L Gerig

On-line rapid palliation using helical tomotherapy : A prospective feasibility study

Rapid delivery of radiation therapy is expected to benefit patients requiring palliation. We investigated the feasibility of employing a helical tomotherapy unit to scan, plan, and deliver a radiation treatment in a single radiation therapy appointment. Eleven patients each had an MVCT scan acquired, a plan created, and delivery completed while the patient was on the treatment couch. Timelines for each step of the process were recorded for each patient, and compared with the conventional process for similar patients. Preliminary results show that patients routinely can be treated within a 1 hour appointment for the first fraction.

Using multileaf collimator interleaf leakage to extract absolute spatial information from electronic portal imaging device images

Electronic portal imaging devices (EPIDs) are potentially valuable tools for linear accelerator quality assurance and for measuring and analyzing geometric variations in radiation treatment delivery. Geometric analysis is more robust if referenced against an absolute position such as the isocenter (collimator axis of rotation), allowing the observer to discriminate between various setup errors and jaw or multileaf collimator (MLC) calibration errors. Unfortunately, mechanical instabilities in EPIDs make such analysis difficult. In the present work, we describe how MLC interleaf radiation leakage, hidden in the background of portal images, can be extracted and analyzed to find the field isocenter perpendicular to leaf travel direction. The signal from the interleaf radiation leakage is extracted to provide a precise and accurate determination of the isocenter location in the direction perpendicular to MLC leaf travel. In the direction of leaf travel, the minimization of residuals between planned and measured leaf positions is used to determine the isocenter. This method assumes that leaf positioning errors are randomly distributed. The validity of the method for determining the angular deviation between EPID image grid lines and collimator angle and for determining the known isocenter position is experimentally established. PACs numbers: 87.53.Oq, 87.53.Xd, 87.57.NK

Sci—Fri PM: Delivery — 08: Total Marrow Irradiation Using Helical Tomotherapy in Treating\ a Multiple Myeloma Patient: A Case Study

The Ottawa Hospital Cancer Centre has embarked on a phase I/II dose escalation study of IG‐IMRT using Helical Tomotherapy (HT) for Total Marrow Irradiation (TMI) of multiple myeloma patients prior to autologous hematopoietic stem cell transplantation. In this work we outline the technical and physical hurdles related to planning and dose delivery and summarize our experience to date. Limitations with the scanning and planning systems required that patients have two CT scans; one of the upper body and one of the lower body with at least a 20 cm overlap. They must also have a separate treatment plan for each region. PTVs and OARs were defined on both CT sets and image fusion using ImageJsoftware was used to link the two scan sets. The treatment plan for the upper body used a 2.5 cm beam to provide good sup‐inf dose conformation, while a 5.0 cm beam was used for the lower body. DQA was planned, delivered and analyzed, showing good agreement between the planned and measured dose distributions in the junction region. We demonstrate the technical feasibility of our method in overcoming the challenges related to the planning system, including junctioning and summing the dose clouds of longitudinally adjacent plans created on different CTdata sets. The treatment was well‐tolerated by the patient and no severe acute toxicity was noted. Scaling to the QUANTEC data (V20 of 30–35%) for lungs, we estimate that with the present CTV‐PTV margins it should be possible to safely deliver 25 Gy TMI.

Poster — Thur Eve — 31: Optimum Frequency of Spatial Registration in Image Guided Radiation Therapy for TMI

Purpose: To assess the influence of the frequency of spatial registration in total marrow irradiation using Helical Tomotherapy to correct for intrafraction variations. Methods: The analysis was performed using TCP and NTCP models in a phantom study. Different cases were investigated: one treatment per fraction (no‐junction), and splitting the treatment along Y‐axis (SUP‐INF direction) into 2 or 3 independent sub‐treatments (one/two‐junction), which allows 2 or 3 spatial registrations per treatment fraction. Linearly increasing margins were added to expand the CTV (ribs and spine) and OARs (lungs) with increasing distance from the registration point (iPTV and iPRV). Margin increases of 5, 10 and 15 mm were analyzed. The prescription was for 95% of the iPTV to receive 20 Gy. Results: The dose to the iPRVs was reduced when the treatment was split; D20 to lung was 22, 16.2 and 13.4 Gy for no‐junction, one‐junction and two‐junction cases respectively. However the dose received by the iPTV also decreased; the volume of the iPTV receiving the prescription was 93.9%, 90.5% and 88.4% respectively. The probability of uncomplicated cure TCP(1‐NTCP) increased 3% from no‐junction to two‐junction cases for a maximum margin of 5 mm. The corresponding difference for maximum margin of 15 mm was 22%. Larger margins combined with splitting the treatment to perform additional patient registrations showed benefit, mainly due to lower NTCP. Conclusion: Increasing uncertainties can be reduced by splitting large treatments and realigning the patient prior to each sub‐treatment. This technique reduces the NTCP and marginal misses of the PTV.

