Volume 33, Issue 6, June 2006
Index of content:
- Therapy Scientific Session: Valencia B
- IMRT Delivery and Applications
TU‐E‐ValB‐01: Helical Tomotherapy Targeting Total Bone Marrow ‐ Initial Clinical Experience at the University of Minnesota33(2006); http://dx.doi.org/10.1118/1.2241629View Description Hide Description
Purpose: We report here the successful use of Tomotherapy at delivering intensity modulated radiotherapy to the bone and bone marrow spaces along the entire axis of a patient and describe a dosimetric analysis of the total marrow irradiation (TMI) treatment. This is part of a dose escalation trial to determine the maximum tolerated dose (MTD) of TMI when given prior to an alkylator‐intensive conditioning regimen for the treatment of high risk or relapsed solid tumors.Method and Materials: A patient enrolled in a dose escalation study trial received 600 cGy in 3 fractions. Two independent CTimage sets (upper and lower part of the body) were obtained. A helical tomotherapy treatment plan was created from this CTimage sets. The quality assurance was evaluated with the use of (a) ion chamber and (b) extended dose range film. The isorad‐p cylindrical diodes were used for in‐vivodosimetry.Results: The patient showed neutrophil engraftment on day 11 and platelet engraftment by day 58. He is currently well at 120 days post transplant with no evidence of disease. The patient developed nausea and vomiting after the first fraction of Tomotherapy TMI. Other than above there were no adverse effects of TMI. The planned radiation conformed to all bone marrow sites. Average doses to lungs,kidneys,heart, and eyes were 50–70% of the prescribed dose for TMI treatments. The dose delivery verifications (pretreatment and in vivodose measurement) were within ±3–5% of the expected dose calculated from the treatment planning station. Conclusions: We show that helical tomotherapy targeting the bone marrow of the whole body is clinically feasible. The clinical implementation of intensity modulated radiation to conform the radiation dose to all active bone marrow of the whole body opened up the possibility of a dose escalation study for high risk patients.
TU‐E‐ValB‐02: Using Magnetically Collimated Electrons and Narrow Intensity Modulated Tangential Photons for Accelerated Partial Breast Irradiation33(2006); http://dx.doi.org/10.1118/1.2241630View Description Hide Description
Purpose: Previous studies have shown that enface magnetic collimation lowers entrance dose and reduces lateral scattering for electron beams. However, dose uniformity is limited at depth for single beam delivery. This study investigates whether adding narrow intensity modulated tangential photonbeams would solve the problem and create a new technique for accelerated partial breast irradiation.Method and Materials: Magnetically collimated electron beams were measured and modeled empirically for treatment planning and combined with photonbeams on a commercial treatment planning system (Pinnacle 7.6, Philips Medical System). Treatment plans were generated for conformal irradiation of the partial breast using a pair of intensity modulated tangential photonbeams plus enface magnetically collimated electron beams. The photonbeams were planned via inverse planning with the beam weights of the electron beam adjusted simultaneously. The first MLC segment of the photonbeams was fixed to cover the entire planning target volume in the beam's eye view to minimize the effect of target motion. Final treatment plans were analyzed for dose uniformity, for the conformity of the target volume and also for the dose to normal tissues including the skin.Results: The combined beam approach significantly improved dose conformity, e.g., narrower separation between peripheral isodose lines, as compared with using either electron beams or a pair of tangential fields alone. The use of enface electron beams significantly improved the target dose uniformity to better than 10%. Due to magnetic collimation, the skin dose was on average lowered by 18% over the use of conventional electron beam for the technique. Conclusion: Intensity modulated tangential photonbeams combined with magnetically collimated electron beams offers a new technique for accelerated partial breast irradiation.
33(2006); http://dx.doi.org/10.1118/1.2241631View Description Hide Description
Introduction: We compare the dosimetric characteristics of simultaneous integrated boost (SIB) technique utilizing IMRT vs. conventional treatment consisting of medial and lateral tangents followed by an electron boost. Materials and Methods: To date, 5 women at our institution have been enrolled and treated using IMRT with SIB. Patients were treated using 25 fractions of 1.8 Gy to the whole breast (45Gy) while concurrently receiving 25 fractions of 2.4 Gy to tumor bed (60Gy). Conventional treatments were planned using the standard fractionation of 1.8 Gy × 25 fractions to the whole breast followed by 2 Gy × 8 fractions to the tumor bed.
