S Feigenberg

Stereotactic IMRT for prostate cancer: dosimetric impact of multileaf collimator leaf width in the treatment of prostate cancer with IMRT.

The focus of this work is the dosimetric impact of multileaf collimator (MLC) leaf width on the treatment of prostate cancer with intensity‐modulated radiation therapy (IMRT). Ten patients with prostate cancer were planned for IMRT delivery using two different MLC leaf widths—4 mm and 10 mm— representing the Radionics micro‐multileaf collimator (mMLC) and Siemens MLC, respectively. Treatment planning was performed on the XKnifeRT2 treatment‐planning system (Radionics, Burlington, MA). All beams and optimization parameters were identical for the mMLC and MLC plans. All the plans were normalized to ensure that 95% of the planning target volume (PTV) received 100% of the prescribed dose. The differences in dose distribution between the two different plans were assessed by dose–volume histogram (DVH) analysis of the target and critical organs. We specifically compared the volume of rectum receiving 40 Gy (V40), 50 Gy (V50), 60 Gy (V60), the dose received by 17% and 35% of rectum (D17 and D35), and the maximum dose to 1 cm3 of the rectum for a prescription dose of 74 Gy. For the urinary bladder, the dose received by 25% of bladder (D25), V40, and the maximum dose to 1 cm3 of the organ were recorded. For PTV we compared the maximum dose to the “hottest” 1 cm3 (Dmax1cm3) and the dose to 99% of the PTV (D99). The dose inhomogeneity in the target, defined as the ratio of the difference in Dmax1cm3 and D99 to the prescribed dose, was also compared between the two plans. In all cases studied, significant reductions in the volume of rectum receiving doses less than 65 Gy were seen using the mMLC. The average decrease in the volume of the rectum receiving 40 Gy, 50 Gy, and 60 Gy using the mMLC plans was 40.2%, 33.4%, and 17.7%, respectively, with p<0.0001 for V40 and V50 and p<0.012 for V60. The mean dose reductions for D17 and D35 for the rectum using the mMLC were 20.4% (p<0.0001) and 18.3% (p<0.0002), respectively. There were consistent reductions in all dose indices studied for the bladder. The target dose inhomogeneity was improved in the mMLC plans by an average of 29%. In the high‐dose range, there was no significant difference in the dose deposited in the “hottest” 1 cm3 of the rectum between the two plans for all cases (p>0.78). In conclusion, the use of the mMLC for IMRT of the prostate resulted in significant improvement in the DVH parameters of the prostate and critical organs, which may improve the therapeutic ratio. PACS number: 87.53.Tf

Randomized phase II trial of cisplatin, etoposide, and radiation followed by gemcitabine alone or by combined gemcitabine and docetaxel in stage III A/B unresectable non-small cell lung cancer

Purpose: Southwest Oncology Group 9504 demonstrated the feasibility and potential benefit of docetaxel consolidation after etoposide, cisplatin, and radiotherapy in patients with locally advanced non-small cell lung cancer. Our study assessed consolidation with either gemcitabine alone or with docetaxel after identical chemoradiation as used in Southwest Oncology Group 9504. Methods: Patients with stage III non-small cell lung cancer and good performance status were included. Treatment consisted of concurrent cisplatin 50 mg/m2 on days 1 and 8 plus etoposide 50 mg/m2 on days 1 to 5 for two 28-day cycles plus radiotherapy (62 Gy, 2 Gy daily in 31 fractions over 7 weeks), followed by randomization to either gemcitabine 1000 mg/m2 on days 1 and 8 (G) or gemcitabine 1000 mg/m2 on days 1 and 8 plus docetaxel 75 mg/m2 on day 1 (GD) every 21 days for three cycles. Results: Eighty-three patients were entered, 81 received induction therapy, and 64 were randomized (32 in each arm). Grade 3 or four events, including neutropenia (56.3% vs. 28.1%, p = 0.03), anemia (18.8% vs. 3.1%, p = 0.05), and fatigue (15.6% vs. 6.3%, p = NS), were more frequent with GD compared with G. Among all patients, median survival from registration was 20.8 months (95% confidence interval: 16.4–33.8), and 2-year survival was 46.7% (95% confidence interval: 35.6–57.1). From randomization, median progression-free survival was 5.4 months for G and 13.4 months for GD, and median survival was 16.1 months for G and 29.5 months for GD. Two-year survival rates were 40.6% for G and 55.7% for GD. Conclusion: The doublet, as expected, resulted in more toxicity, particularly myelosuppression and fatigue. Survival associated with the GD treatment arm of this trial exceeds that of previously reported trials.

