Dosimetric Comparison of Robotic IMRT and Partial-Arc VMAT in Accelerated Partial Breast Irradiation (APBI) for Early Breast Cancer

Poster Viewing E639

Volume 96  Number 2S  Supplement 2016 Purpose/Objective(s): In radiation therapy (RT) of esophageal cancer, CTV to PTV margins are generally isotropic and equal for all patients. However, detailed knowledge of the position variability and tumor motion caused by respiratory motion is lacking. The purpose of this study was to accurately quantify esophageal tumor position variability and respiratory motion and investigate possible surrogate structures for image guidance. Materials/Methods: The first 12 patients enrolled in a prospective cohort study (NCT02139488) were analyzed. Patients were treated with chemo-RT with a radiation dose of 23 to 281.8 Gy combined with weekly carboplatin/ paclitaxel and daily 4D CBCT scans. As soft tissue contrast in CBCT is limited, gold fiducial markers (0.35 x 5 mm) were inserted during endoscopic ultrasonography preferably at the proximal border, in the middle and at the distal border of the tumor before the planning CT was made. The following regions of interest (ROI) were registered for each fraction: bony anatomy (vertebrae), carina, diaphragm, and fiducial markers using a rectangular ROI, and gross tumor volume (GTV), using a shaped ROI (“mask”). These different surrogates for setup and their implication on margins were calculated with statistics of the average residual marker displacement when using the different Pregions as reference (standard deviations of random [s] and systematic [ ] errors). Breathing amplitudes also influence margins; therefore, their distribution within the cohort was determined by the fiducial ROI. Subsequently, a planning target volume margin including the average respiratory motion was determined for these scenarios according to the nonlinear van Herk formula. Since esophageal tumors border both to lung and mediastinal tissue, the parameters for this formula were conservatively chosen to be representative for water. Results: A median of 3 fiducials was placed in 12 tumors located at the mid esophagus (nZ3), lower esophagus (nZ4), and gastroesophageal junction (nZ5). The median (range) peak-to-peak respiratory tumor motion amplitude in the left-right (LR), craniocaudal (CC), and anteroposterior (AP) directions was 0.15 (0.07-0.73), 0.63 (0.39-0.95), and 0.30 (0.08-0.79) cm, respectively. The required margin, depending on surrogate used for setup correction, ranged from LR 0.69-0.88 cm and CC 0.76-1.14 cm to AP 0.56-0.7 cm (Table 1). Overall, the registration on mask results in the smallest margins. Conclusion: Substantial position variability of the GTV during RT of esophageal cancer was observed as well as interpatient variation in respiratory-induced motion. Tumor localization is considerably improved, compared to bony setup, when the GTV mask registration is used in CBCT guidance, despite low soft tissue contrast. Furthermore, patient-specific margins are required to mitigate breathing-induced motion. Abstract 3566; Table 1. Region of interest Bone Carina Diaphragm Mask

Margin LR (cm)

Margin CC (cm)

Margin AP (cm)

0.79 0.77 0.88 0.69

1.09 1.00 1.14 0.76

0.67 0.62 0.70 0.56

Author Disclosure: F.E. Voncken: None. S. Nakhaee: None. L. Wiersema: None. J.M. van Dieren: None. M.E. van Leerdam: None. J. Sonke: Research grants and royalties; Elekta Oncology Systems Ltd. member of international advisory board; PMB. B. Aleman: None. P. Remeijer: None.

3567 Dosimetric Comparison of Robotic IMRT and Partial-Arc VMAT in Accelerated Partial Breast Irradiation (APBI) for Early Breast Cancer S.Y. Kim, J.H. Kim, J.Y. Woo, J.G. Baek, K.S. Park, M.J. Kim, and J.H. CHO; Department of Radiation Oncology, Yonsei Cancer Center, Yonsei University Health System, Seoul, Korea, The Republic of Korea Purpose/Objective(s): We conducted a pilot plan study for APBI to set up an effective and new approach of IMRT using different modalities. This study compares dosimetric and treatment-relevant parameters between partial arc volumetric modulated arc therapy (P-VMAT) and robotic IMRT (R-IMRT/robotic radiosurgery with multileaf collimator).

