2799 - A Feasibility Study of Using ⁶⁴Cu-ATSM PET for Radiation Dose Painting Escalation in Locally Advanced Rectal Cancers

V. Guimas¹, M. le Thiec¹, G. Gibbard², K. A. Al Feghali², D. de Jong², S. Supiot³, E. Rio¹, N. Guillaume¹, R. Movassaghi¹, and A. Moignier¹; ¹Institut de Cancérologie de lOuest René Gauducheau, Nantes, France, ²RefleXion Medical, Inc., Hayward, CA, ³Institut de Cancérologie de lOuest René Gauducheau, Saint-Herblain, France

Purpose/Objective(s): In locally advanced rectal cancer (LARC), which is relatively radioresistant, dose escalation increases clinical and histological tumor response and improves local control ^1-2. However, radiation-induced toxicity is a major limiting factor for dose-escalation. Additionally, the new ⁶⁴Cu-ATSM PET-CT tracer allows for dose-painting RT since it is a potential cellular redox witness of oxidative stress, responsible for radioresistance and tumor aggressiveness ^3-4_. The medical technology company machine (Hayward, CA, USA) is a biology-guided radiotherapy (BgRT)-capable machine that is also cleared for image-guided radiation therapy. This study compares a dose-painting and escalation IMRT plan on the X1 treatment planning system (TPS) based on a ⁶⁴Cu-ATSM biological target volume (BTV), with a conventional VMAT plan for LARC. Materials/

Methods: Pelvic dose distributions were retrospectively calculated for 14 patients who underwent neoadjuvant chemoradiotherapy (CRT) for LARC and had prior-CRT diagnostic ⁶⁴Cu-ATSM PET-CT, ¹⁸FDG PET-CT and MRI. They were planned using two different boost definition and dose-escalation RT techniques: ⁶⁴Cu-ATSM PET-CT-based BTV on the medical technology company TPS and GTV based on ¹⁸FDG PET-CT and MRI for VMAT with technology company. High-risk planning target volume (PTV HR) was defined as BTV or GTV with 10 mm isotropic margin except 7 mm laterally for both plans. Low-risk PTV (PTV LR) was defined as mesorectum and lymph node drainage area according to the recommendations of Valentini et al ^5. The delineated organs at risk (OARs) were small bowel, large bowel, bladder, left and right femoral head and penile bulb or vagina. The prescribed PTV HR and PTV LR doses were 25Gy in 5 fractions and 45Gy in 25 fractions respectively, with homogeneous target dose. Dose constraints from preRADAR trial were used ⁶. The summed EQD2 OARs dose distributions were compared.
Results: The BTV (mean: 4.7cc [min: 0.9cc – max: 18.6cc]) was on average eleven times smaller than the GTV (mean: 52.7cc [min: 10.9cc – max: 253.7cc]). The 2 plans met the V95% requirement with slightly better coverage for medical technology companys IMRT plans compared to VMAT plans (98.8% and 97.6% respectively). A medical technology companys plans resulted in lower dose to OARs compared to VMAT plans.
Conclusion: This dosimetry study is an encouraging precursor to allow clinical dose-painting and dose-escalation for LARC on the medical technology companys system. The benefit has the potential for further increase by the future incorporation of online adaptive BgRT capabilities.

	Volume (cc)	Constraint (Gy) (EQD2 a/ß = 3Gy)	Medical technology company plan (Gy) Mean (Min-Max)	VMAT plan (Gy) Mean (Min-Max)
Bowel Small	0.5	70	45.3 (24.4-85.6)	48.8 (24.8-86.6)
	10	40	36.8 (17.4-57.0)	38.8 (17.6-73.0)
Bowel Large	0.5	60.16	50.1 (43.1-66.1)	59.9 (43.6-84.2)
Bladder	0.5	80.56	54.3 (43.7-83.1)	68.9 (47.5-85.4)
Femoral Heads	0.5	-	30.8 (22.5-40.1)	34.2 (24.4-41.5)
Penil Bulb/vagina	0.5	-	40.1 (6.9-71.4)	45.3 (7.2-81.2)