A method for repositioning of stereotactic brain patients with the aid of real-time CT image guidance.

This note presents a method that recalculates the coordinates of the isocentre for patients undergoing stereotactic radiotherapy to the brain with a relocatable head frame based on a pre-treatment CT scan. The method was evaluated by comparing initial stereotactic coordinates of the isocentre with the recalculated coordinates for eight single-fraction patients. These patients had the Brown-Roberts-Wells (BRW) frame fixed to the outer table of the skull, and therefore the coordinates of any anatomical point should be identical between the initial scan and the pre-treatment scan. The differences between the two sets of coordinates were attributed to errors in the method. The results showed that the systematic errors in the recalculated coordinates were less than 0.05 mm, and they were not statistically significant. The random errors (one standard deviation) were from 0.35 mm (lateral) to 0.58 mm (vertical). The average value of the combined 3D difference was 0.75 mm.

MO‐D‐224A‐04: EPID Transmission Dosimetry for QA Use in Adaptive Radiotherapy

Purpose:Radiotherapy patients treated for H&N cancer often lose weight and have shrinkage of their tumors causing drastic anatomical changes. This can result in changes in dose distribution with respect to PTV coverage and OARs. Monitoring these changes is difficult and presents QA problems for IMRTtreatments. In this work we develop a method to monitor H&N thickness changes and correlate with changes in dose distribution. Method and Materials: Wax was applied to the neck region of an anthropomorphic phantom in 3‐1cm layers. Contours depicting tumor and critical structures were delineated. An IMRT plan was generated to delivery 70Gy to the PTV. The resultant sequence was virtually delivered to the phantom with layers of wax removed. PTV coverage, hot spot, and PRV doses were recorded. A characteristic response curve was generated using the amorphous siliconEPID on a Varian 21EX linear accelerator and slabs of solid water. The phantom was then consecutively imaged removing 1cm layers of wax. All images were acquired for the same number of MU. Results: Decreasing the thickness of the neck region bilaterally by 3cm resulted in an increase in 95% PTV coverage from 70Gy to 72.8Gy. The maximum dose in the PTV increased from 80.6Gy to 86Gy. The PTV volume receiving 110% of the prescribed dose increased from 13.4% to 66.6%. The dose to 0.01cc of the spinal cord and brainstem PRVs increased from 45.8Gy to 47.9Gy and 49.7Gy to 51.2Gy, respectively. Using the EPID we were able to predict changes in the lateral dimension of the phantom to within 4mm. Conclusion: Our results indicate that anatomical changes during treatment may lead to unacceptable dose distributions. By using lateral EPIDimages we can monitor the thickness changes (path length) in the H&N region. These changes may be used to determine when re‐planning is necessary.

SU‐GG‐J‐73: Investigate the Equivalence of Cone Beam CT (CBCT) versus 4‐Dimensinoal (4D) MIP Images for Internal Target Volume (ITV) Definition

Purpose: Maximum intensity projection (MIP) reconstructed 4‐dimensional (4D) computed tomography(CT) reflects the range of target motion and thus is generally used for internal target volume (ITV) definition in lung stereotactic body radiotherapytreatment planning. During treatment delivery, CBCT is used for image guidance by aligning the visualized target in the CBCT with the MIP‐based ITV. This work investigates whether these two image modalities are equivalent in defining an ITV. Materials and Methods: A ball‐shaped polystyrene phantom with different built‐in objects (cube, cone, and sphere) was attached to a motor‐driven platform, which simulates a sinusoidal movement with changeable motion amplitude and frequency. The motion of the platform was set along patient superior‐inferior direction with 1‐cm peak‐to‐peak amplitude. The Varian on‐board Exact Arms kV CBCT system and the GE LightSpeed 4‐slice CT integrated with Respiratory Position Management 4DCT scanner were used to scan the phantom that moved with three frequencies (e.g. 3.4, 4.5, and 5.8 seconds). MIP images were produced. The objects were contoured in the MIP and CBCTimages, respectively, and the volumes of these objects were compared between the two sets of images.Results: It was found that the MIP‐based volumes of these objects increase with the increase of the motion period while the CBCT‐based volumes do not change with the frequency. Even for the fast motion (3.4 sec), the volumes contoured in the MIP images, which are smallest among the volumes of three frequencies, are generally 10% larger than those delineated from CBCT.Conclusions: The delineated volume of an ITV changes as the frequency of target motion changes in the MTP reconstructed 4DCT. The delineated volume of a moving target is not affect by the frequency, but by motion magnitude, in CBCT. In general, CBCT and MIP images are not equivalent in defining an ITV.

