M. Mackenzie

Experimental Validation of the Eclipse AAA Algorithm

The present study evaluates the performance of a newly released photon‐beam dose calculation algorithm that is incorporated into an established treatment planning system (TPS). We compared the analytical anisotropic algorithm (AAA) factory‐commissioned with “golden beam data” for Varian linear accelerators with measurements performed at two institutions using 6‐MV and 15‐MV beams. The TG‐53 evaluation regions and criteria were used to evaluate profiles measured in a water phantom for a wide variety of clinically relevant beam geometries. The total scatter factor (TSF) for each of these geometries was also measured and compared against the results from the AAA. At one institute, TLD measurements were performed at several points in the neck and thoracic regions of a Rando phantom; at the other institution, ion chamber measurements were performed in a CIRS inhomogeneous phantom. The phantoms were both imaged using computed tomography (CT), and the dose was calculated using the AAA at corresponding detector locations. Evaluation of measured relative dose profiles revealed that 97%, 99%, 97%, and 100% of points at one institute and 96%, 88%, 89%, and 100% of points at the other institution passed TG‐53 evaluation criteria in the outer beam, penumbra, inner beam, and buildup regions respectively. Poorer results in the inner beam regions at one institute are attributed to the mismatch of the measured profiles at shallow depths with the “golden beam data.” For validation of monitor unit (MU) calculations, the mean difference between measured and calculated TSFs was less than 0.5%; test cases involving physical wedges had, in general, differences of more than 1%. The mean difference between point measurements performed in inhomogeneous phantoms and Eclipse was 2.1% (5.3% maximum) and all differences were within TG‐53 guidelines of 7%. By intent, the methods and evaluation techniques were similar to those in a previous investigation involving another convolution–superposition photon‐beam dose calculation algorithm in another TPS, so that the current work permitted an independent comparison between the two algorithms for which results have been provided. PACS number: 87.53.Dq

Acute Toxicity in High-Risk Prostate Cancer Patients Treated With Androgen Suppression and Hypofractionated Intensity-Modulated Radiotherapy

PURPOSE To report acute toxicity resulting from radiotherapy (RT) dose escalation and hypofractionation using intensity-modulated RT (IMRT) treatment combined with androgen suppression in high-risk prostate cancer patients. METHODS AND MATERIALS Sixty patients with a histological diagnosis of high-risk prostatic adenocarcinoma (having either a clinical Stage of > or =T3a or an initial prostate-specific antigen [PSA] level of > or =20 ng/ml or a Gleason score of 8 to 10 or a combination of a PSA concentration of >15 ng/ml and a Gleason score of 7) were enrolled. RT prescription was 68 Gy in 25 fractions (2.72 Gy/fraction) over 5 weeks to the prostate and proximal seminal vesicles. The pelvic lymph nodes and distal seminal vesicles concurrently received 45 Gy in 25 fractions. The patients were treated with helical TomoTherapy-based IMRT and underwent daily megavoltage CT image-guided verification prior to each treatment. Acute toxicity scores were recorded weekly during RT and at 3 months post-RT, using Radiation Therapy Oncology Group acute toxicity scales. RESULTS All patients completed RT and follow up for 3 months. The maximum acute toxicity scores were as follows: 21 (35%) patients had Grade 2 gastrointestinal (GI) toxicity; 4 (6.67%) patients had Grade 3 genitourinary (GU) toxicity; and 30 (33.33%) patients had Grade 2 GU toxicity. These toxicity scores were reduced after RT; there were only 8 (13.6%) patients with Grade 1 GI toxicity, 11 (18.97%) with Grade 1 GU toxicity, and 5 (8.62%) with Grade 2 GU toxicity at 3 months follow up. Only the V60 to the rectum correlated with the GI toxicity. CONCLUSION Dose escalation using a hypofractionated schedule to the prostate with concurrent pelvic lymph node RT and long-term androgen suppression therapy is well tolerated acutely. Longer follow up for outcome and late toxicity is required.

