2279 - Assessing and Quantifying Patient External Contour Changes for Node Positive Dose Escalated Prostate Cancer Using Proton Therapy

M. A. Fagundes^1,2, A. Wroe^1,2, Z. Fellows¹, and A. Gutierrez^1,2; ¹Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL, ²Miami Cancer Institute, Baptist Health South Florida, Miami, FL

Purpose/Objective(s): Delivery of prostate proton therapy is dependent on body contour changes which may alter beam range between skin surface entrance and targeted prostate and nodes. Robust optimization, accounting for range uncertainty and set up margin (SM), compensates for some uncertainties but does not account for tissue differences (TD) [difference between CBCT and reference CT external contours] during treatment. In this study, we investigated the frequency and quantified the magnitude of TD in each beam path for each positive pelvic node in high-risk prostate cancer patients. Materials/

Methods: Eighteen (n=18) patients with prostate cancer received dose escalation to 1-8 positive nodes (total 55 nodes). Treatment consisted of 78Gy/39fxs in 2 phases; boost phase: 24 Gy/12fxs to prostate + proximal or positive seminal vesicle when applicable; nodal phase P+SV to 54Gy/27fxs, elective nodes to 48.6Gy with SIB up to 62.1Gy to GTVLN+ (EQD2= 67.4Gy). SM for node positive GTVs were 7 mm cephalo-caudad and 5 mm elsewhere. Nodal phase used lateral opposed and single PA fields. For setup, prostate was matched to fiducials using daily kV/kV, and weekly CBCTs were acquired. CBCTs were used to quantify TD along the beam direction of each beam at the level of the targeted positive nodes. TDs were recorded for each node and for each beam. For analysis, nodes were assumed to be fixed to bony anatomy. Nodes were binned by anatomical location: Iliac, Obturator, Perirectal, Presacral, Common iliac and Retroperitoneal. Histograms and cumulative percentage plots were generated to quantify magnitude and frequency.
Results: A total of 154 CBCTs were evaluated. For all patients, positive TDs were observed in all nodal locations in at least 1 weekly CBCT and the overall median TD was 1 mm [Range: 0-12mm]. Table 1 illustrates a comprehensive summary of the beam specific TD by positive node locations. TD were largest for common iliac and retroperitoneal and smallest for iliac and obturator. For lateral beams, > 95% of TD were < 5 mm for Iliac, < 6 mm for Obturator, < 8 mm for Perirectal or Presacral and < 10 mm for Common iliac and Retroperitoneal nodes.
Conclusion: Tissue differences in the beam path of positive nodes do occur and their magnitudes correlate with anatomical location. Location and beam specific set up margins for positive nodes should be considered during proton planning. Abstract 2279 - Table 1: Summary of Tissue Differences (TD) as a function of Beam Direction and Positive Lymph Node Anatomical Location

		Iliac	Obturator	Perirectal	Presacral	Common	Retroperitoneal
# Pts	18	12	11	7	4	6	1
# Pos. Nodes	55	18	15	9	5	6	2
Lt & Rt Lateral	< 3 mm	85.4%	72.3%	59.6%	69.7%	58.3%	68.7%
	< 4 mm	93.4%	83.7%	70.5%	78.9%	66.6%	75.0%
	< 5 mm	96.7%	92.0%	80.1%	89.4%	75.9%	75.0%
	< 6 mm		95.4%	84.2%	93.4%	80.5%	81.2%
	< 7 mm			93.1%	94.7%	86.1%	93.7%
	< 8 mm			96.6%	96.0%	90.7%	93.7%
	< 9 mm					92.6%	93.7%
	< 10 mm					96.3%	100.0%
PA	< 3 mm	97.8%	97.7%	97.3%	100%	96.3%	93.7%
	< 4 mm	100%	100%	100%		100%	100%