Can Direct Aperture Optimized IMRT using only Conventional Jaws produce satisfactory treatment plans for Head and Neck Cancer?

Guangwei Mu, Ping Xia

Department of Radiation Oncology, University of California San Francisco, San Francisco, CA, USA

INTRODUCTION

DVH pGTV (JOIMRT and MLCIMRT)

100 DVH pCTV (JOIMRT and MLCIMRT)

Background:

 IMRT has become the choice of treatment for disease sites that require critical structure sparing such as head and neck cancer.  It has evolved into a standard practice in many radiation therapy centers. However, due to the cost and knowledge required with the implementation of IMRT, many centers worldwide still use cobalt 60 machines or early versions of LINAC without multi-leaf collimators, and do not have the capability of providing IMRT to the needed patients.  DAO enabled JO-IMRT has been proven to be effective for simpler cases such as breast and prostate. But it was suspected that Head and Neck cancer might be too complex for the JO IMRT.

 It was believed that to reach a clinically acceptable JO-IMRT plan for a relatively complex case, 20 segments per beam angle was required.

Objective:

1. To investigate the feasibility of DAO enabled Jaws-only IMRT (JO-IMRT) treatment planning for complex Head and Neck cancer patients.

2. To determine the proper number of segments needed for a clinically acceptable plan for JO-IMRT.

METHODS AND MATERIALS

 Using a commercial Treatment Planning System Prowess Panther, which utilizes Direct Aperture Optimization (DAO) algorithm, a set of randomly selected six head and neck patients previously treated at our institution were planned for JO-IMRT and MLC based IMRT (MLC-IMRT). The identical clinical acceptance criteria, identical set of beam orientations as the clinical plans were used.  Number of segments per beam from 5 to 9 was experimented to try to achieve acceptable plans. For each patient, the best efforts were recorded and analyzed to verify the validity of possible clinical application.

 The plan acceptance criteria were established according to the RTOG-0225 protocol.

(a) Example of dose distribution on the slice 1 for MLCIMRT (left) and JOIMRT (right).

70 Gy 59.4 Gy 54 Gy



Actual MUs per fraction pGTV D

95%

(cGy) pCTV D

95%

(cGy) SpinalCord D

0.1cc

(cGy) Brainstem D

0.1cc

(cGy) LT-Parotid D

mean

(cGy) RT-Parotid D

mean

(cGy) pGTV COIN pCTV COIN Uniformity

100 80 60 40 20

9beam_5seg 9beam_6seg 9beam_7seg 9beam_8seg 9beam_9seg 9beam_MLC

0 0 2000 4000 Dose (cGy) 6000 8000 (a) pGTV DVHs for various number of segments per beam, and MLC plan.

DVH Brain Stem (JOIMRT and MLCIMRT)

100 80 60 40 9beam_5seg 9beam_6seg 9beam_7seg 9beam_8seg 9beam_9seg 9beam_MLC 20 0 0 1000 2000 3000

Dose (cGy)

4000 5000 6000

(c)Brain Stem DVHs for various number of segments per beam, and MLC plan.

100 80 60 40 20 0 0 1000 2000 3000 4000

Dose (cGy)

5000 6000 7000 Figure 2. DVHs of the targets and some key sensitive structures for the JO-IMRT plans with 9 to 5 segments per beam angle. For comparison reason, the MLC-IMRT plan with 9 segments per beam was also shown.

Patient #1 MLC

443

JO

842 7080 6008 3840 4640 2503 2520 7053 5988 4050 4680 1791 2565

DVH L-Parotid (Jaw Only)

Patient #2 MLC

411

JO

1623 7087 5955 5065 4380 N/A 2518 7003 5965 4470 5390 N/A 2516

Patient #3 MLC

564

JO

1027 7003 6180 4225 3860 2316 4471 9beam_5seg 9beam_6seg 9beam_7seg 9beam_8seg 9beam_9seg 9beam_MLC 7007 6008 3846 5080 1928 2621 19.2% 62.2% 86% 17.3% 56.7% 87.1% 47.1% 65.3% 81.7% 38.2% 59.5% 77.8% 42.1% 65.5% 82.6% 56.4% 69.5% 83.7% 80 60 40 20 0 0 2000 4000

Dose (cGy)

6000 8000

(b) pCTV DVHs for various number of segments per beam, and MLC plan.

DVH Spinal Cord (JOIMRT and MLCIMRT)

9beam_5seg 9beam_6seg 9beam_7seg 9beam_8seg 9beam_9seg 9beam_MLC 100 40 20 80 60 9beam_5seg 9beam_6seg 9beam_7seg 9beam_8seg 9beam_9seg 9beam_MLC 0 0 1000 2000

Dose (cGy)

3000 4000

(d)Spinal Cord DVHs for various number of segments per beam, and MLC plan.

DVH R-Parotid (Jaw Only)

100 80 60 40 20 0 0 1000 2000 3000 4000

Dose (cGy)

5000 6000 7000 9beam_5seg 9beam_6seg 9beam_7seg 9beam_8seg 9beam_9seg 9bean_MLC Figure 3. Part of an actual JO-IMRT delivery sequence, the intensity map and the monitor units (MU) associated with each aperture.

RESULTS

 With a randomly selected patient, 8 or 9 segments per beam enabled the successful reaching a clinically satisfactory JO-IMRT plan for head and neck patients within reasonable time frame.  Six randomly selected HNC patients were re planned with inverse-planned JO-IMRT and MLC IMRT. The JO-IMRT plans were shown to be effective. Of the six HNC patients, all had at least one JOIMRT plan, with 9 segments per beam, to meets the acceptance criteria. Four acceptable MLC-IMRT plans with 9 segments per beam were achieved, with another one very close. The amount of time to reach an acceptable JO-IMRT varied widely but on average required around 15 times of iterations as for other simpler anatomic sites. The number of segments for quality plan did not have to be high. Nine segments per beam was a good choice to arrive at quality plan in most cases.  Among the JO-IMRT and MLC-IMRT plans with identical number of apertures per beam angle, no significant difference was observed in the average DVHs, and the plan conformal index.  The averaged plan quality evaluation metrics:

Metric Plan COIN (pGTV) COIN (pCTV) Uniformity MLC IMRT

37.70% ± 18.67% 63.72% ± 4.44% 82.82% ± 2.46%

JO IMRT

37.95% ± 18.06% 61.32 ± 6.39% 83.55% ± 3.11%

MU

453 ± 69 982 ± 333 Figure 4. Part of an actual MLC-IMRT delivery sequence, the intensity map and the monitor units (MU) associated with each aperture.

45 Gy (b) Example of dose distribution on the slice 2 for MLCIMRT (left) and JOIMRT (right).

Figure 1. Dose distribution of a specific patient at two locations. On left is the IMRT plan delivered with MLC and on the right with conventional jaws only.

Table 1. Example of the key endpoint doses and plan quality evaluation metrics for three nasopharyngeal cancer patient planned with DAO JO-IMRT and MLC-IMRT, with 9 segments per beam angle.

.

CONCLUSIONS

 JO-IMRT is feasible for head and neck tumors. The plan quality of JO-IMRT plans was comparable to that of traditional IMRT or DAO optimized MLC-IMRT plans with regards to plan conformity and uniformity as well as sensitive structure sparing.  JO-IMRT plans require nearly twice numbers of MUs than the MLC IMRT plans.

 Nine segments per beam were sufficient to obtain acceptable jaw-only IMRT plans.

UCSF