R McMahon

IMRT treatment of anal cancer with a scrotal shield

The risk of sterility in males undergoing radiotherapy in the pelvic region indicates the use of a shielding device, which offers protection to the testes for patients wishing to maintain fertility. The use of such devices in the realm of intensity-modulated radiotherapy (IMRT) in the pelvic region can pose many obstacles during simulation, treatment planning, and delivery of radiotherapy. This work focuses on the development and execution of an IMRT plan for the treatment of anal cancer using a scrotal shielding device on a clinical patient. An IMRT plan was developed using Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA), using a wide array of gantry angles as well as fixed jaw and fluence editing techniques. When possible, the entire target volume was encompassed by the treatment field. When the beam was incident on the scrotal shield, the jaw was fixed to avoid the device and the collimator rotation optimized to irradiate as much of the target as possible. This technique maximizes genital sparing and allows minimal irradiation of the gonads. When this fixed-jaw technique was found to compromise adequate coverage of the target, manual fluence editing techniques were used to avoid the shielding device. Special procedures for simulation, imaging, and treatment verification were also developed. In vivo dosimetry was used to verify and ensure acceptable dose to the gonads. The combination of these techniques resulted in a highly conformal plan that spares organs and risk and avoids the genitals as well as entrance of primary radiation onto the shielding device.

An image-guided technique for planning and verification of supine craniospinal irradiation

We present a technique for planning and verification of craniospinal treatment with the patient in the supine position. Treatment delivery and verification is streamlined through the use of modern imaging techniques. Treatments use two lateral brain fields abutted to a single or pair of posterior spine fields. Treatment delivery is simplified by aligning all isocenters in the anterior‐posterior and lateral directions. Patient positioning is accomplished via on‐board kV imaging. Verification of field shape and junctions is accomplished with BB placement and MV portal imaging. Daily treatment is simplified by using only longitundinal couch shifts, which are recorded in the patient chart and RV database. The technique is simple to implement in a clinic that is already using a similar beam arrangement with the patient prone. It requires no additional devices to be fabricated (for immobilization or QA), and it takes advantage of all the existing elements of a modern linac. PACS number: 87.55.‐x

Commissioning of Varian ring & tandem HDR applicators: reproducibility and interobserver variability of dwell position offsets

Studies have shown that source dwells within Varians HDR CT/MR compatible ring applicators can deviate from intended positions by several millimeters. Quantifying this offset is an important part of commissioning. The aims of this study were to: 1) determine the reproducibility of the offset, 2) study the interobserver variation in the offsets measurement, and 3) quantify the dosimetric impact of the offset. Offsets were measured for four ring applicators: two 30°, one 45°, and one 60°. Dwell positions were measured five times for each ring to determine the reproducibility of source positioning. Experiments were done to compare two separate source wires, as well as different time points within a single source wires lifecycle. Data were analyzed by three independent observers. To quantify the dosimetric impact of the offset, a treatment plan was generated using BrachyVision. The dose to point A, and the D2cc metric for rectum and bladder were calculated with and without the offset. For the 45° and 60° rings, measured offsets were 3.0 mm and 3.6 mm, respectively. The 30° ring showed substantial variation in distal dwell positions (maximum difference between the five experiments of 2.9 mm). Subsequent testing of a replacement ring showed an offset of 2.4 mm that was more reproducible. Offsets varied less than 1 mm between different source wires, and changed less than 1 mm over the course of a source wires lifecycle. When comparing observers, the average range in a measurement of a dwell position was 0. 5mm (σ = 0.2 mm, max 1.3 mm). The offset resulted in dose variations to point A, bladder, and rectum of less than 1%, 2%, and 5%, respectively. Results indicate that Varian rings can show systematic and random offsets of more than 3 mm. Some can be considered defective and should be replaced. Each applicator should be individually commissioned and reproducibility should be confirmed with multiple tests. PACS number: 87.56.‐v, 87.53.Jw

