Volumetric modulated arc therapy (VMAT) for prostate cancer

Transcription

1 Planning study Volumetric modulated arc therapy (VMAT) for prostate cancer Institution: Purpose: Swedish Cancer Institute, Seattle, WA, USA In a community cancer center, treating prostate cancer comprises about a third of the volume of a typical practice. VMAT promises to improve the speed of clinical treatments and improve the overal efficiency of the clinic. The purpose of this study was to compare VMAT with step-andshoot IMRT in terms of plan quality and delivery efficiency for patients with prostate cancer. First published in poster format at ASTRO 2008, Boston USA.

2 Volumetric modulated arc therapy for prostate cancer Authors Muhammad K. N. Afghan, PhD Daliang Cao, PhD Vivek Mehta, MD Tony Wong, PhD Jin-Song Ye, MS David M. Shepard, PhD Method and materials Four patients with prostate cancer who were previously treated with step-and-shoot IMRT were included in this study. To generate VMAT treatment plans for each case, we used Philips Pinnacle3 treatment planning system to create a step-and-shoot IMRT plan using a range of 12 to 35 equally spaced beams. The optimized intensity maps were then converted to deliverable arcs using our homegrown software. The software uses simulated annealing based optimization engine to optimize aperture weights and leaf positions that minimize the difference between the optimized and sequenced intensity maps. The VMAT delivery constraints are enforced during the optimization process. The sequenced arcs from the software are then imported into the Pinnacle3 treatment planning system and a final dose calculation is performed. To recover any loss of plan quality due to the arc sequencing, a segment weight optimization in the Pinnacle3 treatment planning system is performed. For each patient, the VMAT plan and the step-and-shot IMRT plan were compared in terms of the dosimetric coverage of the planning target volume (PTV). The doses to the rectum and bladder provided by the two delivery techniques were also compared. The VMAT treatment plans were delivered using PreciseBEAM VMAT control system and verified using the MatriXX system. We also compared the delivery efficiency of VMAT with the fixed-field IMRT. Results Figures 1a and 1b show a single arc VMAT plan created for the patient. Note that a highly conformal plan was produced using a single arc. The plan required monitor units to deliver the prescribed dose of 180cGy per fraction whereas the seven-field IMRT plan required monitor units to deliver the same prescribed dose. Figure 1: VMAT plan for prostate patient 1 1a,left ) axial view of the isodose curves 1b, right ) coronal view of the isodose curves

3 Figure 1c) DVH comparison of a single arc VMAT plan and seven-field IMRT plan. The DVH comparison of the VMAT plan with seven-field IMRT plan is shown in Figure 1c. For the VMAT plan, the percentage volume of the PTV covered by more than 95% of prescribed dose (V95) increased from 97.6% to 99.5%. The standard deviation in the target dose decreased from 4.8 to 3.1cGy. The mean dose per fraction to the bladder increased from to 105.6cGy whereas in the case of rectum it decreased from 72.1 to 10.8cGy, when VMAT was used. Figure 2 shows the comparison of planned and measured dose distribution of the VMAT plan for prostate patient 1. The coronal view of the planned distribution is shown in Figure 2a whereas the measured distribution is shown in Figure 2b. Figure 2c shows the overlay of planned and measured isodose curves. Note that the two dose distributions look very similar. The pass rate using gamma index with 3%/3mm criterion was 96.24%. The treatment time to deliver the VMAT plan was 3 minutes 18 seconds as compared with 7 minutes 44 seconds for the fixed field delivery. Figure 2: comparison of planned and measured dose distribution of the VMAT plan for prostate patient 1. 2a, left) planned distribution 2b, right) measured distribution

4 Figure 2: comparison of planned and measured dose distribution of the VMAT plan for prostate patient 1. 2c) overlay of planned and measured isodose curves. Figure 3 shows a highly conformal single arc VMAT plan for prostate patient 2. The plan required monitor units to deliver the prescribed dose of 180cGy per fraction whereas the seven-field IMRT plan required monitor units to deliver the same prescribed dose. Figure 3: VMAT plan for Prostate patient 2. 2a, left) Axial view of the isodose curves. 2b, right) Sagittal view of the isodose curves. (continued)

5 Figure 4 shows the overlay of planned and measured isodose curves. The pass rate using gamma index with 3%/3mm criterion was 98.98%. The treatment time to deliver VMAT plan was 2 minutes 29 seconds. Figure 4: overlay of planned and measured isodose curves for prostate patient 2. For VMAT plans, V95 increased from 96.7% to 98.9%. In addition, VMAT provided improved tumor dose homogeneity. The average standard deviation in the target dose decreased from 4.7 to 3.4cGy per fraction. The average mean dose per fraction to the bladder and the rectum increased from 81.3 and 79.5cGy to 82.1 and 80.8cGy, respectively, when VMAT was used. For VMAT plans, the average dose received per fraction, by 20% of the bladder, decreased from to 148.4cGy whereas the average dose received per fraction by 20% of the rectum increased from to 129.3cGy. On average, VMAT plans required 536 MUs whereas step-and-shoot IMRT plans required 621 MUs to deliver the prescribed dose of 180cGy per fraction. The average treatment time to deliver the VMAT plans was 3 minutes 28 seconds as compared with 7 minutes 33 seconds for the fixed field deliveries. The average pass rate using gamma index with 3%/3mm criterion of 98.3% was observed. Conclusions From a dosimetric perspective, VMAT treatment plans are equivalent to fixed field IMRT. VMAT, however, provides a more efficient delivery technique with an average time savings of over 4 minutes per patient.

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