Proton Stereotactic Body Radiotherapy as a Nephron-Sparing Approach for Patients with Localized Renal Cell Carcinoma: A Dosimetric Analysis

Abstract

Radiotherapy for renal cell carcinoma (RCC) is limited by the radiosensitivity of both the kidney and nearby structures. Patients with compromised renal function are at further risk of kidney injury from the low-dose bath from VMAT-based SBRT. Our aim was to demonstrate the potential nephron-sparing dosimetry of pencil beam scanning (PBS) proton radiotherapy SBRT for early stage RCC. Three patients previously treated for pancreatic cancer were randomly selected. Spherical CTV contours were drawn to mimic renal tumors at 4 locations (superior and inferior poles, pelvis, and mid-posteriorly) in each kidney and 3 different sizes (1, 2, and 4 cm) per location. Dose prescription was 54 Gy in 3 fractions. Proton plans were robustly optimized to 3 mm. Photon plans had a 3 mm PTV expansion. 2-arc VMAT plans and orthogonal-pair proton plans were generated for every tumor. The subset of renal-pelvis and superior pole tumors additionally had oblique pair and single beam proton plans generated. DVHs for all plans were analyzed through custom software developed in-house. Ipsilateral kidney doses were calculated after subtracting the CTV. 216 treatment plans (72 photon and 144 proton) and corresponding DVHs were generated. The ipsilateral kidney V < 12 Gy was selected as our nephron-sparing metric. Single beam proton plans showed a mean 12.7% absolute improvement in V < 12 Gy, while orthogonal pair plans showed a 4.3% improvement (see table). Oblique-pair proton plans were in-between (data not shown). After accounting for location, the difference seen for orthogonal pair plans was only significant for mid-posterior tumors (mean 18.9% improvement in V < 12 Gy, p = 0.0002). However, mean improvement in V < 12 Gy was significant regardless of location for single-beam plans. Protons also yielded superior duodenum and small bowel dosimetry: Mean V20 Gy was 0.39 cc and 2.31 cc respectively for proton plans, vs. 1.54 cc and 5.54 cc respectively for photon plans. Proton beam angle and single vs. paired proton beams did not significantly alter dosimetry to OARs. Dosimetric parameters did not vary significantly based on laterality or size of the tumor. Other ROIs met dosimetric constraints regardless of treatment modality. PBS proton treatment planning demonstrates promise in the treatment of renal tumors and significantly improves nephron-sparing dosimetry. Notable improvements in dosimetric parameters for critical radiosensitive organs were seen compared to photons in this large treatment-plan analysis. Further analyses identifying optimal patient selection criteria will support future development of clinical protocols.Abstract 3663; Table 1Single Beam PBSOrthogonal Pair PBSCritical ROIsMean Ipsilateral Kidney V < 12 GyMean Ipsilateral Kidney V < 12 GyMean Duodenum V20 GyMean Small Bowel V20 GyProtons84.2 ± 15.4%75.8 ± 16.7%0.39 ± 1.59 cc2.31 ± 8.07 ccPhotons71.5 ± 19.7%71.5 ± 19.7%1.54 ± 5.51 cc5.54 ± 16.37 ccMean difference12.7%4.3%1.15 cc3.23 ccp0.00320.0480.0002910.000041 Open table in a new tab