TU‐C‐BRB‐03: Radiation Dose Response of Plasma Cell Neoplasms

Purpose: To review the available clinical dose response data for extramedullary plasmacytomas (EMP) and solitary plasmacytomas of the bones (SPB), including standard 12 Gy TBI treatments for multiple myeloma (MM), to compute the expected dose response for plasma cell neoplasms and evaluate differences between EMP and SPB dose response. Method and Materials: Articles from 27 published studies on plasmacytomas were analyzed. Local control (LC) was used as the end point. Clinical data are often reported as LC for the median of a dose range — only data from ranges of width ⩽10 Gy were used. The maximum likelihood method (ML) was used to estimate the parameters of a tumour control probability (TCP) model based on Poisson statistics, and approximate likelihood confidence regions (CR) were determined. A Monte Carlo experiment (MC) assessed the parameters uncertainty due to the 10 Gy dose interval. A statistical test based on the ability of the MC distributions of the parameters to discriminate between different kinds of tumors was performed. Results:Radiation therapy was used as the sole treatment in more than 70% of the patients and in 8 of the 12 studies selected. Parameters characterizing TCP and 95% confidence intervals from MC are reported, along with graphical representations of the dose response, and 2D MC histograms and the CRs on the parameter space. Conclusion: An extensive review of plasmacytoma clinical data was performed. Although the data suffer from a lack of low dose data and are mostly reported within a dose range, this approach is a preliminary assessment of dose response relationship for plasma cell neoplasms. The parameters of the TCP model were determined. Significant difference was seen between EMP and SPB dose response. The models could be used to interpolate clinical data and estimate TCP when assessing new therapies and comparing different treatment planning approaches.

Poster — Wed Eve—52: Radiobiological Modeling of a Proposed Dose Escalation in TMI

Purpose: To compare the effectiveness of different approaches to total marrow irradiation (TMI) using Helical TomoTherapy. Methods: TMI treatment was planned on a 55 year‐old male patient (87% of the GTV to receive a prescribed dose ( D p ) of 20Gy). Field sizes (fs) of 25 and 50mm were examined. The normal tissue complication probability (NTCP) was calculated using Lyman‐Kutcher‐Burman model. Tumour control probability (TCP) was evaluated using the Poisson model. Dose escalation analysis was performed by linearly escalating the DVHs from the 20Gy‐TMI plan, to any D p . Results: There was no substantial difference between TCP using 25mm (40±9%) or 50mm fs (42±9%). For organs of the torso, the difference in the prescribed dose to the GTV that would lead to a normal organ complication of 50% from the TMI ( D p /50 ) between 25 and 50mm was less than 3%. For organs in the head, the D p /50 for 50mm fs was consistently lower by up to 15% compared to 25 mm fs. The optimal dose given by maximizing TCP(1‐NTCP), was ∼39Gy for lungs, resulting in 95% (±3%) of tumor control and 3% (0, 16%) rate of pneumonistis. Conclusion: TCP and NTCP were estimated for a TMI patient, originally receiving 20Gy and linearly escalating the DVHs to higher doses.Tissue sparing was seen by using 25mm fs only in the organs of the head. This suggests it would be beneficial to use the small fields in the head only; since using small fields for the whole treatment would lead to long treatment times.

SU‐GG‐T‐63: Feasibility Study of Longitudinal Field Junctioning with Helical Tomotherapy

Purpose: To examine junctioning of longitudinally adjacent PTVs treated with helical tomotherapy (HT). Method and Materials: Cylindrical PTVs were defined in an elliptic cylindrical homogeneous phantom. Dose distributions (95% PTV to receive 2 Gy) created using 2.5 and 5.0 cm long HT fields were calculated and verified dosimetrically. Cranial — Caudal (CC) dose profiles were summed to study the junctioning of PTVs to create a single contiguous PTV. Junctioning adjacent PTVs with different inter‐PTV spacing, created by equal or different field sizes was studied for dose homogeneity. The use of dose stepped PTVs near the junction region was also examined. Here the SUP end of the INF PTV or the INF end of the SUP PTV was divided into smaller subPTVs of decreasing prescription dose. The resulting dose distributions were summed as a function of inter‐PTV spacing. Simulated dose profiles were verified by film dosimetry.Results: The most homogenous dose resulted when adjacent PTVs had the same CC dose profile (field size). Independent of the Inter‐PTV spacing, PTVs of different CC dose profiles could not produce homogeneous doses. Minimizing the volume dose excursion from prescription resulted in cold spots (−26%) and hot spots (+29%) with 8% of the PTV receiving < 95% of prescription. Dividing each PTV into four multiple contiguous subPTVs, with constantly decreasing prescribed dose (2, 1.5, 1.0, 0.5Gy) allowed PTV matching with dose homogeneity similar to junctioning PTVs of equal CC slope. 95% of the PTV received at least 101% of the prescribed dose, with dose excursions of −19% to +13% from prescription, (1% of the PTV received less than 95% of prescribed dose). Conclusion: Junctioning adjacent PTVs is possible, but PTVs created by different field widths present a challenge. Homogeneity is improved by breaking PTVs into multiple contiguous subPTVs modified to feather (broaden) the effective junctioning region.

