1. Impact of individualized target volumes on stereotactic body radiotherapy
(SBRT) treatment planning: Dosimetric analysis
CJ Hampton, WT Kearns, JJ Urbanic, KP McMullen, AW Blackstock, WH Hinson
Department of Radiation Oncology • Wake Forest University School of Medicine • Winston-Salem, North Carolina
Introduction either in the absence of (patients 1-3), or as a complement to the only one patient (patient #4, Figure 2a). The worst case indicated
compression apparatus (patients 4-6). Our SBRT treatment planning underdosing of PTVInd by 14% (patient #1). The portion of PTVInd not
Multimodality and 4D imaging provide temporal, anatomic and protocol introduces a hybrid internal target volume, ITVInd, defined by the overlapped by PTV0236 was on average covered by a mean dose of 96% of
physiological information that help reduce treatment planning combination of the 4DCT-derived MIP with gross tumor volumes (GTV1 & the prescription isodose. PTVInd, however, contained “cold” areas where
uncertainties. Our institution has performed over 200 CT or PET/CT SBRT
GTV2) contoured from the registered helical CT acquisitions and GTVPET the prescription dose was reduced by as much as 54% (Patient 6, Figure
simulations since 2003 using the commercially available Elekta
when available. Isotropic margins of 5mm were added to ITVInd as a 2b). When treatment plans created using PTV0236 and PTVInd as targets are
Stereotactic Body Frame (Elekta, Inc., Norcross, GA). While the
“safety margin" creating PTVInd(Figure 1). 3D treatment plans were compared, the impact of individualized margins on irradiated lung volumes
bodyframe employs stereotactic principles to immobilize patients for
calculated with heterogeneity corrections delivering D95%= Rx isodose for is exhibited by the ability to achieve dose coverage for the larger PTVInd
reproducible setup, a small amount of setup uncertainty can be quantified
by analyzing registered initial and repositioning CT scans. The bodyframe PTVInd, and minimal dose to normal tissues, especially the normal lung volume without sacrificing minimization of the irradiated lung volume
includes an abdominal compression apparatus which is used to force (V20<10%). For comparison, treatment plans were also created for a (Table 2). In fact, for all patients, V13 and V20 differ by no greater than 2%
shallow breathing by the patient, thereby minimizing physiological tumor for the two plans even when no abdominal compression is available and
second PTV (PTV0236) based on the expansion of GTV2 with population-
motion. A subpopulation of eligible patients, however, is unable to the PTVInd is largest (Patient 1). All treatment plans were able to achieve
based margins of 5mm (10mm S/I) as recommended by RTOG 0236.
Patient
tolerate the abdominal compression or, limited by anatomy, is unable to Abdominal Volume (cc) Dmax (%) V20 less than 10% as targeted by the protocol. When individualized margins
avoid collisions with the device. 4DCT, has emerged as a viable means of Compression? PTV0236 PTVIND
PTV0236 PTVIND are used for planning along with abdominal compression, the mean lung
creating patient-specific target volumes (PSTV) capturing a sample of 125 150 dose is decreased by an average of 11 cc with patient 4 achieving the
1 No 18.2 28.4
intrafraction motion. This work assesses the impact of individualized 125 122 largest reduction (-27cc).
2 No 23.8 30.2
target volumes derived from multimodality simulation scans on treatment
3 No 28.3 35.1 128 130
plans for 6 lung SBRT patients. The dosimetric impact of individualized Irradiated Lung Volume (% of total)
Patient
146 154 MeanLD (cc)
4 Yes 11.1 9.9 V13 V20
target volumes on target and normal tissues is contrasted with patient
5 Yes 52.1 56.9 120 132 PTV0236 PTVIND PTV0236 PTVIND PTV0236 PTVIND
population-based target volumes recommended by the recently closed
RTOG 0236 phase II clinical trial which investigated SBRT in the treatment 6 Yes 89.3 98.2 129 133
1 8 9 4 5 343 362
of medically inoperable stage I/II non-small cell lung cancer. (a) (b) 2 12 12 5 5 403 437
Table 1 – A comparison of target volumes (a) and Dmax (b) for plans 3 7 7 3 4 344 356
covering D95% of PTVInd and PTV0236 with the prescription isodose surface. 4 4 3 2 1 180 153
5 10 9 6 6 371 352
(a) (b) 6 17 17 8 10 565 579
Deviation (% of DRx)
Table 2 – A comparison on the impact of target coverage for PTVInd and
Cold Spot (% of DRx)
PTV0236 on the irradiated lung volume
Figure 1 – Individualized target volumes: the combination of GTV1 Conclusions
(orange), GTV2 (green), GTVPET (khaki), and MIP (yellow) plus margin • DVH analysis of comparative plans indicates that a reduction in
form PTVInd (light blue). PTV0236 (navy blue) is an expansion of GTV2. Patient Patient dosimetric coverage of the individualized planning target volume by
14% of the prescription dose may occur for some patients.
Figure 2 – (a) Deviation in coverage for PTVInd for a treatment plan
• Due to steep falloff gradients, non-overlapped regions of PTVInd
targeting PTV0236. (b) Cold spot (Dmin) as a percentage of DRx resulting
Methods and Materials from deviation in target coverage
contained “cold” spots with a 54% reduction in the prescription dose
coverage.
Three of the six SBRT patients met the criteria for the use of abdominal Results
• Coverage of the larger PTVInd can be achieved mostly by increasing
compression. Our SBRT simulation protocol involved the acquisition of 3-4 heterogeneity within the tumor volume with minimal increases in
The volume of PTVInd was greater than PTV0236 for 5 of 6 patients (Table
imaging studies per patient using a GE Discovery PET/CT scanner (GE irradiated lung volume.
1a). Separate treatment plans providing the desired coverage of PTV0236
Healthcare, Waukesha, WI), interfaced with the Varian Real-time • Individualized planning target volume margins taking into account data
Positioning (RPM) system (Varian Medical Systems, Palo Alto, CA): 2 and PTVInd were successfully created. For the group, the average increase
quantifying setup uncertainties and physiological tumor motion may be
sequential helical CT scans interspaced by patient repositioning, a non- in Dmax as a consequence of covering PTVInd was 8% (Table 1b). DVH used for stereotactic body radiotherapy without increasing irradiated
gated PET scan (optional) and a cine CT scan which was retrospectively analysis of target coverage demonstrates the potential risk of underdosing lung volumes beyond accepted values.
gated to produce a 4DCT dataset. 4DCT, was used for all patients to when population-based models are used. When the plan providing • The volumetric impact of individualized target volumes on SBRT was
create patient-specific maximum intensity projections (MIP) coverage of PTV0236 was spatially overlaid on top of PTVInd, target coverage the subject of a related investigation, the results of which are discussed
in ASTRO poster presentation # 2998 (CJ Hampton et al.)
was on average reduced by 8.3% with coverage maintained for
Presentation #3016