How Much Target Average Positions in Thorax Region Change Daily

Purpose: Targets in the thorax regions are subject to respiratory motion and the average position maybe changed daily. This study aims to assess the degree of target average position change in respect to the bony structure using cone-beam computed tomography (CBCT). The work further validates the planning target volume (PTV) margin used in the stereotactic body radiotherapy (SBRT) of lung cancers.

SU‐FF‐J‐90: Target Relocalization Accuracy and PTV Margin Verification Using Three‐Dimensional Cone‐Beam Computed Tomography (CBCT) in Stereotactic Body Radiotherapy (SBRT) of Lung Cancers

Purpose: To assess the target relocalization accuracy in respect to the bony structures using daily CBCT and thus to validate the planning target volume (PTV) margin used in the lungSBRT.Methods and Materials: All patients underwent 4D CT scanning in preparation for lungSBRT. The internal target volume (ITV) is outlined from the reconstructed 4D data using the maximum‐intensity projection (MIP) algorithm. The clinical target volume (CTV) is defined as the ITV plus 3 mm margin and an additional 3 mm margin is added to the CTV to make the PTV. Conformal treatment planning is performed on the helical images, to which the MIP images were fused. Prior to each treatment, CBCT is used to align with the simulation CT (helical) based on bony anatomy. The necessary shifts are recorded. The treating physician then checks and modifies the alignment based on target relocalization within the PTV (soft tissue alignment). The final shifts are derived based on the soft tissue alignment. Two sets of shifts are compared here for the purpose of the study. Results: For 8 consecutive patients, treating 9 targets for a total of 36 fractions, it was found that the treatment setup errors (based on bony anatomy alignment) are 0.23±0.47 cm in the anterior‐posterior (AP), 0.06±0.5 cm in the lateral (Lat), and 0.18±0.39 cm in the superior‐inferior (S‐I) directions, respectively. After the setup error correction, the targets were found to be within the PTV margin with the average shifts of 0.064±0.64 cm in AP, 0.089±0.41 cm in Lat, and 0.21±0.38 cm in S‐I directions, respectively. Conclusions: For this patient population, the target relocation accuracy is generally satisfactory after patient setup error correction. This demonstrates that the PTV margin designed based on the ITV outlined on MIP images is appropriate to account for intra and inter‐fractional tumor motions.

SU‐FF‐J‐40: A Simple Method for Dosimetric Assessment in H&N Radiotherapy with Rotational Setup Errors

Purpose: Rotational setup error for H&N patients is detected using cone‐beam CT. Fusion with 3D image can determine a sequence of three Euler angles for the best matching with the planned patient position. Currently, correction for rotational errors is not available except with the use of a 6‐degree‐of‐freedom treatment couch. However, it is possible to compensate an arbitrary rigid‐body rotation by just a simple couch rotation in addition to gantry and collimator angle changes. We introduce a formulation for compensating a rotational error with respect to a given planned beam position for adaptive treatment or easy dosimetric evaluation for no correction. Method: For each planned beam position, the beam axis in patient coordinates upon rotational error is calculated, pointing out of the beam source plane. A couch rotation is first applied to move the planned beam axis back to the source plane. The adapted gantry and collimator angles are then calculated to align with the planned beam axis and radiation portal. The inverse transformation determines the effective beam position in planning CT for dosimetric evaluation with no correction. Results: Our method was validated by phantom experiments. Light fields were compared for various beam positions before and after phantom rotations. The delivered dose distribution for two patients with 2∼3 degrees of rotations were calculated and compared with the planned distribution, showing marginal deterioration with 1.5% target dose reduction and 2% parotid dose increase. Conclusions: A simple “theoretical” solution is available for handling rotational setup error when rigid‐body approximation applies. Its practical application is to evaluate the dosimetry for no correction, and our data showed that rotational error was less of a concern compared to as large as over 5% dose variation with patient anatomy change due to weight loss or tumor shrinkage.