Arc-modulated radiation therapy based on linear models

This paper reports an inverse arc-modulated radiation therapy planning technique based on linear models. It is implemented with a two-step procedure. First, fluence maps for 36 fixed-gantry beams are generated using a linear model-based intensity-modulated radiation therapy (IMRT) optimization algorithm. The 2D fluence maps are decomposed into 1D fluence profiles according to each leaf pair position. Second, a mixed integer linear model is used to construct the leaf motions of an arc delivery that reproduce the 1D fluence profile previously derived from the static gantry IMRT optimization. The multi-leaf collimator (MLC) sequence takes into account the starting and ending leaf positions in between the neighbouring apertures, such that the MLC segments of the entire treatment plan are deliverable in a continuous arc. Since both steps in the algorithm use linear models, implementation is simple and straightforward. Details of the algorithm are presented, and its conceptual correctness is verified with clinical cases representing prostate and head-and-neck treatments.

TU‐E‐304A‐06: A 3D Solution for Advanced Photon Arc Therapy Quality Assurance

Purpose: Recently, radiation treatment technology has out‐paced the advances in QA technology, and a critical gap has been created within the arc motion delivery systems such as Rapid Arc®, High Art®, and VMAT®. Gantry position, speed and MU rate create extra degrees of complexity unaccounted for by several traditional 2D QA verifications due to directional dependence and/or inadequate spatial resolution. We present the 1st 3D dosimetry verification of a Rapid Arc treatment and general dosimetry technique based on a radio‐chromic plastic (PRESAGE™)/optical‐CT (OCTOPUS™) combination suitable for todays complex treatment technology. Method and Materials: A radio‐chromic plastic cylinder 17cm diameter × 11cm height was used to verify a simplistic Rapid Arc 6MV prostate plan. The radiation induced OD change (proportional to dosedelivered) was acquired by an optical‐CT scanner with a voxel size of 1×1×2.5mm3. The measured distribution was then compared with the corresponding dose distribution calculated by the treatment planning system (Eclipse® AAA, voxel size 2.5×2.5×2.5mm3). Comparisons between the two dose distributions were made using dose profiles and stacked 2D gamma maps (with criteria 3% dose difference and 3mm distance to agreement) for a quasi 3D gamma volume. Results: The 3D dose distribution measured profiles in the dosimeter showed agreement amongst the calculated treatment plan down to lower dose regions (down to ∼35%). Gamma map comparisons show the dosimeter measurements generally agree with the calculated treatment plan with a few low dose bath problem areas superior and inferior. More traditional QA techniques showed the same discrepancy areas of the plan. This aided in the verification of the system commissioning process. Conclusion: Radio‐chromic plastic/optical‐CT dosimetry techniques are capable of providing 3D, high spatial resolution verification for patient QA with advanced treatment techniques or commissioning, and the potential for its use at other treatment sites where the fluence is more complex.

SU‐E‐T‐419: Clinical Protocol for the Use of Scrotal Shields in IMRT Treatment of Anal Cancer

Purpose: To implement a clinical protocol for the use of scrotal shields for anal cancerIMRT treatment. Methods and Results: (1) Simulation The patient is immobilized in a full‐body alpha‐cradle mold in the frog‐leg position. Further, the mold is built up near the shield to provide a stable and reproducible setup. The planning CTimages are taken in 2 sets with patient in the same position: first one with shield to capture the actual position and shape of the shield, and the second without shield for actual planning and dose calculation. These two image sets are then fused for treatment planning and DRR generation.(2) IMRT planning To avoid irradiation of the PTV through the shield, jaw sizes and collimator rotations are manually optimized for each beam. Each beam only covers a portion of the projected PTV while avoiding the shield. When it is favorable to cover the entire PTV from the anterior directions, zero fluence is enforced in the region where the field overlaps with the shields projection. In the inverse planning process, the shield structure is given a high priority as an avoidance structure so that the fluence through the shield is minimized. With the combination of these steps, the resulted IMRT has a highly conformal dose distribution with 95% of the target volume covered by the prescription dose and a global maximum dose of 112%. (3) IGRT Daily KV imaging ensures the proper alignment of the bony anatomy and the positioning of the shield, with the shield projected on the DRR. Day1 and weekly CBCTimaging further confirms soft tissue target alignment Conclusions: .A clinical procedure for using a scrotal shield in IMRT treatment of anal cancer is described in detailed steps from simulation and planning, to treatment verification and is demonstrated with a clinical patient case.

