Proton LET Optimization Via Iterative Convex Relaxation Method

PURPOSE/OBJECTIVE(S)

If not optimized, the LET distribution can greatly impact normal tissue toxicity near tumor targets, because the LET often peaks at the distal edge of Bragg peak. LET optimization can account for biological effectiveness of protons during treatment planning, for minimizing biological proton dose and hot spots to normal tissues. However, the LET optimization is nonlinear and nonconvex to solve, which poses a great challenge in optimization. This work will develop an effective LET optimization method via iterative convex relaxation (ICR).

MATERIALS/METHODS

In contrast to the generic nonlinear optimization method, such as Quasi-Newton (QN) method, that does not account for specific characteristics of LET optimization, ICR is tailored to LET modeling and optimization in order to effectively and efficiently solve the LET problem. Specifically, nonlinear dose-averaged LET term is iteratively linearized and becomes convex during ICR, while nonconvex dose-volume constraint and minimum-monitor-unit constraint are also handled by ICR, so that the solution for LET optimization is obtained by solving a sequence of convex and linearized convex subproblems. Since the high LET mostly occurs near the target, a 1cm normal-tissue expansion of clinical target volume (CTV) (excluding CTV), i.e., CTV1cm, is defined to as an auxiliary structure during treatment planning, where LET is minimized.

RESULTS

ICR was validated in comparison with QN for abdomen, lung, and head-and-neck (HN) cases. ICR was effective for LET optimization, as ICR substantially reduced the LET and biological dose in CTV1cm the ring, with preserved dose conformality to CTV. Compared to QN, ICR had smaller LET, physical and biological dose in CTV1cm, and higher conformity index values; ICR was also computationally more efficient, which was about 3 times faster than QN. A lung case is presented in the table, where the quantities from top to bottom are computational time T (unit: second); total objective F, dose objective Fd and LET objective FL (unit: 10-3); conformity index for physical dose CId and biological dose CIb; mean LET L (unit: keV/μm), mean physical dose d and mean biological dose b (in ratio to prescription dose) for CTV1cm; mean LET L (unit: keV/μm), mean physical dose d and mean biological dose b (in ratio to prescription dose; unit: 10-1) for the heart.

CONCLUSION

A LET-specific optimization method based on ICR has been developed for solving proton LET optimization, which has been shown to be more computationally efficient than generic nonlinear optimizer via QN, with better plan quality in terms of LET, biological and physical dose conformality.

Multi-Institutional Experience of Proton Therapy for Rhabdomyosarcoma and Ewing Sarcoma in the Proton Collaborative Group (PCG) Prospective Registry

PURPOSE/OBJECTIVE(S)

To report on outcomes, acute toxicities, and the use of dose-escalation with proton therapy (PT) in patients with rhabdomyosarcoma and Ewing sarcoma in a prospective multi-institutional registry (PCG).

MATERIALS/METHODS

Data on patients with primary rhabdomyosarcoma and Ewing sarcoma treated with definitive PT (defined as ≥45 Gy) were queried from the PCG registry. A similar query was performed of our institutional database with IRB approval. Overall survival rates were calculated by Kaplan-Meier. Toxicities were scored using CTCAE v4.0.

RESULTS

A total of 354 patients across 10 institutions (203 rhabdomyosarcoma, 151 Ewing sarcoma) met the eligibility criteria. Median age was 9 years (Interquartile Range: 5-15). Median dose was 50.4 GyRBE for rhabdomyosarcoma patients (Range: 45-66 GyRBE) and 55.8 GyRBE for Ewing sarcoma patients (Range: 45-66 GyRBE). Median follow-up was 2.4 years (Range 0.3-12.3 years). Two-year overall survival rates were 81.1% (95% CI: 73.7%-88.5%) for rhabdomyosarcoma and 79.1% (95% CI: 71.7%-86.2%) for Ewing sarcoma. The Table lists the prescription doses delivered by tumor histology; 28.1% of rhabdomyosarcoma and 21.9% of Ewing sarcoma patients, respectively, received dose-escalated radiotherapy (defined as >50.4 Gy for rhabdomyosarcoma and >55.8 Gy for Ewing sarcoma). Excluding alopecia and skin desquamation, 153 patients (43.2%) developed any acute grade 2+ non-hematologic toxicity, while 49 patients (13.8%) developed one or more grade 3 toxicities. The most common grade 3 toxicities were anorexia/weight loss (7.3%), pain (7.3%) mucositis/esophagitis (4.8%), and nausea/vomiting (3.1%). One grade 4 toxicity (esophagitis) and no deaths were reported during treatment.

CONCLUSION

In this multi-institutional prospective registry, 28.1% of rhabdomyosarcoma and 21.9% of Ewing sarcoma patients received dose-escalated PT, with 13.8% of patients developing grade 3 toxicities. Long-term outcomes for disease control and late toxicity and anticipated cooperative group trial results are needed to fully assess the benefits and risks of dose-escalated radiotherapy for these tumors.

Factors related to outcome of early and delayed prosthetic joint infections

In this prospective study, we evaluate the impact of adherence to a diagnostic and therapeutic protocol on prosthetic joint infections (PJI) diagnostic accuracy and outcome.

PATIENTS AND METHODS

Patients with early or delayed PJI referred over a 5-year period were included. Diagnosis was based on characteristic clinical signs, radiographic findings and microbiological evidence. Antibiotics were chosen on the basis of microbiological findings, and drugs active against methicillin-resistant staphylococci were administered if no microbiological evidence had been obtained.

RESULTS

Inclusion criteria were met in 159 cases (median age 64 years, males 45%). 56 were early infections and 103 delayed infections. Comorbidities were reported in 99 (62%) cases. Positive cultures were obtained in 122/159 (77%), coagulase-negative staphylococci were cultured in 20%, Staphylococcus aureus in 28%, and Pseudomonas aeruginosa in 7%. In early infections, cure rate after debridement and antibiotic therapy was 80%. In delayed infections, cure rate after two-stage exchange was 85%. Of 28 patients with delayed infection treated with antibiotics without surgery, only 8 (29%) infections were suppressed 48 weeks after treatment discontinuation. Rifampin afforded a better outcome.

CONCLUSION

Appropriate diagnostic and surgical procedures and microbiologically driven antibiotic therapy including rifampin are recommended to improve diagnostic accuracy and outcome.