SU‐EE‐A1‐02: Image Guided Hypofractionated IMRT for Multiple Intracranial Metastases — An Alternative to Arc‐Based SRS

Purpose:Stereotactic radiosurgery(SRS) uses an invasive head‐frame and involves long treatment times when multiple isocenter plans are treated, which becomes intolerable for some patients. This study evaluates intensity‐modulated radiotherapy(IMRT) for treating multiple (⩾4) intracranial tumors in comparison to arc‐based SRS.Method and Materials: This study involved 3 patients treated with image‐guided hypofractionated IMRT (Eclipse) and 3 treated with SRS using arc therapy (XKnifeRT). There were 28 intracranial lesions in total with planning target volumes (PTVs) ranging from 0.195cc to 5.64cc. For this study, 3 IMRT patients were re‐planned for arc therapy, while 3 SRS patients were re‐planned for PTVs (=GTVs+2mm) for both IMRT and arc therapy. By keeping the minimum surface dose of the PTVs the same between the IMRT and arc plans, we have compared maximum doses and PITV (the ratio between the volume of Prescribed Isodoseline and the Target Volume) for the two methods. We also compared the doses received by 10%, 20% and 50% of the whole brain, as well as the maximum doses to the critical structures (eyes and brainstem). Results: The average maximum doses to the 28 targets were 125.85%+/− 18.4% for arc plans and 115.2%+/−4.6% for IMRT plans. The average PITVs were 2.65+/−1.22 and 1.93+/−0.52 for the arc and IMRT plans, respectively. For critical structures, average doses received by 10%, 20% and 50% of the brains, as well as the maximum doses to other structures, were higher for IMRT plans. However, the treatment time for IMRT is significantly reduced (e.g. 15 min) compared to that for multiple isocenter SRS (up to hours). Conclusion:IMRT plans appeared to achieve comparable conformality compared to arc based SRS. This provides a good alternative for poor performance status patients. However, further study needs to be conducted to ensure the PITV calculation accuracy for small targets.

WE‐D‐351‐05: Track‐Based Treatment Planning for Radiosurgery: A Modified McGill Technique

Purpose: Linac‐based radiosurgery is performed using arcs delivered at different couch angles. The goal of this study is to present the idea of track‐based, instead of arc‐based planning. Various beam tracks can be delivered to a given isocenter if the couch and gantry are rotating simultaneously. Such a technique would be a modification of the McGill technique introduced in 1987. Method and Materials: In theory, track‐based planning should have advantages over the arc‐based planning because every set of acceptable arc‐based plans is a subset of all acceptable track‐based plans. The total‐arc‐degree parameter in arc‐based planning would be replaced by the track length over a sphere with a unity radius. In order to illustrate track‐based planning, several different plans using large number of 10‐degree arcs and appropriate couch rotations between adjacent arcs were generated. The dose distributions were calculated using the XKnife TPS (Radionics, Boston, MA). Several treatment plans were created for several targets shapes and sizes. A track‐based plan for a lesion with AP elongation (the most difficult ellipsoid target) was compared with two conventional single isocenter plans (15‐mm cone). Results: The three plans for the AP‐elongated lesion achieved the same conformality (PITV 1.4). However, the track‐based plan delivered significantly lower doses to the surrounding normal brain defined by the volume receiving 20% and 50% of the prescription dose (20.8 cc & 4.7vs. 32.3 cc & 5.6). The size of the isodose lines was also smaller for the track‐based plan (5.9 cm & 3 vs. 19.7 cm & 4.2). Similarly, the track‐based planning achieved similar conformalities with all targets. Conclusion: There is no doubt that the track‐based planning will be more flexible and offer a greater variety of solutions compared to arc‐based planning. Track‐based planning could be the future of automated forward or inverse planning.