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A Dosimetric Comparison of TBI with a Dedicated Co-60 Irradiator to Tomotherapy-Based TBI and TMLI

S Ahmed1*, M Dominello1,2, A Hammoud2, A Nalichowski1,2, J Burmeister1,2, (1) Wayne State University School of Medicine, Detroit, MI, (2) Karmanos Cancer Institute, Gershenson ROC, Detroit, MI

Presentations

(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.

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Keywords

TBI, Dose Volume Histograms, Bioeffect Dose

Taxonomy

TH- External Beam- Photons: General (most aspects)

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