SU‐GG‐T‐137: Similarities between Static and Rotational Intensity Modulated Plans

Purpose: To explore similarities between intensity modulated radiotherapy(IMRT) and intensity modulated arc therapy (IMAT) techniques in the context of the number of multi‐leaf collimator(MLC) segments required to achieve plan objectives, the major factor influencing plan quality. Materials and Methods: Three clinical cases with increasing complexity were studied: (a) prostate only, (b) prostate and seminal vesicles, (c) prostate and pelvic lymph nodes. Initial “gold‐standard” plans with the maximum possible organ‐at‐risk sparing were generated for all three cases. For each case, multiple IMRT and IMAT plans were generated with varying intensity levels (IMRT) and arc control points (IMAT), which translates to varying MLC segments in both modalities. The IMAT/IMRT plans were forced to mimic the organ‐at‐risk sparing and target coverage in the gold‐standard plans, thereby only allowing the target dose inhomogeneity to be variable. A higher target dose inhomogeneity (quantified as D5 — dose to the highest 5% of target volume) implies that the plan is less capable of modulation. Results: For each case, given a similar number of MLC segments, both IMRT and IMAT plans exhibit similar target dose inhomogeneity, indicating there is no difference in their ability to provide dose painting. Target dose inhomogeneity remained approximately constant with decreasing segments, but sharply increased below a specific critical number of segments (70, 100, 110 for cases a, b, c, respectively). Conclusions: For the cases studied, IMAT and IMRT plans are similar in their dependence on the number of MLC segments. A minimum critical number of segments is required to ensure adequate plan quality. Future studies are needed to establish the range of minimum critical number of segments for different treatment sites and target‐organ geometries. Acknowledgment: Research supported in part by a master research grant from Varian Medical Systems.

SU‐EE‐A1‐03: Arc Modulated Radiation Therapy Based on Linear Models

Purpose: The challenge for implementing promising Arc Modulated Radiation Therapy (AMRT) is its effectiveness of treatment planning tools to find optimal multileaf collimator(MLC) sequence and the fluence map. This study provides a new approach to the above challenge using linear models. Method and Materials: AMRT is implemented with a three‐step procedure. First, fluence maps for 36 fixed‐gantry beams are generated using a linear‐model based IMRT optimization algorithm. Second, the 2D fluence maps are decomposed into 1D fluence profiles, sorted according to each leaf pairs. These sorted fluence maps are referred to as Rotational Fluence Maps (RFMs). Third, a mixed integer linear model constructs the leaf motions of an arc delivery that reproduce the RFMs previously derived. The MLC sequence takes into account the starting and ending leaf positions in between the neighboring apertures, such that the MLC segments of the entire treatment plan are deliverable in a continuous arc. Results: Patient studies with a prostate case and a neck‐and‐neck show that the AMRT can reproduce the 36‐beam IMRT plan using 1 arc and 3 arcs with negligible dosimetrical variance. Furthermore, the HN AMRT plan is compared with the nine‐beam IMRT plan that had been used for the patient treatment in the clinic (generated using the Varian Eclipse Treatment planning system). Consider the nine‐beam IMRT plan as the gold standard, the AMRT plan shows comparable target coverage and ORA sparing. Hence, the AMRT plan can be considered a clinically acceptable IMRT plan.Conclusions: A new inverse AMRT planning technique using linear models was developed for effective planning of IMRT treatment. (Research sponsored by Varian Corporation)