The patient data was used to compile population dose volume histograms (pDVH), based on the mean values and include error bars which represent the 1σ uncertainty of the mean. Results: The mean percent of the BoostPTV volume receiving 60Gy, was slightly higher for the conventional technique compared to SIB (Conventional = 98±1.4%, SIB=95.0±1.0%). The mean volume of the treated breast outside the BoostPTV, PTV45‐Boost, that received greater than 120% of the prescribed dose was more in the Conventional technique then SIB (Conventional=15.9±3.4%, SIB=6.1±1.6%). The mean volume of lung receiving 20Gy was equivalent in both techniques (Conventional=19.8±5.8%, SIB=17.1±5.2%). The mean ipsilateral lung dose was equivalent between techniques (Conventional=10.7±2.2 Gy, SIB=11.9±4.2 Gy). The mean dose to contralateral breast was systematically higher in SIB technique compared to the conventional technique (Conventional=69±39 Gy, SIB=191±126 Gy). The mean volume of the heart that received greater then 30Gy was small for both plans (Conventional=2.8% and SIB=0.4%). Conclusion:IMRT with SIB is feasible and allows patients to complete EBRT in about 20% less time than the standard treatment course. It offers improved dose homogeneity to treated breast, comparable normal tissue sparing, and excellent short term cosmesis.
TU‐E‐ValB‐04: Impact On Nodal Dose Distribution From Daily Fiducial Tracking for IMRT Prostate and Pelvic Node Treatments33(2006); http://dx.doi.org/10.1118/1.2241632View Description Hide Description
Purpose: In prostate only treatments, fiducial seeds can be implanted in the prostate and used for accurate daily targeting of the prostate. For high‐risk disease, the regional nodes are simultaneously treated with the prostate using intensity‐modulated radiotherapy thereby sparing the rectum, bladder, bowel, and femoral heads. However, the implanted fiducials do not indicate the position of the nodes as the nodes are anchored to the patient's bony anatomy.Method and Materials: We measured the displacement of the fiducials with respect to the bony anatomy for each treatment fraction as a surrogate for the displacement of the nodes with respect to the prostate from their original planning position. The displacements, taken from the clinical cases were measured to the nearest 0.01 cm, in LR, AP, and SI directions and were used to perform a forward calculation of the IMRT plan that the patient received during treatment. The dose coverage to the nodes, femoral heads and bowel were evaluated using isodose curves and dose volume histograms. Results: Our results indicate for an average patient, the maximum shifts of the seeds with respect to the original DRR position was 0.78 cm inferior, 0.28 cm left, and 0.46 cm posterior. A weighted average of the same directions would be: 0.46 cm, 0.05 cm and 0.13 cm. Forward planning using the weighted average shifts, shows changes in the nodal maximum dose of −0.6% and mean dose −0.2%. The maximum bowel dose changed by −0.5% and the mean dose by −1%. The dose to the left and right femoral heads increased by +2% and +1.2%, respectively. Conclusion: The results indicate that for an average patient the difference in the dose planned to the nodal targets and the organs‐at‐risk are not clinically significant to those received during treatment using fiducial tracking of the prostate.
33(2006); http://dx.doi.org/10.1118/1.2241633View Description Hide Description
Purpose: Long volume sarcomas are usually treated with a two‐isocenter technique. Due to the sharp dose gradients at the field edges, 3 mm setup uncertainties can cause up to 30% dose variations in the overlap region, potentially increasing dose to the bone and risking subsequent fracture. This dose variation is conventionally reduced by using junction shifts. We have developed a two‐isocenter IMRT technique to spare bone and simultaneously control the junction dose. The technique uses extended dose gradients throughout a junction region to improve the tolerance to setup uncertainties and inverse planning to develop complimentary dose gradients. Methods: An initial dose gradient across a 4–6 cm junction region is induced either with segmented inferior parallel‐opposed beams or using variable dose target junction volumes. Superior IMRT fields, which overlap the junction region, are put and optimize doses superiorly and establish a complementary dose gradient to the initial gradient. Then, segments beams are removed and replaced by inferior isocenter fields to complete dose coverage in the junction region and inferior target. Results: Four lower extremity sarcoma cases have been planned with this technique in Pinnacle 7.6c, demonstrating the following advantages over the conventional technique: Improvement in junction dose uniformity: ±5% target dose heterogeneity for the segments‐induced method and ±3% for the variable‐dose‐volumes method vs. 7% for the conventional moving junction techniques; Improvement in potential error control: for 3 mm setup uncertainties, a): ±5% junction target dose shifts vs. up to ±10% hot or cold spots in the conventional treatment; b): Maintained bone sparing dose in junction vs. risk of increased bone dose. Conclusion: A technique has been developed to permit the application of IMRT to long limb sarcomas with both critical structure sparing and stable junctioning. This technique has applications to other sites where abutting fields offer treatment or efficiency advantages.