Materials/Methods: Eight patients were selected (LT nZ4, RT nZ4). Two different plans using VMAT and R-IMRT were created. Surgical clips were used as fiducials for R-IMRT with synchrony tracking. A total of 34 Gy in 10 fractions was prescribed to the planning target volume (PTV). Dosimetric parameters including conformity index (CI), homogeneity index (HI), and dose gradient range (DGRZReff50%-ReffRX, ReffRXZ((3VRX)/4P)1/3) were used for plan evaluation. PTV coverage and organs at risk (OAR) dose were also evaluated according to NSABP B39/RTOG-0413 guideline. For clinical application, monitor units (MUs), delivery time, and segments number were also compared. Results: Both plans satisfied the goal of the guideline. 95% of the PTV was to receive 100% of the prescription dose. P-VMAT significantly reduced heart volumes receiving 1.7 Gy versus R-IMRT (the mean V1.7Gy was 1.6% vs 0.1% and 29.1% vs 11.1% for RT and LT, P<0.01). No other significant difference was observed both in PTV coverage and OAR dose (Table 1). The mean CI and HI value of PTV were 1.05  0.05 and 1.09  0.03 in PVMAT plans and 1.04  0.03 and 1.12  0.02 in R-IMRT plans. The mean DGR was 1.22  0.17 cm and 1.49  0.3 cm for R-IMRT and P-VMAT plans. In addition, the mean MUs for P-VMAT (710.7  42.3) were less 56.5% than those for R-IMRT (1634.2  132.8). Delivery time takes about 14 times longer for R-IMRT (33  6.9 minutes) than P-VMAT (2.3  0.4 minutes) without considering patients’ setup time. The mean segments number was 333.6  18.3 for P-VMAT and 130.6  37.7 for R-IMRT. Conclusion: Both modalities achieved clinically acceptable plans and similar outcomes. Considering the DGR values, R-IMRT rapidly decreases the high dose region in normal tissue adjacent to the target which means effective sparing of normal breast. Though we used the same PTV for both plans to compare dosimetric parameters, R-IMRT with synchrony tracking is a very attractive system that can reduce the target margin in actual treatment. In spite of these advantages, R-IMRT requires more MUs and beam delivery time to deliver similar target prescription to the tumor than P-VMAT. PVMAT would be considered useful modality in terms of efficiency. Abstract 3567; Table 1. Dosimetric parameters (meanSD) PTV

Ipsi_Breast Ipsi_Lung Contr_Breast Contr_Lung Heart Rt Lt Skin ChestWall

Dmean D2% V100% V50% V100% V5Gy V10Gy Dmax V1.7Gy e V1.7Gy V1.7Gy Dmax Dmax

P-VMAT

R-IMRT

Unit

35.10.7 36.11.0 950.1 478.4 22.16.8 104.9 3.12 0.90.1 1.53.2 e 0.10.2 11.113.2 34.51.4 35.11.6

35.70.5 37.30.7 950.1 45.610.2 22.16.7 13.35.8 2.31.7 0.90.2 0.81.5 e 1.61.3 29.115.6 35.12.3 36.31.1

Gy Gy % % % Gy %

Gy

Author Disclosure: S. KIM: None. J. Kim: None. J. Woo: None. J. Baek: None. k. Park: None. m. Kim: None. J. CHO: None.

3568 Regression Model of Tumor and Diaphragm Position for MarkerLess Tumor Tracking in Carbon Ion Scanning Therapy for Hepatocellular Carcinoma R. Hirai,1 Y. Sakata,1 Y. Taguchi,1 and S. Mori2; 1Toshiba Corporation, Kawasaki, Japan, 2National Institute of Radiological Sciences, Chiba 2638555, Japan Purpose/Objective(s): To allow tumor targeting in the context of liver treatment planning, it is important to estimate the tumor position dynamically. In fluoroscopy, a multiple template-based method has been proposed for tumor tracking without fiducial markers. This method requires ground-truth tumor positions on training images to estimate tumor templates; however, these positions have to be manually set. Moreover, the low contrast of fluoroscopy images makes tumor position marking difficult. To solve this problem, we propose a semiautomatic tumor position tracking method for the training images to reduce both the positional error