SU‐GG‐T‐58: Commissioning of Varian Ring & Tandem HDR Applicators: Reproducibility and Inter‐Observer Variability of Dwell Position Offsets

Purpose: Studies have shown that dwells within Varian HDR ring applicators can deviate from intended positions by several millimeters. Quantifying this offset is an important part of commissioning. The aims of this study were to 1) determine the reproducibility of offsets, 2) study the inter‐observer variation in the measurement, and 3) quantify the dosimetric impact. Method and Materials: Offsets were measured for 4 ring applicators. Applicators were taped to a piece of film and exposed on a simulator. An HDR plan with 10 dwells (1cm spacing) was delivered. The resulting image shows delivered dwell positions superimposed on the rings lumen. Using this technique, dwell positions were measured five times for each ring to determine the reproducibility of source positioning. Data were analyzed by two independent observers. A cervical treatment plan was used to quantify the dosimetric impact of the offsets. Results: For the 45 and 60 degree rings, measured offsets were 3.0mm and 3.6mm, respectively. The 30 degree ring showed substantial variation in distal dwell positions (maximum difference between experiments of 2.9mm), and was replaced by the vendor. Subsequent testing of the new ring showed an offset of 2.4mm that was more reproducible. When comparing observers, the average difference in measured dwell positions was 0.04mm (StDev = 0.4mm, max 1.3mm). Dosimetric consequences were generally small. Point A, bladder, and rectum doses varied by less than 1%, 2%, and 5%, respectively. Conclusion: Results indicate that Varian rings can show systematic and random offsets of >3mm. Some can be considered defective and should be replaced. Each applicator should be individually commissioned and reproducibility should be confirmed with multiple tests. Inter‐observer variability in the determination of dwell positions was less than 1.3mm. Offsets translated to < 5% variation in dose to critical structures, which is less than vendor‐reported results (up to 20% variation).

SU-FF-T-567: Volumetric Modulated Arc Therapy for Spine Body Radiotherapy: Comparison with Static Intensity Modulated Treatment

Purpose: Volumetric modulated arc treatment (VMAT) reduces treatment time over static beam intensity modulated treatment (IMRT) by deliveringIMRT using arcs. This clinical study evaluates the feasibility of using VMAT for spine stereotactic body radiotherapy(SBRT) to achieve highly conformal dose distributions that spare adjacent OARs with reduced treatment time. Methods and Materials: Ten spine SBRT patients were studied retrospectively. IMRT and VMAT plans (one and two arc plans) were generated for each patient. PTV dose coverage, OAR dose sparing, and normal tissue integral dose were measured and compared. Differences in treatment delivery were also analyzed by comparing MLC segments, total MUs, and total treatment time. Results: The PTV DVHs were comparable between the VMAT and the IMRT plans in the shoulder (D99%‐D90%), slope (D90%‐D10%), and tail (D10%‐D1%) regions. When averaged over all patients, only VMAT2arc had a better conformity index than IMRT (1.09 vs.1.15, p=0.007). In terms of cord sparing, IMRT was the best and VMAT1arc was the worst. IMRT achieved >10% more D1% cord sparing for 6 out of 10 cases and 7–15% more D10% sparing over the VMAT1arc. For cord sparing, the difference between IMRT and VMAT2arc were smaller and statistically insignificant at all dose levels. Differences for other OARs were small and statistically insignificant. The mean MUs and treatment times were (8711MU, 15.86min), (7730MU, 8.56min) and (6317MU, 7.88min) for IMRT, VMAT1arc and VMAT2arc plans, respectively. The average integral dose was the lowest for IMRT and the highest for VMAT1arc. However, the difference was statistically insignificant. Conclusions: Although VMAT provided comparable PTV coverage and esophagus sparing for spine SBRT, VMAT1arc showed significantly worse cord sparing than IMRT, while VMAT2arc was comparable to IMRT. Treatment efficiency is substantially improved with the VMAT technique.