has been tested on a set of artificial patients and on four clinical cases: prostate, pancreas, esophagus, and glomus. All demonstrated consistent results, indicating that the code is a reliable optimizer. Additional experiments compared the results for a full set of open beams to the geometrically selected set and the GA code with simulated annealing. Geometric selection of beam directions did not significantly compromise optimization quality. Compared to simulated annealing, the genetic algorithm was equally able to optimize the objective function, and calculations suggest it may be the faster method when the number of beams to be considered exceeds approximately 70.

Practical aspects of transmission cord blocks in radiotherapy

We present the advantages of using partial transmission cord blocks throughout treatment, as opposed to adding full-thickness blocks near the end. Such blocks reduce the risk of block omission or mispositioning and require less total time for construction. We also present an argument for the existence of an optimal width for cord blocks used in mediastinal treatments. A figure of merit has been derived which quantifies the tradeoff between narrow blocks, which increase the variation in dose across the block shadow and the risk of positioning errors, and wide blocks, which may unnecessarily shield potential tumor sites. For 60Co, 4- and 10-MV beams, the figure of merit peaks at block widths of 2.0-2.5 cm at the level of the cord. Effective transmission data for cord blocks constructed of cerrobend are given for those three beams. Quality assurance studies show that transmission through cerrobend blocks can be controlled to the required precision.

Transmission blocks: clinical and biological rationales

This paper describes the clinical and biological rationales for the use of transmission blocks. Clinical advantages over the use of full-thickness blocks applied part way through the course of therapy include the use of only one set of fields, blocks, and beam calculations, and less complex chart recording. There is a net saving in time required for the preparation and treatment of the patient. There is also a quality assurance advantage since the impact of a potential error in block positioning is reduced. In terms of biological advantages, it is demonstrated that the linear-quadratic iso-effect model can be applied to predict an improvement of up to 10% in the therapeutic ratio if transmission blocks are used instead of full-thickness blocks.