Treatment Planning of Lung Cancer: Dosimetric Considerations Indrin J. Ch hetty, PhD Henry Ford Health System
Disclo osure My department receives research support from: NIH/NCI Varian Medical Systemss Philips HealthCare
Learning Obje ctives/outline To discuss the physics related to lung tumor dose coverage, with special emphasis on small tumor sizes and location To review dose calculation accurac cy with different algorithms for lung planning/dose calculation To present approaches to improve plan quality especially when using simplistic calculation algorithms for lung cancer planning To review example volumetric arc technical lissues therapy plans and discuss
Pre Test Question. For the treatment of small lung tu umors located peripherally, which dos algorithm is contra-indicated for SBRT-based treatment planning:. 1-D Pencil Beam. 3-D Pencil Beam. Superposition/Convolution. Monte Carlo. Both A and B
The Primary Issue: Underdosage of the PTV mparison of the 100% IDLs, Pencil beam (dashed) and MC (solid
Lateral Scattering of electrons: Monte Carlo simulati ion, 10 MV pencil beam
Impact of electron scattering on beam penumbra: conf formal llung plan Note the differences in dose gradient due to penumbral broadening PB MC
Loss of Charged Particle Equilibrium (CPE) PE exists in a volume if each char ged particle (electron) leaving the olume is replaced by an identical electron entering the volume broad photon field narrow photon field volume volume n narrow field, CPE is lost and dose reduction can be severe
Small field central axis depth dose: slab phantom 1.0 0.8 Ion Chamber MC (DPM) 6x, 2x2 cm 0.5 ρ=1 ρ=0.2 ρ= =1 0.3 0 4 8 12 16 20 24 Build down effect severee dose reduction caused by scattering of electrons into the lung tissue Dose builds up in the tumor resulting in underdosage at tumor periphery.
Implications for island tumors.0 Ion Chamber.8.5 ρ=1 ρ=0.2 ρ=1 MC (DPM) 6x, 2x2 cm ρ = 0.2 ρ =10 1.0 ρ = 1 Ring of underdosa rebuildup of dose.3 0 4 8 12 16 20 Depth (cm) 24 ing of underdosage gets larger r for smaller tumors (as size proaches the e range) and higher energies due to larger e- rang
Pencil Beam (6 MV) The Energy Effect (low vs. high MV) Pencil Beam (18 MV) 95% 95% 90% PTV 90% PTV C-S (6 MV) C-S (18 MV) 95% PTV PTV 90% 95% 90%
The Energy Effect (6 MV) AAA PB PTV DVHs (PB vs. AAA), 6 PB: mean = 70.2 Gy AAA: mean = 68.9 Gy Diff. in min. PTV dose MV = 11%
The Energy Effect (18 MV) AAA PB PTV DVHs (PB vs. AAA), 18 PB: mean = 70.5 Gy AAA: mean = 64.7 Gy 8 MV Diff. in min. PTV dose = 16%
Accuracy of dose calcu ulations for lung SBRT PTV diam. = 3.2 cm PTV vol. = 14.6 cc B PTV 80% MC 80% PTV ITV PTV 95%
DVH comparis on for the PTV MC plan recomputed using MUs from PB plan 58% 98% MC PB
PTV mean dose diff. vs. PTV diam. (mm): 100 patients PTV diam. (mm) 20 40 60 80 100-5.5 TV mean ose diff.% C-PB] -15.5-25.5
MLD (MC) as a function of MLD (PB): 50 patients 10 LD (Gy) MC 8 6 4 y = 0.9093x - 0.0755 R 2 = 09948 0.9948 2 0 0 2 4 MLD (Gy) PB 6 8 10 12
Comparative Dose Calculation Study Purpose: To investigate dosimetric differences between treatment plans computed with: 1-D Pencil beam (PB) (iplan BrainLAB) 3-D Pencil beam (Eclip pse, Varian) Anisotropic Analytic Algorithm (AAA, convolution-type, Eclipse) Pinnacle Collapsed Co ne Convolution (CCC, convolution-type, Pinnacle, Philips) Monte Carlo (iplan BrainLAB)
6X, central tumor: M U for the 1D-PB plan 100 AAA 80 60 MC CCC 1D-PB 3D-PB 40 20 0 Dose (Gy) 20 25 30 35 40 45 50 55
6X, peripheral tumor: 12 Gy x 4 100 80 MC AAA 1D-PB 60 CCC 3D-PB 40 20 0 0 10 20 30 40 50
% difference in PTV min. dose vs. MC, 11 lung cancer plans %diff diff. 1D PB 3D D-PB AAA CCC D x -D mc 1D-PB 3D Ave 29.2 21.6 1.5-2.0 STD Dev 12.11 12.8 46 4.6 51 5.1 Max Diff. 63.2 48.5 12.7 8.7