33(2006); http://dx.doi.org/10.1118/1.2241634View Description Hide Description
Purpose: Validation of respiration gated IMRT on Siemens linear accelerator has not been reported. This work investigates the reliability, accuracy and efficiency of the delivery of respiratory gated IMRT on a Siemens accelerator. Method and Materials: A Siemens Primus accelerator was interfaced with a pressure sensor belt (Anzai Gating system) to deliver step‐and‐shoot IMRT. A series of IMRT fields, including actual patient, as well as custom segmented fields including the “Picket Fence” were delivered with and without gating (interruption) using a variety of different gating parameters (e.g., duty cycle). Radiographic films and 2D diode array (MapCheck , Sun Nuclear) were used to measure dose distributions. The dose distributions measured with and without gating were compared to identify any delivery error from gating. IMRT with multiple beam angles was also delivered, with and without gating, on a cubic motion phantom. Special measurements were made to individually evaluate dark current, small MU non‐linearity and flatness degradation and their cumulative effects on multiple times interrupted fields. Delivery times for MLC and compensator IMRT plans with various segmentations for sample lung and breast cases were compared. Results: Beam characteristics for the Siemens accelerator was not altered by gating and gated IMRT with Siemens/Anzai systems was found to be accurate and reliable. Measured dose distributions agreed with the calculated results and/or with those delivered without gating. Picket fence results with and without beam gating indicated same MLC positioning accuracy during the gating. The delivery times for the DAO‐based IMRT plans that had small numbers of segments were shorter than those for any other types of IMRT plans including compensators and were comparable with those for 3DCRT. Conclusion: The delivery of gated‐IMRT with Siemens/Anzai systems is reliable and accurate. The DAO‐based IMRT is preferred for gated delivery in terms of treatment times.
33(2006); http://dx.doi.org/10.1118/1.2241635View Description Hide Description
Purpose: To develop an intensity segmentation and leaf sequencing algorithm specifically for intensity‐modulated arc therapy (IMAT), which can be applied to optimized intensity patterns derived from existing commercial IMRT inverse planning software. Methods and Materials: Three phantom cases, as well as a clinical case were planned using a Hi‐Art II (Tomotherapy Inc, WI.) planning station. The end of planning sinograms were then extracted and inputted into our IMAT conversion algorithm. The number of required arcs, deliverable MLC segments for each arc and the relative intensity weighting of each arc were outputted. The number of arcs (modulation) could be controlled by a user parameter, α. The resulting MLC segments were then fed into a fast monte‐carlo dose calculation algorithm, NXEGS (NumeriX, LLC) to obtain 3D dose distributions. Dose statistics (max, min, mean) and dose volume histograms of relevant structures were calculated and compared against the results generated by the Hi‐Art II system. Results: Each plan was converted in under three minutes on a typical desktop PC, with the arc numbers varying between 4 and 15 360° arcs. Qualitatively, the dose distributions obtained from the IMAT plans were similar to the tomotherapy results, as well as planned doses. Quantitatively, the IMAT plans were slightly degraded, with the average dose to normal structures being 7.5% higher for IMAT vs. tomotherapy. However, the IMAT plans generally met planned values, being 9.1% below for maximum doses to normal structures. The number of arcs and therefore the resulting dose distribution could be varied according to α. Conclusions: IMAT segmentation and leaf sequencing produced deliverable IMAT MLC segments and relative arc weights directly from Hi‐Art II optimized plans. The algorithm was computationally efficient, and produced similar dose distributions. Additional optimization could improve resulting dose distributions further. IMAT back‐up for tomotherapy is another potential application.