Assessment of Extended-Field Radiotherapy for Stage IIIC Endometrial Cancer Using Three-Dimensional Conformal Radiotherapy, Intensity-Modulated Radiotherapy, and Helical Tomotherapy

PURPOSE To perform a dosimetric comparison of three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and helical tomotherapy (HT) plans for pelvic and para-aortic RT in postoperative endometrial cancer patients; and to evaluate the integral dose (ID) received by critical structures within the radiation fields. METHODS AND MATERIALS We selected 10 patients with Stage IIIC endometrial cancer. For each patient, three plans were created with 3D-CRT, IMRT, and HT. The IMRT and HT plans were both optimized to keep the mean dose to the planning target volume (PTV) the same as that with 3D-CRT. The dosimetry and ID for the critical structures were compared. A paired two-tailed Student t test was used for data analysis. RESULTS Compared with the 3D-CRT plans, the IMRT plans resulted in lower IDs in the organs at risk (OARs), ranging from -3.49% to -17.59%. The HT plans showed a similar result except that the ID for the bowel increased 0.27%. The IMRT and HT plans both increased the IDs to normal tissue (see Table 1 and text for definition), pelvic bone, and spine (range, 3.31-19.7%). The IMRT and HT dosimetry showed superior PTV coverage and better OAR sparing than the 3D-CRT dosimetry. Compared directly with IMRT, HT showed similar PTV coverage, lower Ids, and a decreased dose to most OARs. CONCLUSION Intensity-modulated RT and HT appear to achieve excellent PTV coverage and better sparing of OARs, but at the expense of increased IDs to normal tissue and skeleton. HT allows for additional improvement in dosimetry and sparing of most OARs.

Patient specific treatment verifications for helical tomotherapy treatment plans

We performed two-dimensional treatment verifications for ten patients planned and treated with helical tomotherapy. The treatment verification consisted of a film measurement as well as point dose measurements made with an ion chamber. The agreement between the calculated and the measured film dose distributions was evaluated with the gamma index calculated for three sets of criteria (2 mm and 2%, 4 mm and 3%, and 3 mm and 5%) as recommended in the literature. Good agreement was found between measured and calculated distributions without any need of normalization of the dose data but with dose map registration using reference marks. In this case, 69.8 +/- 17.2%, 92.6 +/- 9.0%, and 93.4 +/- 8.5% passed the 2 mm and 2%, 4 mm and 3%, and 3 mm and 5% criteria, respectively. Agreement was excellent when both normalization and manual registration of the dose maps was employed. In this case 91.2 +/- 5.6%, 99.0 +/- 1.4%, and 99.5 +/- 0.8% passed the 2 mm and 2%, 4 mm and 3%, and 3 mm and 5% criteria, respectively. The mean percent discrepancy for the point dose measurements was -0.5 +/- 1.1%, -2.4 +/- 3.7%, -1.1 +/- 7.3% for the high dose, low dose, and critical structure point, respectively. Three criteria for a satisfactory treatment verification in the high dose regions of a plan were established. For the un-normalized reference mark registered data 80% of pixels must pass the 3 mm and 5% criteria. For the normalized and manually registered data, 80% must pass the 2 mm and 2% criteria, and the point dose measurement must be within 2% of the calculated dose. All low dose region/critical structure point dose measurements were evaluated on a patient by patient basis. The criteria we recommend can be useful for the routine evaluation of treatment plans for tomotherapy systems.

Adaptive radiotherapy using helical tomotherapy for head and neck cancer in definitive and postoperative settings: initial results.