Poster — Thurs Eve‐32: Dose errors related to the treatment couch

Modern radiotherapy linacs often use carbon fibre for their couch tops due to its radio translucent properties. Beam attenuation by the couches is often ignored during planning and MU calculation. This work examines beam attenuation and loss of skin sparing (dose build up region) when various photon beams transit either the MedTec (Siemens) or Medical Intelligence (Elekta) couches. Additionally, measured doses were compared to CMS treatment planning system (XiO version 4.33.02) predictions. We found the two couches to have different structures, resulting in different attenuation signatures as a function of gantry angle. For normal beam incidence the Siemens and Elekta couches had radiological thicknesses of 4.5 mm and 6.0 mm, respectively. For a normal incidence 10×10 cm2 6MV beam the surface dose after couch transmission was 93% vs. 83% for Elekta and Siemens, respectively. Conversely, the increased mass on the lateral edge of the Siemens couch resulted in a maximum attenuation (6 MV 5×5 cm2 beams) of 8% compared to 5% by the Elekta couch. Incorporating the treatment couch as part of the patient planning CT allowed the CMS TPS model to calculate couch attenuation within 1% of measurement, except at the very edge of the Siemens couch, where the attenuation is strongly gantry angle dependent. The CMS beam model was also able to predict the loss of skin sparing within 1%. In conclusion, the two patient couches are different, but both can significantly affect patient dose which can be accounted for in the CMS TPS.

SU‐FF‐T‐412: The Reliability of Surrogates in Predicting Tumour Motion: A Comparison of Surrogate Based and Non‐Surrogate Based Approach

Purpose: To assess the interfraction reproducibility of external surrogates in predicting lungtumor motion and to compare that with the reliability of the non‐surrogate approach Methods: 10 patients with fluoroscopically visible non‐operable lungtumors were studied. Data were acquired twice for each patient, pretreatment and post 20 fractions. Chest and abdominal wall surface motion was measured with 3D photogrammetry of retro‐reflective skin markers. Simultaneous time‐stamped A/P fluoroscopic images were used to determine tumor motion in two dimensions. A linear model was formulated to describe the relationship between the surface markers and tumor motion and then applied to predict tumor motion during both the first and second visit. Quantification of the difference between the predicted and actual tumor position demonstrated the surrogates ability to predict tumor motion. This difference defines the Internal Margins (IM) that must be applied when the surrogate is employed for tracking or gating. Comparison of IMs thus obtained over a four week course of radiotherapy would give a measure of the reliability of the surrogate. Results: Comparison of surrogate based margins on day1 and day20 indicated that prediction ability of the surrogate deteriorated during this time and the required IM increased by an average of 1.27mm. Furthermore, analysis of non‐surrogate‐based case showed that tumor motion amplitude had increased significantly form day1 to day20 requiring an average increase of 1.16 mm in the IMs. Statistical analysis showed that for both cases assuming a 6mm additional margin would provide the desired tumor coverage with a 99% C.L. Conclusions: Predictive models of motion can reduce the IM component of the PTV by a significant volume, however interfraction reproducibility is poor and the chance of geometric miss is high. Our observations of tumor motion variability suggest that static field margins should also be reassessed over a standard treatment course.

SU‐FF‐T‐396: TCP and NTCP Variation with the Percentage of Prostate Treatment Fractions Delivered Under Image Guidance

Purpose: In image guided radiotherapy, a treatment course for prostate cancer often consists of a mix of initial IGRT and conventional 3DCRT/IMRT fractions. This study examines how the number of IGRT fractions impacts the TCP and rectal NTCP. Method and Material: We simulated a standard six‐field prostate XRT technique consisting of a total of 76 Gy over 38 daily fractions. Dose distribution within the pelvic region for a simulated patient were calculated using Theraplan® plus for a margin of 5 and 10 mm corresponding IGRT and 3DCRT respectively. The dose distributions for two plans were then exported to Matlab 6.5 for radiobiological simulation. In the simulation, the prostate shifts were sampled from our shift database using Monte Carlo technique. Overall dose distribution to CTV and Rectum was obtained by summing fractional dose voxels over 38 fractions. A Poisson model coupled with linear‐quadratic model was used to calculate TCP while Lyman model was used to calculate the rectal complication. A moderate value of 3.1 Gy for α/β was applied for both prostate and rectum. Results: 2000 treatment courses were simulated for possible number of IGRT fractions from 0 to 38. TCP and NTCP were subsequently calculated. Our simulation suggests that rectal complication is continually improved with the increasing IGRT fractions. However, TCP is optimized only when 7–10 IGRT fractions are applied as an adaptive measure. Conclusion: The number of IGRT fractions is a variable which can impact TCP, NTCP, and throughput. This simulation shows that the use of 7 imaged/repositioned fractions as an IGRT technique can safely allow dose escalation of 4 Gy to the prostate while imaging and repositioning all fractions can reduce rectal complication by up to 50% compared to treatment without image guidance.