(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
To present biological and physical dosimetric comparisons between Total Body Irradiation (TBI) with a dedicated Co-60 irradiator (Gamma-TBI), Tomotherapy-based TBI (Tomo-TBI) and Total Marrow and Lymphatic Irradiation (Tomo-TMLI).
Methods:
For Gamma-TBI, a prescribed dose of 12Gy is delivered in 6-8 AP/PA fractions BID, 1.5-2Gy/fraction. Cerrobend lung blocks are used to reduce mean lung dose to approximately 9Gy. For 5 patients treated with Gamma-TBI, comparative Tomo-TBI and Tomo-TMLI plans were generated retrospectively for 12Gy in 6 fractions BID delivered head-first-supine.
For Tomo-TMLI, the PTV consisted of bones, lymphatics, brain, spleen, liver, and gonads (PTV-TMLI) in contrast the Tomo-TBI PTV (PTV-TBI) included the entire patient body excluding a 1 cm outer rind, lungs, and heart. Since the AP/PA fields for Gamma-TBI were calculated on supine and prone CT scans, respectively, we used the intersection of these two PTVs for target dose comparison between TBI techniques.
Results:
The PTV-TBI comparison reveals better prescription dose coverage with Tomo-TBI (V12Gy=76-88%, median doses 12.5Gy-13.0Gy) than with Gamma-TBI (V12Gy=31-61%, median doses 11.6Gy-12.2Gy). Lung dose was significantly reduced with Tomo-TBI (EQD2=4.7Gy±0.3SD) relative to Gamma-TBI (EQD2=7.9Gy±0.5SD). Similarly, EQD2 for heart was reduced from 9.3Gy±0.6SD to 4.1Gy±0.6SD. EQD2 for kidneys was reduced to 3.3Gy±0.3SD for Tomo-TMLI relative to 13.3Gy±0.4SD for Tomo-TBI without kidney-sparing.
With Tomo-TMLI, V12Gy for PTV-TBI (excluding kidneys) ranged from 38-54% (compared to 90% with Tomo-TBI) showing significantly reduced dose to uncontoured normal tissues. Conversely, V12Gy for PTV-TMLI with Tomo-TBI ranged 84-92% (compared to 90% with Tomo-TMLI) indicating greater dose heterogeneity in the Tomo-TBI plan.
Conclusion:
Tomotherapy-based TBI can reduce lung and heart EDQ2 to <5 Gy with better target volume coverage than a large-field TBI unit. Tomo-TMLI can provide uniform dose to marrow and lymphatic regions while substantially lowering dose to remaining normal tissues. Both techniques facilitate possible dose escalation without additional toxicity over conventional TBI.
Keywords
TBI, Dose Volume Histograms, Bioeffect Dose
Taxonomy
TH- External Beam- Photons: General (most aspects)
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