What can be done to improve the plan? ncrease the prescription dose non-uniformity in target ncrease the PTV margin and hence the field size will not ix the lateral scattering problem but will help restore CPE - ncreases unintended dose to healthy lung tissue se beams with smallest path length through lung and low nergies (choose 6X over highe her MV)
What can be done to improve the plan? on-coplanar beams may be helpful l depending di on location electron lateral scattering is cylindrically symmetric; dding more co-planar beams will not improve the coverage One size does not fit all! ach case must be judged by location and field size
VMAT (RapidArc) Example Case
RapidArc (2 partial arcs) IMRT
DVHs for normal lung and R peripheral p PTV V20 (RA) = 26.4%; MLD = 14.2 Gy V20 (IMRT) = 27.5%; 147Gy 4.7 IMRT RA IM MRT RA
VMAT vs. IMRT Case: Summary arget coverage was somewhat better with RA vs. IMRT Lungs MLD (Gy) Esophagus Mean (Gy) Heart Mean (Gy) Cord Max (Gy) MU and (Beam-on Time) RA 14.7 17.5 11.3 35.8 475 (0.8 min IMRT 14.2 20.3 8.2 36.5 1563 (2.6 min
VMAT and the Interplay Effect escribes the interaction between organ motion and MLC leaf otion rom Bortfeld et al. Physics in Medicine and Biology: 47, 2002 Interplay effect in IMRT is generally small (~1%) especially for highly fractionated treatments
VMAT and the Interplay Effect Motion amplitude: 1.3 cm in S/ /I, 0.2 cm in R/L, 0.4 cm in A/P Rapid Arc (RA) plan two 180 deg. Arcs; IMRT plan Interplay effect incorporated an nd compared to static case for both RA and IMRT plans
Tumor Motion: Exhale and Inhale States INH (Axial EXH (Axial INH (Cor) EXH (Cor)
DVHS (PTV and normal lung) with and without interplay IMRTplan PTV min. dose diff. = 18% with interplay p y
DVHS (PTV and normal lung) with and without interplay RapidA Arc plan with interplay
Overall Summary Simulation, planning and delivery of lung cancer is confounded by motion and het terogeneity in tissue density Lung tissue density impacts tumor dose deposition significantly requiring accurate dose algorithms mor size is important pay special attention to small tumo ith field sizes close to the elec ctron range Location! Location! Location! onvolution/superposition or MC-based methods should be sed for lung cancer treatmentt planning
Overall Summary High energy photon beams (> 6 MV) should be avoided VMAT/RapidArc offers an effic cient solution to RT delivery, particularly in the context of SBRT Interplay effects tend to be small in the context of IMRT or VMAT, however, further investigation is needed to fully understand these effects in th he VMAT setting
Acknowledgements Colleagues at HFHS Colleagues at Un iv of Michigan Assistance from Industry BrainLab (Feldkirchen, Germany) Philips Radiation Oncology Systems Varian Medical Systems NIH/NCI Grant Support r. Akila Viswanathan (Chair) and Dr. Ramesh Rengan (Co-Chair) Cristin Watson and other ASTRO HQ staff
Post Test Question. For the treatment of small lung tu umors located peripherally, which dos algorithm is contra-indicated for SBRT-based treatment planning:. 1-D Pencil Beam. 3-D Pencil Beam. Superposition/Convolution. Monte Carlo. Both A and B
Test Question Answer E: Under small field conditions, and lower density lung tissue, electron scattering becomes th dominant factor influencing dose to the tumor. Pencil beam algorithms (1-D or 3-D) do not proper account for electron scattering either implicitly or explicitly and have been shown to be quit inaccurate under such circumstances. Thereforee pencil beam algorithms are contra-indicated in th context of SBRT planning. References: S. Benedict, K. Yenice, D. Followil, et al. " Stereotactic body radiation therapy: The report of AAPM Task Group 101", Med Phys 37, 4078-4101 (2011). I. Das, G. Ding, A. Ahnesjo, " Small fields: Nonequilibrium radiation dosimetry", Med Phys 35 (1 206-215215 (2008).