AIMS To assess whether routine mid-treatment replanning in head and neck squamous cell carcinoma patients results in meaningful improvements in target or normal tissue dosimetry and to assess which patients derive the greatest benefit. MATERIALS AND METHODS Twenty patients treated with either postoperative chemoradiotherapy or definitive chemoradiotherapy with primary or nodal disease ≥3cm in size were included in this prospective pilot study. Seven patients received adjuvant chemoradiotherapy and 13 received definitive chemoradiotherapy. Patients were planned and treated on a helical tomotherapy system. All patients had a second computed tomography scan after 15 fractions and a new plan based on this was initiated from fraction 20. RESULTS Relative volume changes between computed tomography scans were: GTV 29%; CTV60 (adjuvant patients) 4%; parotid volume 17.5%; median reduction in neck separation 6-7 mm; weight loss 3%. For the group overall and for the definitively treated patient cohort, respectively, adapted plans resulted in reductions in PTV66 D(1) (0.3Gy, P=0.01 and 0.5Gy, P=0.01); PTV54 D(1) (0.6Gy, P<0.0001 and 0.9Gy, P=0.0002); spinal cord maximum (0.5Gy, P=0.004 and 0.6Gy, P=0.04) and volume of skin receiving ≥50Gy (16 cm(2), P=0.01 and 19 cm(2), P=0.001). Definitively treated patients also had a reduction in mean parotid dose (0.6Gy, P=0.046) and volume of normal tissue receiving ≥50Gy (67 cm(3), P=0.02). Patients with nasopharyngeal carcinoma received the greatest benefits with treatment adaptation with reduction in spinal cord maximum 1.2Gy, mean parotid dose 1.2Gy and parotid V(26) 6.3%. There was no significant benefit for adjuvant patients. Other factors associated with greater benefits were greater weight loss and greater reduction in neck separation and higher T stage. CONCLUSIONS There is minimal benefit to routine adaptive replanning in unselected patients, and no benefit in adjuvantly treated patients. Patients with nasopharyngeal carcinoma or with greater weight loss or reduction in neck separation did have clinically significant benefits. These patients should be targeted for adaptive strategies.

Evaluation of a lung tumor autocontouring algorithm for intrafractional tumor tracking using low‐field MRI: A phantom study

PURPOSE The first aim of this study is to investigate the feasibility of online autocontouring of tumor in low field MR images (0.2 and 0.5 T) by means of a phantom and simulation study for tumor-tracking in linac-MR systems. The second aim of this study is to develop an MR compatible, lung tumor motion phantom. METHODS An autocontouring algorithm was developed to determine both the position and shape of a lung tumor from each intra fractional MR image. To initiate the algorithm, an expert user contours the tumor and its maximum anticipated range of motion (herein termed the Background) using pretreatment scan data. During treatment, the algorithm processes each intrafractional MR image and automatically contours the tumor. To evaluate this algorithm, the authors built a phantom that replicates the low field contrast parameters (proton density, T(1), T(2)) of lung tumors and healthy lung parenchyma. This phantom allows simulation of MR images with the expected lung tumor CNR at 0.2 and 0.5 T by using a single 3 T scanner. Dynamic bSSFP images (approximately 4 images per second) are acquired while the phantom undergoes a series of preprogrammed motions based on patient lung tumor motion data. These images are autocontoured off-line using our algorithm. The fidelity of autocontouring is assessed by comparing autocontoured tumor shape and its centroid position to the actual tumor shape and its position. RESULTS The algorithm successfully contoured the shape of a moving tumor model from dynamic MR images acquired every 275 ms. Dices coefficients of > 0.96 and > 0.93 are achieved in 0.5 and 0.2 T equivalent images, respectively. Also, the algorithm tracked tumor position during dynamic studies, with root mean squared error (RMSE) values of < 0.55 and < 0.92 mm for 0.5 and 0.2 T equivalent images, respectively. Autocontouring speed is approximately 5 ms for each image. CONCLUSIONS Dices coefficients of > 0.96 and > 0.93 are achieved between autocontoured and real tumor shapes, and the position of a tumor can be tracked with RMSE values of < 0.55 and < 0.92 mm in 0.5 and 0.2 T equivalent images, respectively. These results demonstrate the feasibility of lung tumor autocontouring in low field MR images, and, by extension, intrafractional lung tumor tracking with our laboratorys linac-MR system.

Relative biological damage and electron fluence in and out of a 6 MV photon field

Scattered radiation in the penumbra of a megavoltage radiation therapy beam can deposit a non-negligible dose in the healthy tissue around a target volume. The lower energy of the radiation in this region suggests that its biological effectiveness might not be the same as that of the open beam. In this work, we determined the relative biological damage in normal human fibroblasts after megavoltage irradiation in two geometries. The first was an open-beam irradiation and the second was a blocked configuration in which only scattered radiation could reach the target cells. The biological damage was evaluated by the gamma-H2AX immunofluorescence assay, which is capable of detecting DNA double-strand breaks in individual cells. We report that the scattered radiation is more effective at producing biological damage than the open beam radiation. We found a 27% enhancement in the net mean nuclear gamma-H2AX fluorescence intensity at 2 Gy and a 48% enhancement at 4 Gy. These findings are of interest due to the increased doses of penumbral radiation close to target volumes both in dose escalation studies and in IMRT treatment deliveries where high dose gradients exist for the purpose of conformal avoidance of healthy tissues.

A Monte Carlo study of the variation of electron fluence in water from a 6 MV photon beam outside of the field

Existing studies have suggested some debate on whether the quality of radiation that delivers dose outside of the primary field of a radiotherapy photon beam can be considered the same as that inside the primary field. We used a Monte Carlo approach to simulate the electron fluence differential in energy inside a water phantom in response to irradiation by a 6 MV photon beam. The goal was to quantify how significantly the electron fluence changes when moving from a volume exposed to the primary field to one outside of the primary field, and understand any potential biological implications. We scored the electron fluence outwards in annular volumes in response to a 5 cm radius 6 MV beam and at the central axis in response to a rectangular 6 MV beam partially blocked by an MLC. The resulting fluence spectra were compared to different low-LET sources for which biological response in the form of chromosomal aberrations has been published. Our results show a significant increase in the low energy component of the fluence spectra outside of the primary field, which increases the mean LET to values similar to that seen in response to a 137Cs photon source. In turn, it is shown that this has the potential to increase the RBE.

Intensity modulated arc deliveries approximated by a large number of fixed gantry position sliding window dynamic multileaf collimator fields.

The intensity modulated arc has been proposed as an alternative to tomotherapy. Treatment planing systems more typically model the conventional step and shoot or sliding window dynamic multileaf collimator (DMLC) deliveries, and may not support intensity modulated arc therapy (IMAT). As well, another potential drawback to this technique is that increasing the number of intensity levels required to achieve certain dose distributions necessitates increasing the number of gantry passes, as may occur if the desired dose distribution is complex (e.g., concave or bifurcated), potentially increasing the overall treatment time. A technique is presented here for the delivery of tomotherapy like dose distributions in a single gantry pass by the use of a large number of fields modulated by a sliding window DMLC technique from fixed equally spaced gantry positions. This serves as a good approximation to either IMAT or tomotherapy deliveries. The planning of these fields is achieved using iterative filtered back projection. Measured results of deliveries of varying degrees of complexity on a homogeneous phantom are compared to desired distributions.

Dosimetric verification of inverse planned step and shoot multileaf collimator fields from a commercial treatment planning system

An inverse treatment planning (ITP) module on a commercial treatment planning system (TPS) (Helax AB, Uppsala, Sweden) is being used for an in‐house clinical trial for treatment of nasopharyngeal cancer with contralateral parotid sparing. Intensity modulated radiation therapy (IMRT) fields are delivered by step and shoot multileaf collimator (MLC) with a DMLC enabled Varian 2300 CD (Varian Associates, Palo Alto, CA). A series of testing procedures have been devised to quantify the modeling and delivery accuracy of routine clinical inverse planned IMRT using Helax TMS and the Varian step and shoot MLC delivery option. Testing was done on specific aspects of the TPS modeling germane to DMLC. Measured relative dose factors (head scatter plus phantom scatter) for small MLC fields, normalized to a 10×10cm2 non‐MLC field, were found to differ by 2–3% from the TPS values for the smallest of the fields tested. Relative distributions for small off axis fields were found to be in good agreement. A process for the routine clinical verification of IMRT fields has been implemented. Each IMRT field in an inverse plan is imported into a flat water tank plan and a “beams eye view” (BEV) dose distribution is generated. This is compared to the corresponding measured BEV dose distribution. The IMRT verification process has also been performed using an anthropomorphic phantom. Large clinical fields (i.e., greater than 14.5 cm in the leaf direction) caused difficulties due to a vendor specific machine restriction, and several techniques for dealing with these were examined. These techniques were (i) the use of static stepping of closed junctions, (ii) the use of two separate IMRT fields for a given gantry angle, and (iii) restricting the overall maximum field size used. The overall process has allowed implementation of an in‐house protocol for IMRT use on an initial clinical site. Results of the verification measurements for the first ten patients treated at this center reveal an average maximum dose per IMRT field delivered of 71.0 cGy, with a mean local deviation from the planned dose of – 1.2 cGy, and a standard deviation of 2.4 cGy. PACS number(s): 87.53.Dq, 87.53.Tf